4. THE ZONATION OF ROCKY LITTORAL AREAS AROUND LITTLE CAYMAN
G.W. P o t t s
A b s t r a c t
The d i s t r i b u t i o n o f t h e r o c k y l i t t o r a l f a u n a a t s i t e s r e p r e s e n t i n g d i f f e r e n t d e g r e e s o f e x p o s u r e were examined around L i t t l e Cayman, I t was found t h a t t h e m o l l u s c a n s p e c i e s c o m p o s i t i o n , s p e c i e s morphology and v e r t i c a l d i s t r i b u t i o n was a f f e c t e d by t h e e x p o s u r e o f a p a r t i c u l a r s i t e .
I n t r o d u c t i o n
During J u l y a n d Augusc 1975 t h e Royal S o c i e t y and Cayman I s l a n d Government s u p p o r t e d a n e x p e d i t i o n t o L i t t l e Cayman where a s p e c t s o f t h e e c o l o g y o f t h e i s l a n d were s t u d i e d . The Cayman I s l a n d s (between 1g015'N and 19O45'N l a t i t u d e and 7 g 0 4 4 ' and 81°27'W l o n g i t u d e ) a r e composed o f t h r e e l i m e s t o n e i s l a n d s which a r e formed b y t h e p r o j e c t i n g p e a k s o f a r a n g e o f s u b m a r i n e m o u n t a i n s t h a t form t h e Cayman Ridge t o
t h e n o r t h w e s t o f J a m a i c a . According t o Matley (1926) t h e c e n t r a l p a r t o f e a c h i s l a n d i s comprised o f a n o l d " B l u f f Limestone" w h i l e t h e
p e r i p h e r y i s made up o f a m i x t u r e o f c o r a l s a n d , m a r l , and e l e v a t e d l i m e s t o n e r e e f which h a s been weachered t o form what i s c a l l e d t h e
" I r o n s h o r e f o r m a t i o n " (Doran i 9 5 4 , I d y l l , 19663. The l i m e s t o n e s h o r e i s e x t e n s i v e l y e r o d e d b y t h e a c t i o n o f r a i n , t h e s e a , and a l s o t h e g r a z i n g a c t i o n o f some l i t t o r a l m o l l u s c s a n d t h e a b r a s i v e a c t i o n o f t h e s h e l l s o r s p i n e s o f some s p e c i e s o f m o l l u s c and echinoderm, a s d e s c r i b e d by Ginsburg (1953) f o r t h e F l o r i d a Keys r e g i o n . I t was t h i s r o c k t h a t was examined i n t h e p r e s e n t work on l i t t o r a l z o n a t i o n . The Cayman I s l a n d s a r e p a r t i c u l a r l y i n t e r e s t i n g b e c a u s e t h e y form a d i s c r e t e i s l a n d u n i t l y i n g i n d e e p w a t e r midway between J a m a i c a , Cuba and t h e n o r t h e r n c o a s t o f S o u t h America and y e t t h e r e have been few s t u d i e s s p e c i f i c a l l y r e l a t i n g t o t h e l i t t o r a l r e g i o n t o t h e s e i s l a n d s . A b b o t t (1958)
p r o v i d e s t h e most i m p o r t a n t c o n t r i b u t i o n t o t h e m a r i n e m o l l u s c s o f Grand Cayman and i n c l u d e s b r i e f e c o l o g i c a l n o t e s i n c o n j u n c t i o n w i t h a
r e a s o n a b l y comprehensive s p e c i e s l i s t . The z o n a t i o n o f l i t t o r a l s p e c i e s on L i t t l e Cayman h a s n o t b e e n s t u d i e d b e f o r e a l t h o u g h i t i s t o b e
A t o l l R e s e a r c h B u l l e t i n No-. 241: 23-42, 1980.
e x p e c t e d t h a t i n g e n e r a l terms it would b e s i m i l a r t o t h a t d e s c r i b e d f o r Grand Cayman ( A b b o t t 1958) and f o r o t h e r a r e a s o f t h e C a r i b b e a n
( S t e p h e n s o n and S t e p h e n s o n (1950, 1 9 5 2 ) , Coomans (19581, Voss a n d Voss ( 1 9 5 5 ) , Bakus (1968)
,
Bakus (1975),
and o t h e r s ) . N o n e t h e l e s s t h e s u r v e y d i d p r o d u c e new r e c o r d s from a h i t h e r t o u n s t u d i e d i s l a n d w i t h d a t a on t h e i n f l u e n c e o f e x p o s u r e on l i t t o r a l r o c k f a u n a s .Methods
A number o f a r e a s were s e l e c t e d a r o u n d L i t t l e Cayman t o d e m o n s t r a t e t h e z o n a t i o n o f l i t t o r a l a n i m a l s ( F i g u r e 11). Each a r e a was examined f o r s p e c i e s c o m p o s i t i o n , d i s t r i b u t i o n and abundance and t h e n t h e
p r o f i l e s were drawn o f t h e s h o r e from t h e low w a t e r n o t c h t o above t h e s t r a n d l i n e which o c c u r r e d a p p r o x i m a t e l y one m e t r e above t h e l e v e l o f h i g h w a t e r . S p e c i a l a t t e n t i o n was p a i d t o t h e d i s t r i b u t i o n o f s p e c i e s i n r e l a t i o n t o t h e i r v e r t i c a l z o n a t i o n a t s i t e s on t h e l e e w a r d and windward s h o r e s . The i n f l u e n c e o f e x p o s u r e and wave a c t i o n were s t u d i e d by s e l e c t i n g s u i t a b l e b o u l d e r s and a s s e s s i n g t h e d i s t r i b u t i o n o f m o l l u s c s on t h e seaward and landward f a c e s o f t h e s e r o c k s .
U n f o r t u n a t e l y t i m e d i d n o t p e r m i t t a k i n g p h y s i c a l measurements o f e x p o s u r e which i s h e r e b a s e d upon t h e i n t e r p r e t a t i o n o f s u c h f e a t u r e s a s c o r a l t y p e , p r e s e n c e and e x t e n t o f a s t o r m b e a c h , r e e f development, n a t u r e o f p a r t i c u l a t e s u b s t r a t e s a s w e l l a s m e t e o r o l o g i c a l d a t a .
No p u b l i s h e d t i d a l d a t a was found f o r L i t t l e Cayman, however i t i s u n l i k e l y t h a t i t w i l l d i f f e r s i g n i f i c a n t l y from t h e A d m i r a l t y T i d e T a b l e p r e d i c t i o n s f o r George Town, Grand Cayman ( f i g . 1 2 ) and t h i s h a s b e e n u s e d a s t h e b a s i s f o r a s s e s s i n g t h e t i d a l l e v e l s .
F i g . 12 g i v e s t h e f l u c t u a t i o n s i n t i d a l h e i g h t t h r o u g h o u t t h e p e r i o d o f t h e e x p e d i t i o n t o g e t h e r w i t h t h e h o u r by h o u r c h a n g e s i n t i d a l h e i g h t on a s p r i n g t i d e and on a n e a p t i d e . I n a r e a s w i t h s u c h a s m a l l t i d a l a m p l i t u d e i t i s p r o b a b l e t h a t l o c a l c o a s t a l morphology and m e t e o r o l o g i c a l c o n d i t i o n s a r e more i m p o r t a n t i n i n f l u e n c i n g t h e z o n a t i o n o f t h e f a u n a and f l o r a t h a n t h e t i d a l r a n g e ( G o s l i n e , 1965)
.
The s i t e s s e l e c t e d f o r e x a m i n a t i o n r e p r e s e n t a r a n g e o f e x p o s u r e s from t h e e x t r e m e s h e l t e r o f m e n I s l a n d w i t h i n S o u t h Hole Sound t o t h e exposed B l u f f a t E a s t End, and s e v e r a l s i t e s r e p r e s e n t i n g t h e i n t e r m e d i a t e e x p o s u r e t y p e s . The s i t e s examined were; i n o r d e r from s h e l t e r e d t o more exposed s i t e s ; Owen I s l a n d w i t h i n S o u t h Hole Sound, Jackson" Bay Rocks, P r e s t o n Bay, Rubble r i d g e t h a t e n c l o s e s S o u t h Hole Sound, West End P o i n t r o c k s a n d t h e B l u f f a t E a s t End.While s e v e r a l g r o u p s o f m a r i n e b i o t a were r e p r e s e n t e d i n t h e
l i t t o r a l o f L i t t l e Cayman t h e p r e s e n t s u r v e y d e a l s i n t h e main w i t h t h e m o l l u s c a and decapod c r u s t a c e a . Where p o s s i b l e p r o v i s i o n a l f i e l d
i d e n t i f i c a t i o n s were made and samples t a k e n f o r c h e c k i n g upon r e t u r n . The f o l l o w i n g a u t h o r i t i e s h a v e been c o n s u l t e d when i d e n t i f y i n g s p e c i f i c g r o u p s ; m o l l u s c s , Warmke and A b b o t t (1961) M o r r i s ( 1 9 7 3 ) ; N e r i t i d a e , R u s s e l l (1941) ; t h e L i t t o r i n a z i c z a c "complex", Borkowski and Borkowski
( 1 9 6 9 ) ; h e r m i t c r a b s , Provenzano ( 1 9 5 9 ) ; s t o m a t o p o d s , Manning ( 1 9 6 8 ) ; m a r i n e a l g a e , Chapman ( 1 9 6 3 ) ; and m a r i n e a n g i o s p e r m s , den H a r t o g
( 1 9 7 0 ) . The t e r m i n o l o g y u s e d t o d e s c r i b e t h e s h o r e zones f o l l o w s
Stephenson and Stephenson (1950)
.
R e s u l t s
The r e s u l t s o f t h e f i e l d surveys a r e o u t l i n e d below, f i r s t
d e s c r i b i n g t h e d i f f e r e n t survey s i t e s , a comparative n o t e on t h e s i z e d i s t r i b u t i o n of i n t e r t i d a l mollusca i n r e l a t i o n t o exposure. F i n a l l y t h e work i s d i s c u s s e d i n r e l a t i o n t o o t h e r surveys from t h i s r e g i o n . P r e s t o n Bay
P r e s t o n Bay ( P l a t e 22) i s on t h e South West s i d e of L i t t l e Cayman and c o n s i s t s of a sand and c o r a l r u b b l e beach w i t h o c c a s i o n a l o u t c r o p s o f eroded limestone. Beyond t h i s l i t t o r a l zone i s t h e s u b l i t t o r a l upper r e e f t e r r a c e which v a r i e s between 200 and 400 m wide and which t e r m i n a t e s on t h e seaward s i d e w i t h a c o r a l b u t t r e s s zone which i t s e l f i s followed by a lower r e e f t e r r a c e . I n a d d i t i o n t o t h e limestone beach o u t c r o p a s m a l l c o n c r e t e j e t t y was examined.
Fig. 3 shows t h e p r o f i l e of a r e p r e s e n t a t i v e rock o u t c r o p from t h i s r e g i o n and a l s o i n d i c a t e s t h e d i s t r i b u t i o n p a t t e r n o f t h e most i m p o r t a n t s p e c i e s . Above t h e beach rock i s t h e s u p r a l i t t o r a l f r i n g e comprising c o r a l sand and c o a r s e c o r a l r u b b l e . T h i s mobile s u b s t r a t e was devoid of macrofaunal elements e x c e p t i n some a r e a s o f sand i n which t h e burrowing c r a b , O c y p o d e sp. (probably 0 . q u a d r a t a ( F a b r i c i u s ) ) was t o b e found. The only mollusc t h a t occurred i n t h i s r e g i o n was
T e c t a r i u s m u r i c a t u s on wood on t h e s t r a n d l i n e and o c c a s i o n a l l y on c o a s t a l shrubs, such a s S u r i a n a m a r i t i m a (L. )
,
B o r r i c h i a a r b o r e s c e n s( L . ) DC. and o t h e r s .
The r o c k s o f t h e upper m i d l i t t o r a l zone c o n t a i n s p e c i e s t h a t
depend upon p e r i o d s of submergence f o r t h e i r s u r v i v a l . The most a c t i v e i s t h e c r a b , G r a p s u s g r a p s u s ( L . ) which was common wherever r o c k s extend i n t o t h i s r e g i o n . One specimen of G o n i o p s i s c r u e n t a t a ( L a t r e i l l e ) was a l s o found although i t i s more f r e q u e n t l y a s s o c i a t e d w i t h e s t u a r i n e muds (Chace and Hobbs, i 9 6 9 ) . The l i t t o r a l gastropods E c h i n i n u s n o d u l o s u s , N o d i l i t t o r i n a t u b e r c u l a t a ( P l a t e 24) a r e t o l e r a n t of t h e more exposed c o n d i t i o n s found on t h e d r y s u r f a c e s of upward f a c i n g r o c k s , w h i l e L i t t o r i n a z i c z a c and L . l i n e a t a a r e u s u a l l y found i n c r e v i c e s , coming o u t and moving over t h e rock s u r f a c e s when t h e r o c k s a r e wet o r a t high t i d e . Where L . z i c z a c i s found it may occur i n l a r g e numbers and e s t i m a t e s of 1700 p e r square m were made where t h e s u b s t r a t e type and c r e v i c e s provide s u i t a b l e h a b i t a t s . I t was n o t p o s s i b l e t o d i s t i n g u i s h between L . z i c z a c , L . l i n e o l a t a and L . l i n e a t a
i n t h e f i e l d s o f o r t h e s e and subsequent p o p u l a t i o n e s t i m a t e s they have been grouped t o g e t h e r ( s e e Borkowski and Borkowski 1 9 6 9 ) . Although
samples from t h e d i f f e r e n t a r e a s i n d i c a t e d which s p e c i e s were p r e s e n t . E. n o d u l o s u s was n o t a s numerous and reached a maximum d e n s i t y o f approximately 500 p e r square m. Both L . z i c z a c and E . n o d u l o s u s had patchy d i s t r i b u t i o n s and over most of t h e a r e a t h e i r d e n s i t i e s were much
lower than t h o s e mentioned above. N o d i l i t t o r i n a t u b e r c u l a t a l i k e E . n o d u l o s u s was d i s t r i b u t e d over t h e upper f a c e s of t h e r o c k s and
occupied much t h e same t i d a l l e v e l . I t was i n t e r e s t i n g t o note t h a t *
d e s p i t e t h e s i m i l a r i t y i n h a b i t a t between t h e s e s p e c i e s , i t was u n u s u a l t o f i n d t h e s e s p e c i e s t o g e t h e r on t h e rock ( s e e Abbott 1954 and comments on t h e d i s t r i b u t i o n o f t h e s e s p e c i e s )
.
The lower m i d l i t t o r a l zone i s occuped i n t h e main b y n e r i t e s
( P l a t e 2 6 ) , b u t a l s o c o n t a i n s f a u n i s t i c e l e m e n t s o f t h e upper m i d l i t t o r a l zone on r o c k s t h a t d r a i n and d r y q u i c k l y , and i n f r a l i t t o r a l f r i n g e
s p e c i e s l e s s t o l e r a n t o f exposure t o a i r . A t P r e s t o n Bay N e r i t a versicolor o c c u r r e d i n t h e upper m i d l i t t o r a l zone and e x t e n d e d down t o n e a r l y low w a t e r mark. T h i s s p e c i e s had t h e w i d e s t r a n g e o f any o f t h e l i t t o r a l g a s t r o p o d s and was dominant i n t h e upper l e v e l s o f t h e lower m i d l i t t o r a l zone. The mid-tide l e v e l rock p o o l s were o c c u p i e d i n t h e main b y N e r i t a t e s s e l l a t a which o v e r s m a l l a r e a s was found i n d e n s i t i e s of o v e r 2 0 0 p e r s q u a r e m. On t h i s p a r t i c u l a r beach N . p e r o r o n t a was i n f r e q u e n t and a few o n l y were found a t a b o u t mid-tide l e v e l i n r o c k p o o l s o r a t t h e e d g e s o f them t o g e t h e r w i t h L i t t o r i n a m e l e a g r i s .
On t h e lower p a r t o f t h e m i d l i t t o r a l zone i n b o t h p o o l s and on t h e open r o c k s was t h e c h i t o n , A c a n t h o p l e u r a g r a n u l a t a which w i t h
N . t e s s e l a t a were t h e dominant s p e c i e s a t low w a t e r . The i n f r a l i t t o r a l f r i n g e c o n t a i n e d a wave c u t n o t c h t h a t was s u b j e c t e d t o s t r o n g wave a c t i o n and which showed an i n c r e a s e d s p e c i e s d i v e r s i t y . Here t h e f i r s t macroscopic a l g a e were found and t h e i n t r u s i o n o f some members of t h e immediate s u b l i t t o r a l b e n t h i c communities. On t h e low w a t e r n o t c h C i t t a r i u m p i c a and P u r p u r a p a t u l a were found, t h e former r e a c h i n g d e n s i t i e s i n e x c e s s o f 40 p e r s q u a r e m; b u t t h e c o n v o l u t e d n a t u r e o f t h e s h o r e made i t i m p o s s i b l e t o do more t h a n e s t i m a t e a t t h e p o p u l a t i o n d e n s i t y . P . p a t u l a was l e s s common. On t h e l a r g e r specimens o f
C . p i c a specimens o f t h e l i m p e t Acmaea l e u c o p l e u r a were f r e q u e n t l y t o b e found a t t h e edge of t h e body w h i r l a d j a c e n t t o t h e c o l u m e l l a . None o f t h e s e s u c k e r l i m p e t s were found on C . p i c a t h a t was l e s s t h a n 4 cm i n s h e l l l e n g t h a l t h o u g h some were found f r e e on t h e n e a r b y r o c k s of t h e low w a t e r r i d g e . I n t h e low l e v e l r o c k p o o l s and a l o n g t h e
i n f r a l i t t o r a l f r i n g e specimens o f E c h i n o m e t r a l u c u n t e r ( L . ) and i e s s commonly Diadema a n t i l l a r u m ( P h i l i p p i ) were found, w h i l e i n t h e
s u b l i t t o r a l r e g i o n E c h i n o m e t r a r e a c h e d d e n s i t i e s o f a p p r o x i m a t e l y SO p e r s q u a r e m i n t h e numerous c r e v i c e s and f i s s u r e s o f t h e s u b l i t t o r a l r o c k .
A s i n g l e specimen o f each o f T e g u l a f a s c i a t a and C l a t h r o d i l l i a m e l a n e s i a n a were found i n t h e lower m i d l i t t o r a l zone. Beneath l a r g e s t o n e s i n t h e lower m i d l i t t o r a l zone and i n t h e i n f r a l i t t o r a l f r i n g e t h e T r e e o y s t e r , I s o g n o m o n r a d i a t u s was n o t uncommon. Small specimens o f t h e b r i t t l e s t a r , Ophiocoma e c h i n a t a (Lam.) were sometimes found b e n e a t h s t o n e s i n l i t t o r a l p o o l s w h i l e l a r g e r specimens were found b e n e a t h s t o n e s o v e r t h e upper r e e f t e r r a c e t o g e t h e r w i t h 0 . p u m i l a
( L a m . )
.
The rock p o o l s and t h e immediate s u b l i t t o r a l r o c k c o n t a i n e d most commonly t h e a l g a e A c e t a b u l a r i a c r e n u l a t a Lamx., P a d i n a s a n c t a e - c r u c i s Bbrg. and an u n i d e n t i f i e d s p e c i e s o f D i c t y o t a s p . , t o g e t h e r w i t h p a t c h e s o f T u r b i n a r i a t u r b i n a t a B a r t . which was a l s o f r e q u e n t l y washed a s h o r e i n wave swept a r e a s .
The i n f r a l i t t o r a l f r i n g e shows t h e b e g i n n i n g o f t h e f a u n a o f t h e u p p e r r e e f t e r r a c e w i t h c o r a l s t o l e r a n t o f t u r b i d c o n d i t i o n s ,
S i d e r a s t r a e a s i d e r e a ( E l l i s & S o l . ) , M i l l e p o r a c o m p l a n a t a L. and o t h e r s . Where t h e s u b s t r a t e i s f i n e l y p a r t i c u l a t e t h e e e l g r a s s T h a l a s s i a
t e s t u d i n u m Kijnig may come c l o s e t o low w a t e r mark d e n s e l y c o v e r i n g t h e a r e a . S y r i n g o d i u m f i l i f o r m i s K i i t z
.
was a l s o found a s s o c i a t e d w i t h T. t e s t u d i n u m i n i n s h o r e a r e a s .P r e s t o n Bay J e t t y
W i t h i n P r e s t o n Bay was a s m a l l c o n c r e t e j e t t y 1 8 m l o n g and 1 . 6 0 m wide. The a l i g n m e n t was a p p r o x i m a t e l y North S o u t h and i t s s l o p e was
20 cm o v e r i t s l e n g t h e n d i n g 20 cm above LWMST a t t h e s o u t h e r n e n d . The p r e v a i l i n g wind was from t h e S o u t h E a s t and t h e w a t e r movement on t h e r e e f f l a t was from E a s t t o West s o t h a t t h e e a s t s i d e o f t h e j e t t y was s u b j e c t e d t o g r e a t e r t u r b u l e n c e t h a n t h e w e s t s i d e .
The d i s t r i b u t i o n o f g a s t r o p o d s r e f l e c t e d t h e d i f f e r e n c e i n t h e e x p o s u r e on t h e two s i d e s o f t h e j e t t y and c l e a r l y d e m o n s t r a t e d t h a t wave a c t i o n and w a t e r movement w i l l h a v e a g r e a t e r e f f e c t on t h e d i s t r i b u t i o n o f l i t t o r a l s p e c i e s t h a n t h e t i d a l a m p l i t u d e . F i g . 1 4 i n d i c a t e s how t h e d i s t r i b u t i o n v a r i e d a l o n g t h e j e t t y and from t h e exposed E a s t s i d e t o t h e s h e l t e r e d West s i d e . T h i s d i s t r i b u t i o n i s p a r t i c u l a r l y s t r i k i n g a s t h e j e t t y p r e s e n t e d a r e l a t i v e l y u n i f o r m
s u b s t r a t e t y p e w i t h o u t t h e numerous c r e v i c e s o f t h e s u r r o u n d i n g r o c k s . The s p e c i e s r e c o r d e d from t h e j e t t y i n c l u d e d L . z i c z a c ,
N . v e r s i c o l o r , N . p e l o r o n t a , N . t e s s e l a t a , A . g r a n u l a t a , C . p i c a
( P l a t e 30) and P. p a t u l a . These g a s t r o p o d s were mainly l i v i n g on t h e v e r t i c a l w a l l s o f t h e j e t t y a t e a c h s i d e . A t h i g h t i d e o r when wave a c t i o n was s u f f i c i e n t t o c a u s e t h e t o p o f t h e j e t t y t o b e awash, t h e g a s t r o p o d s would move a b o u t o v e r t h e h o r i z o n t a l s u r f a c e . The c r a b , C a l l i n e c t e s s p . was s e e n below t h e t i d e l e v e l on t h e s i d e s o f t h e j e t t y w h i l e G r a p s u s g r a p s u s was found i n t h e upper m i d l i t t o r a l r e g i o n .
West End Rocks
West End P o i n t ( P l a t e 2 3 ) c o n s i s t e d o f a r o c k p l a t e a u below a sandy beach. The r o c k was s e v e r a l hundred m e t r e s i n e x t e n t and
c o n t a i n e d s e v e r a l l a r g e rock poo1.s which were examined. A p r o f i l e o f t h e r o c k i s g i v e n i n f i g . 1 5 t o g e t h e r w i t h t h e d i s t r i b u t i o n o f t h e most common l i t t o r a l s p e c i e s . To t h e S o u t h and E a s t o f t h e a r e a a s t e e p l y s l o p i n g s h i n g l e and c o r a l r u b b l e b e a c h e x i s t s and which j o i n s a s e r i e s o f l o n g i t u d i n a l l i m e s t o n e o u t c r o p s which s l o p e t o t h e seawards and which p r o j e c t from t h e s h a l l o w s u b l i t t o r a l o f t h e u p p e r r e e f t e r r a c e
t o a h e i g h t a p p r o x i m a t e l y e q u a l t o HTL. These l e d g e s j o i n t h e r u b b l e b e a c h and e x t e n d E a s t w a r d s t o j o i n P r e s t o n Bay and t h e P r e s t o n Bay I n s h o r e f o r m a t i o n s d e s c r i b e d above. To t h e North o f West End P o i n t r o c k s t h e p a r t i c u l a t e b e a c h c o n t i n u e d t o j o i n up w i t h t h e e x t e n s i v e l i m e s t o n e s h o r e a t S a l t Rocks.
A t P r e s t o n Bay t h e beach i s p r o t e c t e d from t h e f u l l wave e n e r g y by t h e b u t t r e s s zone o f t h e upper f o r e - r e e f t e r r a c e . A t West End P o i n t
t h e r e i s no s u c h p r o t e c t i v e b a r r i e r and t h e waves b r e a k d i r e c t l y on t h e r o c k s below t h e Tower l i g h t house. I t i s t h i s wave a c t i o n t h a t
r e p l e n i s h e s r o c k p o o l s i n t h e m i d l i t t o r a l zone w i t h s e a w a t e r more f r e q u e n t l y t h a n would b e e x p e c t e d by t i d a l movement a l o n e .
The s u p r a l i t t o r a l f r i n g e c o n t a i n e d a s t r a n d l i n e o f wood d e b r i s w i t h few m a r i n e s p e c i e s , and o n l y T e c t a r i u s m u r i c a t u s was found i n abundance, a l t h o u g h even t h i s s p e c i e s was a b s e n t i n a r e a s o f sand and where t h e d r i f t wood r e s t e d on s a n d . The sand i t s e l f was h i g h l y mobile and y e t c o n t a i n e d a s m a l l p o p u l a t i o n o f Ocypode s p . The sand movement had s c o u r e d t h e upper m i d l i t t o r a l rock o u t c r o p s which were d e v o i d o f l i f e , b u t below i t s i n f l u e n c e T e c t a r i u s m u r i c a t u s was found on t h e open r o c k and i n dense numbers on t h e s e a w a t e r t o l e r a n t S e s u v i u m p o r t u l a c a s t r u m L. ( P l a t e 2 5 ) . The l i t t o r a l zone c o n t a i n e d N o d i l i t t o r i n a t u b e r c u l a t a and E c h i n i n u s n o d u l o s u s which were found on t h e exposed r o c k f a c e s and i n m o i s t c r e v i c e s t o g e t h e r w i t h L i t t o r i n a z i c z a c and L . l i n e a t a .
The lower m i d l i t t o r a l s p e c i e s were more numerous t h a n a t P r e s t o n Bay and s e v e r a l s p e c i e s were abundant t h a t weie h a r d l y r e p r e s e n t e d a t t h i s former s i t e . Once more t h e n e r i t e s , N . p e l o r o n t a , N . v e r s i c o l o r and N . t e s s e l l a t a were common ( P l a t e 2 6 ) and i n p a r t i c u l a r N - t e s s e l l a t a which f a v o u r s t h e r o c k p o o l h a b i t a t . I n a d d i t i o n t o t h e s e , t h e z e b r a n e r i t e ( P u p e r i t a p u p a ) a n d L i t t o r i n a m e s p i l l u m were found i n l a r g e numbers i n rock-pools ( s e e P l a t e s 34 and 3 5 ) .
The lower m i d l i t t o r a l zone was more d i s s e c t e d t h a n a t P r e s t o n Bay Rocks and c o n t a i n e d a g r e a t e r d e n s i t y o f m o l l u s c s . N . t e s s e l l a t a e x t e n d s down t o t h e low w a t e r n o t c h where C . p i c a , A . y r a n u l a t a become t h e dominant rnollusca. I n t h e low w a t e r p o o l s o c c a s i o n a l E c h i n o m e t r a 1 u c u n t e r and Diadema a n t i l l a r u m were found. The low w a t e r
c o n c e n t r a t i o n s o f E c h i n o m e t r a i n c r e a s e d i n t h e s u b l i t t o r a l and r e a c h e d d e n s i t i e s e q u i v a l e n t t o t h e s u b l i t t o r a l r e g i o n o f P r e s t o n Bay ( 5 0 / s q m ) . Owing t o t h e t u r b u l e n t c o n d i t i o n s on West End P o i n t i t was n o t p o s s i b l e t o examine t h e immediate s u b l i t t o r a l from t h e seaward, b u t i n s p e c t i o n a t extreme low w a t e r showed t h a t t h e a l g a e were s i m i l a r t o t h o s e a t P r e s t o n Bay w i t h A c e t a b u l a r i a c r e n a t a , P a d i n a s a n c t a e - c r u c i s , D i c t y o t a s p . , and T u r b i n a r i a t u r b i n a t a were common, growing s p a r s e l y i n t h e low l e v e l rock p o o l s .
I n t h i s r e g i o n t h e e x t e n t o f t h e d i s t r i b u t i o n o f m o l l u s c s i s a f f e c t e d by t h e i n c r e a s e d e x p o s u r e of t h i s s i t e . The g r e a t e r wave
a c t i o n n o t o n l y i n c r e a s e s t h e h e i g h t t o which c e r t a i n m i d l i t t o r a l s p e c i e s w i l l e x t e n d , b u t a l s o h a s a profound i n f l u e n c e upon t h e p o o l f a u n a s . The mollusca P l a n a x i s n u c l e u s , P. p l i n e a t u s , L i t i o p a m e l a n o s t o m a and T e y u l a e x c a v a t a were o n l y found a t t h i s s i t e a t low t i d e l e v e l .
The h e r m i t c r a b C l i b a n a r i u s t r i c o l o r (Gibbes) was common and o f t e n v e r y abundant i n m i d - t i d e l e v e l p o o l s i n t h e s h e l l s o f E c h i n i n u s
n o d u l o s u s , N o d i l i t t o r i n a t u b e r c u l a t a , L i t t o r i n a z i c z a c and C e r i t h i u m v a r i a b i l e .
Bloody Bay and J a c k s o n ' s P o i n t
I n t h i s r e g i o n t h e u p p e r s h o r e c o n s i s t s o f a p o o r l y d e v e l o p e d s t o r m b e a c h o r c o r a l b o u l d e r s and r u b b l e which r i s e s t o a h e i g h t o f 2 m above m i d - t i d e l e v e l . Below t h i s i s a s a n d o r s a n d and r u b b l e beach t h a t e x t e n d s i n t o t h e l i t t o r a l r e g i o n and i n some a r e a s o n t o t h e upper f o r e - r e e f t e r r a c e o f t h e s u b l i t t o r a l r e g i o n ( F i g . 1 6 ) . O c c a s i o n a l o u t c r o p s o f r o c k p r o v i d e t h e s o l i d s u b s t r a t e e s s e n t i a l f o r t h e development o f a l i t t o r a l z o n a t i o n and two o f t h e s e were examined.
The c o a s t a l f r i n g e c o n s i s t e d m a i n l y o f s e a g r a p e ( C o c c o l o b a u v i f e r a ( L . ) L.) e x c e p t where a n a r e a had been c l e a r e d t o p r o v i d e t h e b a s i s f o r a c o c o n u t p l a n t a t i o n . I n some s u i t a b l e a r e a s S c a e v o l a p l u m i e r i ( L . ) V a h 1 , S u r i a n a . m a r i t i m a and S e s u v i u m p o r t u l a c a s t r u m L. extended down t o h i g h w a t e r mark and on t h e s e T e c t a r i u s m u r i c a t u s were found i n some members. T h i s h i g h l e v e l g a s t r o p o d was a l s o found up t o 2 m above MTL upon t h e d r i f t w o o d o f t h e s u p r a l i t t o r a l f r i n g e . The f i r s t t r u l y
l i t t o r a l m o l l u s c s , E. n o d u l o s u s , L . t u b e r c u l a t a , L . z i c z a c a n d l . l i n e a t a +;ere o n l y p r e s e n t i n s m a l l numbers on t h e r o c k s , b u t d i d o c c u r on t h e t o p s o f l a r g e b o u l d e r s i n t h e l i t t o r a l r e g i o n where t h e y would b e
s u b j e c t e d t o t h e p e r i o d i c s p l a s h from waves. Ti-ie lower m i d l i t t o r a l i s more e x t e n s i v e t h a n a t West End P o i n t o r P r e s t o n Bay and N e r i t a
v e r s i c o l o r and f i c a n t h o p i e u r a g r a n u l a t a ( P l a t e 33) were p a r t i c u l a r l y a b u n d a n t b o t h r e a c h i n g d e n s i t i e s i n e x c e s s o f 40-50 p e r s q m.
I f - t e s s e l l a t a and N. p e l o r o n t a were p r e s e n t i n t h e permanent p o o l s o f t h e lower m i d l i t t o r a l zone and C i t t a r i u m p i c a was found a l o n g t h e
i n f r a l i t t o r a l f r i n g e under and among t h e s t o n e s and l e d g e s o f t h i s zone, J a c k s o n ' s P o i n t was n o t s u b j e c t e d t o heavy wave a c t i o n d e s p i t e t h e
a b s e n c e o f r u b b l e r i d g e . F i g . 1 6 shows a s e c t i o n o f t h e s h o r e a t J a c k s o n ' s P o i n t and t h e d i s t r i b u t i o n o f t h e most common s p e c i e s .
A t J a c k s o n ' s P o i n t t h e d i s t r i b u t i o n o f g a s t r o p o d s around a l a r g e b o u l d e r was examined t o s e e i f t h e a s p e c t and o r i e n t a t i o n had an e f f e c t u p r ! t h e i r c ! , i s t - r i b u % i o n . r ~ g . n: 17 s h o x s a p r o i i i e C E t h i s b o u l d e r and -the d i s t r i b u t i o n o f some s p e c i e s t h a t wers on i - t . It i s s e e n t h a t t h e
" . ?
iLi>per midl.it-to-ra:l sp:.(:ir-;s E . ~ i o d u i o s u s and L. z i c z a c a:<e ur:affccted i~
t h e i r t i d a l l e v e l r e g a r d l e s s of t h e a s p e c t o f t h e b o u l d e r . However,
f i l s z o n a t i o n o f i;:le lo-vse- m i d i f ~ - t . t a r a l s p e c i e s A. y r a r j u l a t a 236 C . p i c a ri..r?di.cates 3 h i g h e r t i d a l d i s t r i b u - t i o n or! t h e seaward s i d e where t h e y w o ~ i d be s ~ - i e c . t e d - t o gre;li;er wave s p l a s h , By c o n t r a s t t h e n e r i t e s o c c u r r e d on t h e s h e l t e r e d landward s i d e o f t h e r o c k w i t h N. t e s s a l l a t a s l i g h t l y h i g h e r t h a n $7, v e r s i c o l o r and N. p e l o r o n t a . The i n f r a l i t t o r a l f r i n g e c o n t a i n e d t h e a l g a V a l o n i a s p . and i n r o c k c r e v i c e s E c h i n o m e t r a l u c u n t e r was common around t h e b o u l d e r .
The B l u f f - E a s t End
The E a s t End o f L i t t l e Cayman ( P l a t e 29) c o n s i s t s o f t h e B l u f f L i m e s t o n e , a w e a t h e r e d and d i s s e c t e d r o c k o u t c r o p i n t h e l i t t o r a l r e g i o n and w i t h a s t o r m b e a c h o f c o r a l r u b b l e b e h i n d t h e i r o n s h o r e . The s i t e i s v e r y exposed t o s o u t h e a s t winds.
A diagram of t h e d i s t r i b u t i o n of t h e common s p e c i e s i s g i v e n i n F i g . 18. While t h e d e n s i t i e s o f m o l l u s c a a r e v e r y v a r i a b l e some
maximum f i g u r e s were r e c o r d e d a t d i f f e r e n t l e v e l s on t h e s h o r e . I n t h e s u p r a l i t t o r a l f r i n g e on d r i f t wood T e c t a r i u s m u r i c a t u s was v e r y common
( P l a t e 32) r a c h i n g d e n s i t i e s o f between 144 and 152 p e r s q u a r e m w h i l e on t h e r o c k s of t h e upper m i d l i t t o r a l zone d e n s i t i e s o f between 32 and 56 p e r s q u a r e m were t y p i c a l . N o d i l i t t o r i n a t u b e r c u l a t a r e a c h e d d e n s i t i e s o f 60 p e r s q u a r e m and i n rock p o o l s L . m e s p i l l u r n o c c u r r e d a t o v e r 168 p e r s q u a r e m . N e r i t e s were p r e s e n t i n t h e l i t t o r a l zone, b u t i n s m a l l e r c o n c e n t r a t i o n s t h a n t h e more s h e l t e r e d s h o r e s i n t h e s o u t h and w e s t o f t h e I s l a n d . They t e n d e d t o c o n c e n t r a t e a t t h e margins o f rock p o o l s of t h e m i d l i t t o r a l zone. N . versicolor was t h e most common and N . t e s s e l l a t a occupying t h e same r e g i o n was l e s s
a b u n d a n t . N. p e l o r o n t a was more w i d e l y d i s t r i b u t e d a l t h o u g h a t lower d e n s i t i e s . The o t h e r s p e c i e s commonly a s s o c i a t e d w i t h rock p o o l s was t h e s m a l l l i t t o r i n i d , L i t t o r i n a m e s p i l l u m which was u s u a l l y e v e n l y d i s t r i b u t e d o v e r t h e p o o l . A c a n t h o p l e u r a g r a n u l a t a and C . p i c a were common a t low w a t e r l e v e l o c c u r r i n g i n d e n s i t i e s o f 24 and 36 p e r s q u a r e m r e s p e c t i v e l y w h i l e P u r p u r a p a t u l a was o n l y found a t low w a t e r and on t h e low w a t e r n o t c h a t d e n s i t i e s between 2 and 10 p e r s q u a r e m.
With t h e i n c r e a s e i n wave a c t i o n a t t h e e a s t end of L i t t l e Cayman s e v e r a l s p e c i e s were p r e s e n t t h a t were n o t r e c o r d e d from more s h e l t e r e d s i t e s . The most abundant i n t h e lower m i d l i t t o r a l r e g i o n was t h e mussel B r a c h i d o n t e s e x u s t u s t h a t was common i n c r e v i c e s o f t h i s r e g i o n . O t h e r s r e c o r d e d were Isoynornon a l a t u s , C o r a l l i o p h y l l a a b b r e v i a t a and an u n i d e n t i f i e d s p e c i e s o f v e r m e t i d .
Ruhble Ridge
On t h e s o u t h s i d e o f L i t t l e Cayman r u n n i n g a l m o s t t h e whole l e n g t h of t h e i s l a n d t h e f r i n g i n g r e e f i s s e p a r a t e d from t h e seaward t e r r a c e s by a r i d g e of c o r a l rock and r , A b l e ( P l a t e 2 9 ) . T h i s r i d g e does n o t seem t o be comparab1.e t o "he a l g a l rid.ges o f Indo P a c i f i c a t o l l s , an6 comprises of b o u l d e r s r a n g i n g from a f e w cms up t o a b o u t a m e t r e i n d i z i i e t e r . 'The r i d g e was b r e a c h e d i n s e v e r a i p l a c e s forming c h a n n e l s
f r o n t h e s e a i n t o t h e s h a l l o w l a ~ o o n , South Hole Sound. VJaves s p l a s h e d o v e r t h e r i d g e i n many p l a c e s and. a t h i g h t i d e a l m o s t t h e e n t i r e zone .gas covered in s e s wa.ter and subjected t o c:oiisi.derable wave a c t i o x . Small p a t c h e s o f t'ne b l a c k and r e d mangrove grew on t h e i n s i d e edge of t h e r u b b l e r i d g e . The molluscan z o n a t i o n on t h e r i d g e was s i m i l a r t o t h e i r o n s h o r e o u t c r o p s d e s c r i b e d above e x c e p t t h a t t h e r e was no
e q u i v a l e n t t o t h e s u p r a l i t t o r a l zone. The z o n a t i o n was o n l y found on t h e seaward s i d e o f t h e r i d g e and n o t on t h e l a g o o n a l s i d e where
a p p a r e n t l y t h e l a c k o f w a t e r movement and t h e consequent s i l t i n g p r e v e n t e d t h e development o f a rocky s h o r e molluscan f a u n a .
The l a g o o n a l beach of t h e r u b b l e r i d g e c o n s i s t e d o f sand o r f i n e mud g e n e r a l l y w i t h T h a l a s s i a t e s t u d i n u r n growing i n d e n s e p a t c h e s
immediately below low w a t e r . T h i s was r e p l a c e d by s i l t e d c o r a l r u b b l e upon which a y e l l o w a l g a e f i l m c o v e r e d l a r g e a r e a s and i n which mangroves had begun t o grow. The c e n t r a l p a r t o f t h e r i d g e c o n t a i n e d t h e l a r g e s t b o u l d e r s and i t was t h i s r e g i o n t h a t c o n t a i n e d t h e upper m i d l i t t o r a l
f a u n a w i t h T e c t a r i u s m u r i c a t u s and i n c r e v i c e s L i t t o r i n a z i c z a c and
L . l i n e a t a . A t t h e same l e v e l , b u t i n more s h a d e d s i t u a t i o n s , E c h i n i n u s n o d u l o s u s and N . t u b e r c u l a t a were f o u n d , w h i l e under s t o n e s and i n t h e m o i s t a r e a s between t h e b o u l d e r s N e r i t a t e s s e l l a t a and N . versicolor were common t o g e t h e r w i t h g r a p s i d and p o r c e l l a n i d c r a b s . O n t h e o u t e r r u b b l e b e a c h on t h e seaward s i d e o f t h e r i d g e , n e r i t e s were s t i l l common b u t w i t h t h e i n c r e a s e i n w a t e r movement C i t t a r i u m p i c a and A c a n t h o p l e u r a q r a n u l a t a became more a b u n d a n t u n t i l a t low w a t e r mark and i n low a r e a s o f t h e r i d g e where w a t e r s t r e a m e d from t h e seaward s i d e o f t h e r i d g e t o t h e l a g o o n t h e s e two s p e c i e s became t h e dominant m o l l u s c s . C i t t a r i u m p i c a on t h e r u b b l e r i d g e was c o v e r e d i n c a l c a r e o u s a l g a e w h i l e t h o s e a s s o c i a t e d w i t h t h e P r e s t o n Bay j e t t y were r e l a t i v e l y c l e a n ( P l a t e s 30
& 3 1 ) . I t i s i n t e r e s t i n g t o n o t e t h a t d e s p i t e e x t e n s i v e s e a r c h e s
n e i t h e r N e r i t a p e l o r o n t a n o r P u r p u r a p a t u l a were found. Beneath s t o n e s a t t h e l o w e s t l e v e l s E c h i n o m e t r a l u c u n t e r and I s o q n o m o n r a d i a t u s , were f o u n d , t h e e c h i n o i d e x t e n d i n g i n g r e a t e r d e n s i t i e s i n t o t h e s u b l i t t o r a l . Owen I s l a n d
Owen I s l a n d l i e s w i t h i n S o u t h Hole Sound and i s s h e l t e r e d from t h e open s e a by t h e r u b b l e r i d g e ( s e e a b o v e ) . The I s l a n d was n o t
e x t e n s i v e l y s t u d i e d , b u t samples o f t h e common m o l l u s c a were c o l l e c t e d . The t y p i c a l r o c k y s h o r e m o l l u s c a were p r e s e n t , b u t r e p r e s e n t a t i v e s o f t h c low w a t e r s p e c i e s , C i t t a r i u m p i c a and P u r p u r a p a k u l a were a b s e n t a s were t h e l i t t o r a l p o o l s p e c i e s . I n t h e b l a c k mangrove a r e a s t o t h e n o r t h o f Owen I s l a n d L i t t o r i n a a n q u l i f e r a was common and among t h e r o o t s t h e c r a b C a l l i n e c t e s m a r g i n a t u s was f r e q u e n t l y s e e n .
The r e m a i n i n g molusc C e r i t h i u m l i t e r a t u m r e c o r d e d from Owen I s l a n d i s one t h a t i s c h a r a c t e r i s t i c o f t h e T h a l a s s i a b e d s t h a t a r e p l e n t i f u l a r o u n d t h i s I s l a n d .
S i z e d i s t r i b u t i o n o f l i t t o r a l m o l l u s c a i n r e l a t i o n t o e x p o s u r e I n Table 3 t h e mean s i z e s o f t h e common l i t t o r a l mollusca a r e
r l
g i v e n a g a i n s t t h e s i t e s from which t h e y were c o l l e c t e d . h e s i t e s a r e a r r a n g e d f r s m t h e most s h e l t e r e d a t C)r,\me I s l a n d t o t h e n o s t exposed on t h e B l u f f end 9% t h e i s l a n d a t E a s t End. While seine o f t h e sarnpl-e s i z e s a r e i n a d e q u a t e t o draw c l e a r c o n c l u s i o n s from, some g e n e r a l t r e n d s a r e p r e s e n t which i r l c i i c a t e t h e r e l a - t i o n s h i p between e x p o s u r e a n d
m o l l u s c a n d i s t r i b u t i o n and morphology, I n t h e f i r s t p l a c e t h e h i g h l e v e l T e c t a r i u s m u r i c a t u s d o e s n o t h a v e any s i g n i f i c a n t s i z e d i f f e r e n c e s between t h e s h e l t e r e d and exposed s i t e s . Only on t h e Rubble Ridge o f t h e S o u t h Hole Sand was a s m a l l r e c o r d found from a s i n g l e specimen.
The s i t e i s g e n e r a l l y t o o low f o r t h i s s p e c i e s , n e v e r e x t e n d i n g a s h i g h a s t h e s u p r a l i t t o r a l f r i n g e and t h e specimen found c a n n o t b e c o n s i d e r e d r e p r e s e n t a t i v e . L i t t o r i n a m e s p i l l u m was found a t t o o few s i t e s t o g i v e a n y i n d i c a t i o n o f t h e e f f e c t o f e x p o s u r e on body s i z e , b u t i t would b e e x p e c t e d t o b e s l i g h t i n view o f t h e f a c t t h e s p e c i e s i s m o s t l y
r e s t r i c t e d t o p o o l s . The same can b e s a i d o f P u p e r i t a pupa which i s a l s o c o n f i n e d t o r o c k p o o l s and was o n l y found a t t h e r o c k s a t West End.
T a b l e 3. S i z e d i s t r i b u t i o n of t h e Common l i t t o r a l Mollusca around L i t t l e Cayman
1 6 . 8 Owen I s l a n d
( 3 2 ) J a c k s o n Bay 1 6 . 3 ( 8 ) P r e s t o n Bay -
8 . 0 l e Ridge
Rubb (1) West
1 7 . 2 E a s t End
( 2 1 )
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-The f i g u r e s I n t h e body of l n c t & le ~ e p r e s e n t t h e mean s t z e I n (mm) o f named m o l l u s c s .
The sample s i z e 1s g i v e n 1 , ) blaclteLs b e n e a t h each s l z e . The <absence o f f i g u r e s d o e s n o t mean t h a t a s p e c i e s d i d n o t o c c u r a t a s s t n b u t t h a t a sample was n o t c ~ l l c c t e d and measured.
With t h e e x c e p t i o n o f E c h i n i n u s n o d u l o s a t h e l i t t o r i n i d s showed a g r a d a t i o n i n s i z e w i t h i n c r e a s i n g e x p o s u r e . Thus ~ o d i l i t t o r i n a t u b e r c u l a t a , L i t t o r i n a z i c z a c , L. l i n e a t a and L . l i n e o l a t a a l l showed a d e c r e a s e i n s i z e on more exposed s i t e s . The same g e n e r a l t r e n d was shown w i t h P u r p u r a pa t u l a and A c a n t h o p l e u r a q r a n u l a t a
.
The n e r i t e s,
N e r i t a v e r s i c o l o r , N . t e s s e l l a t a and N . p e l o r o n t a d i d n o t show a c l e a r r e l a t i o n s h i p w i t h t h e e x p o s u r e o f a s i t e , b u t e a c h o f t h e s e s p e c i e s was v e r y l a r g e a t Owen I s l a n d , t h e most s h e l t e r e d s i t e . I t can o n l y b e assumed t h a t p e r h a p s t h e s h e l t e r e d n a t u r e o f t h i s s i t e o r some o t h e r f a c t o r t h a n t h e d i r e c t i n f l u e n c e o f e x p o s u r e was r e s p o n s i b l e f o r t h e l a r g e Owen I s l a n d specimens. C i t t a r i z a m p i c a i s most f r e q u e n t l y found a t t h e i n f r a l i t t o r a l n o t c h and i n two f a i r l y e x t e n s i v e s e a r c h e s and c o l l e c t i o n s a t P r e s t o n Bay and E a s t End showed t h a t specimens were l a r g e r a t t h e more exposed E a s t End. C i t t a r i u m a p p a r e n t l y f a v o u r s t u r b u l e n t c o n d i t i o n s and c o n s t a n t water movement a s i s shown by i t s h a b i c a t p r e f e r e n c e and s i z e d i s t r i b u t i o n . What i s s u r p r i s i n g i s t h e
l a r g e mean s i z e o f t h o s e i n d i v i d u a l s on t h e Rubble Ridge of South Hole Sound whrch a p p e a r s t o r e s u l t from t h e absence o f v e r y s m a l l i n d i v i d u a l s found a t he o t h e r s i t e s .
I t i s i n t e r e s t i n g t o n o t e t h a t w i t h t h e a b s e n c e of b r a c k i s h a r e a s around t h e c o a s t , t h e n e r i t e N e r i t a f u l y z a r a n s was a b s e n t . T h i s was p r e d i c t e d by R u s s e l l (1941) ~ 1 1 0 poiii-ked o u t t h a t w i t h i k s p r e f e r e n c e f o r b r a c k i s h e s t u a r i n e and ii&our c o n d i t i o n s i t was u n l i k e l y t o be found on t h e s m a l l e r i s l a n d s o f t h e C a r i b b e a n .
Rock Pool Faunas
D e s p i t e a s m a l l t i d a l r a n g e t h e r e were s e v e r a l a r e a s where
permanent l i t t o r a l rock p o o l s e x i s t e d and which had an i n t e r e s t i n g and v a r i e d . f a u n a and f l o r a . The environment t h a t t h e s e 9001s o f f e r e d must have been s u b j e c t e d t o a wide r a n g e o f t e m p e r a t u r e s and s a l i n i t y and t h e s e f a c t o r s a r e l i k e l y t o be i z p o r t a n t i n i n f l u e n c i n g t h e s p e c i e s found i n t h e s e poo3.s
.
Tsm a r e a s c o n t a i n i n q -rock p o o l s were exm2heCip one a t West End r o c k s {PLa-Le % 3 j and t h e o t h e r j.ri j - a c k s 3 n 2 s s a y andfrom b o t h ,li.re,lis s p e c i n e n s of t h e g a s t r o p o d s ;.-ere co.!..l~cted an5 t h e i r
- ,.\",
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I ',+-!Z~I : L I . O ~ n o t ~ d .
r- he L i t t o r a l roc!< p o o l s f a l l in?-- ti.1ree maic t y r ; e s ; tile :n.iqh ~ : ~ v ~ ~ . ~ - . mid t i d e m a low l e v e l p o o l s , and e a c h i s c h a r a c t e r i s e d by the t y p e o f
fauna and f l o r a p r e s e n t . I t i s n o t p o s s i b l e t o g i v e p r e c i s e l i m i t s upon t h e h e i g h t above c h a r t datum t h a t each c a t e g o r y may o c c u r i n , a s t h i s v a r i e s w i t h t h e d e g r e e of e x p o s u r e t o which t h e s h o r e i s s u b j e c t e d , b u t i n g e n e r a l t h e g r e a t e r t h e e x p o s u r e and wave a c t i o n t h e h i g h e r w i l l e a c h p o o l t y p e o c c u r , High l e v e l p o o l s were found i n t h e s u p r a l i t t o r a l f r i n g e and a r e f i l l e d o n l y b y s p r a y from wave a c t i o n o r by r a i n , and a r e c h a r a c t e r i s e d by g r e e n and brown a l g a l s l i m e s and by t h e l a c k of an o b v i o u s macrofauna. These p o o l s o c c u r a t t h e l e v e l o f T e c t a r i u s
m u r i c a t u s which may b e found on a d j a c e n t r o c k s , s t r a n d e d wood and s h r u b s of t h e s u p r a l i t t o r a l f r i n g e , b u t never i n t h e p o o l s . O c c a s i o n a l l y t h e c r a b G r a p s u s q r a p s u s r e t r e a t e d i n t o t h e h i g h l e v e l p o o l s when d i s t u r b e d , b u t m o s t l y t h e y remained w i t h o u t a macrofauna.
The p o o l s o f t h e m i d l i t t o r a l zone were s u b j e c t e d t o changes o f w a t e r w i t h e a c h t i d e and on t h e exposed w e s t e r n end r o c k s t o o c c a s i o n a l r e p l e n i s h m e n t from wave a c t i o n . These p o o l s were c h a r a c t e r i s e d by a v a r i e d g a s t r o p o d f a u n a and o c c a s i o n a l l y some c o n t a i n e d v e r y l a r g e numbers o f t h e h e r m i t c r a b Clibanarius tricolor. Young f i s h were common and t h e l i t o r a l goby Bathygobius soporator was p r e s e n t i n a l l permanent r o c k p o o l s . The p o o l s i n t h e lower m i d l i t t o r a l zone were
s u b j e c t e d t o c o n s t a n t wave a c t i o n and t h e f a u n a l e l e m e n t s may c o n t a i n an o c c a s i o n a l r e p r e s e n t a t i v e washed i n d u r i n g rough c o n d i t i o n s from t h e i n f r a l i t t o r a l f r i n g e and t h e s u b l i t t o r a l r e g i o n . O c c a s i o n a l a l g a e were found i n c l u d i n g Acetabularia crenulata which was n o t i c e a b l y more
common i n t h e low l e v e l p o o l s which w i t h Dictyota s p . , Turbinaria turbinata, Padina sanctae-crucis and o t h e r s were t y p i c a l o f t h e i n f r a l i t t o r a l f r i n g e and upper r e e f t e r r a c e . Stomatopods o f t h e
g e n e r a Gonadactylus and Pseudosquilla were a l s o f r e q u e n t l y found i n t h e c r e v i c e s w i t h i n t h e p o o l s .
D i s c u s s i o n
The w a t e r e r o d e d l i m e s t o n e o f t h e l i t t o r a l r e g i o n p r o v i d e s an
i n t e r e s t i n g and d i v e r s e h a b i t a t f o r marine m o l l u s c s a r o u n d many t r o p i c a l a t o l l s and i s l a n d s . D e s p i t e t h e o f t e n l i m i t e d t i d a l r a n g e a d i s t i n c t z o n a t i o n i s found from h i g h t o low w a t e r mark, and h a s been t h e s u b j e c t of r e v i e w s by Stephenson and S t e p h e n s o n ( 1 9 5 0 , 1 9 5 2 ) , Southward ( 1 9 5 8 ) and Lewis ( 1 9 6 0 ) . These works t o g e t h e r w i t h more d e t a i l e d s u r v e y s such a s t h o s e by Voss and Voss ( 1 9 5 5 , 1 9 6 0 ) , Arnow, S t C1ai.r and Arnow
(1963)
,
Houbrick (1968) and o t h e r s have d e s c r i b e d t h e d i s t r i b u t i o n and v e r t i c a l z o n a t i o n o f l i - t t o r a l a n i m a l s i n t h e C a r i b b e a n . Vermeij ( 1 9 7 3 ) a l s o c o n s i d e r s t h e d i s t r i b u t i o n o f l i t t o r a l m o l l u s c s i n h i s a t t e m p t t o r e l a t e t h e i r m o r p h o l o g i c a l d i f f e r e n c e s wi-th h a b i t a t p r e f e r e n c e s .However, a s y e t , l i t t l e work h a s been done on t h e Cayman I s l a n d s
l i t t o r a l r e g i o n a p a r t from a s m a l l and n o n - r e p r e s e n t a t i v e c o l l e c t i o n o f m o l l u s c s made d u r i n g t h e OxEord U n i v e r s i t i e s E x p e d i t i o n t o t h e Cayman
I s l a n d s in 1938 ( S a l i s b u r y , 1953) and t h e i m p o r t a n t work o f K b b o t t (1958) w h o d e s c r i b e d t h e mari-ne molluscan f a u n a o f Grand Cayman. No s p e c i f i c r e c o r d s e x i s t f o r L i t t l e Cayman.
While t h e p r e s e n t work d o e s i ? o t r e p r e s e n t a cornprehe~?sive s p e c i e s l i s t o f -- the . j < i .
.
0 - Lfored by -Ebbc)tt (1958),
it d c e s i n d i c a t e t l i pa i s t r j - b u t i o n and z o n a t i o n o f t h e con~mon l i t t o r a l s p e c i e s on the b e a c h r o c k around L i t t l e Ca-pan i n r e l a t i o n t o t h e d e g r e e o f e x p o s u r e .
The way i n which t h e d e g r e e oE e x p o s u r e i n f l u e n c e s t h e v e r t i c a l d i s t r i b u t i o n o f s p e c i e s depends upon a number o f f a c t o r s , i n c l u d i n g t h e b e h a v i o u r a l h o m e o s t a t i c mechanisms employed by t h e a n i m a l a s w e l l a s t h e i r p h y s i o l o g i c a l t o l e r a n c e s (Lewis 1963, F r a e n k e l 1 9 6 8 , B a r d i n 1968, and Borkowski 1 9 7 1 ) . These f a c t o r s t o g e t h e r w i t h m o r p h o l o g i c a l a d a p t a t i o n s ( N o r t h 1954) e n a b l e s p e c i e s t o c o l o n i s e and e x p l o i t w i d e l y d i f f e r e n t a r e a s a l t h o u g h i t s h o u l d b e r e c o g n i s e d t h a t d i f f e r e n t s p e c i e s w i l l o f t e n employ d i f f e r e n t mechanisms f o r m a i n t a i n i n g t h e m s e l v e s w i t h i n t h e i r optimum r a n g e . I n a d d i t i o n t o t h e l o c a l v a r i a t i o n s i n s p e c i e s d i s t r i b u t i o n t h e r e a r e a l s o z o o g e o g r a p h i c v a r i a t i o n s o f t h e k i n d d e s c r i b e d by Vermeij ( 1 9 7 3 ) . The p r e s e n t work c o n f i r m s t h e g e n e r a l
s i m i l a r i t y and l o c a l v a r i a b i l i t y i n s p e c i e s c o m p o s i t i o n and z o n a t i o n a r o u n d L i t t l e Cayman w i t h o t h e r a r e a s w i t h i n t h e C a r i b b e a n . ( S t e p h e n s o n and S t e p h e n s o n 1950, 1 9 5 2 ) . The o v e r a l l z o n a t i o n i n c l u d e s a b e l t of m a r i t i m e s h r u b s a l o n g t h e upper p a r t o f t h e s h o r e . Where t h e s e e x t e n d down towards t h e s u p r a l i t t o r a l f r i n g e T e c t a r i u s r n u r i c a t u s i s found e i t h e r on s h r u b s o r wood on t h e s t r a n d l i n e o r on h i g h l e v e l r o c k s . I n t h i s s u p r a l i t t o r a l f r i n g e g r a p s i d c r a b s may be p r e s e n t a n d on r o c k y s u b s t r a t e s a r a n g e o f upper m i d l i t t o r a l l i t t o r i n i d s t h a t e x t e n d t o below mid t i d e l e v e l . I n t h e u p p e r m i d l i t t o r a l zone t h e f i r s t n e r i t e s a r e p r e s e n t g e n e r a l l y n e a r p o o l s o r m o i s t c r e v i c e s where t h e r e i s l i t t l e r i s k o f d e s i c c a t i o n . These e x t e n d t o t h e i n f r a l i t t o r a l n o t c h where t h e g r a z i n g A c a n t h o p l e u r a g r a n u l a t a and C i t t a r i u r n p i c a become t h e most
a b u n d a n t s p e c i e s t o g e t h e r w i t h t h e c a r n i v o r o u s P u r p u r a p a t u l a . I n more exposed r e g i o n s t h e lower m i d l i t t o r a l zone i s c h a r a c t e r i s e d b y v e r m i t i d s and t h e m u s s e l B r a c h i d o n t e s . The deveiopment o f m a c r o s c o p i c m a r i n e a l g a e i s u s u a l l y r e s t r i c t e d t o low l e v e l p o o l s and i t i s o n l y i n t h e s u b l i t t o r a l r e g i o n o f t h e u p p e r r e e f t e r r a c e t h a t t h e y become
s i g n i f i c a n t .
On L i t t l e Cayman i t h a s been p o s s i b l e t o show how around t h e s h o r e s o f a s i n g l e i s l a n d i t i s p o s s i b l e t o g e t a d i f f e r e n - t s p e c i e s c o m p o s i t i o n and z o n a t i o n p a t t e r n t h a t r e l a t e s t o t h e d e g r e e o f e x p o s u r e .
T h i s h a s been r e c o r d e d w i t h i n t h e Mollusca and would p r o b a b l y b e e v i d e n t i n o t h e r g r o u p s when s u b j e c t e d t o d e t a i l e d e x a m i n a t i o n . Not o n l y do d i f f e r e n t s h o r e s e x h i b i t d i f f e r e n t s p e c i e s c o m p o s i t i o n and z o n a t i o n , b u t a l s o w i t h i n s m a l l a r e a s o f one s h o r e i t i s p o s s i b l e t o e x h i b i t t h i s e f f e c t . Thus on t h e seaward and shoreward s i d e o f a b o u l d e r , o r t h e exposed o r s h e l t e r e d s i d e s o f a j e t t y , d i f f e r e n t z o n a t i o n l e v e l s a r e e x h i b i t e d . By m e a s u r i n g m o l l u s c s from d i f f e r e n t b e a c h e s i t h a s a l s o been p o s s i b l e t o show t h a t t h e r e i s a d i f f e r e n c e o f s i z e between i n d i v i d u a l s found on s h o r e s o f d i f f e r e n t e x p o s u r e . I n f a c t t h e i n d i v i d u a l s p e c i e s s i z e r a n g e s were o f t e n l a r g e r between d i f f e r e n t s i t e s arou2.d t h e I s l a n d t h e n Vermeij (1.973) found f o r t h e same s p e c i e s taken from d i f f e r e n t a r = a s o f t h e i r z o o g e o g r a p h i c r a n g e , Genera1l.y s p e a k i n g t h e r e v:as a t e n d e n c y f o r s p e c i ~ i i e n s t.o b e s m a l l e r on rrloi?.. expose6 s h o r e s , b u t t h i s was no-t always c l e a r m d f u r t h e r :wrk i s needed t o c o n f i r m t h i s . Only w i t h C i t t a r i u m p i c a was t h e r e v e r s e t r u e .
The e f f e c t o f e x p o s u r e on a c o a s t l i n e i s v e r y v a r i a b l e , bur- w i t h i n c r e a s e d e x p o s u r e t h e r e i s g e n e r a l l y more wave a c t i o n which e f f e c t i v e l y i n c r e a s e s t h e t i d a l r a n g e ( G o s l i n e , 1 9 6 5 ) . T h i s e n a b l e s a s p e c i e s l i m i t e d o n l y by t h e need f o r a b r i e f p e r i o d o f submersion t o i n c r e a s e i t s v e r t i c a l r a n g e , p r o v i d e d i t h a s t h e n e c e s s a r y a d a p t a t i o n t o t o l e r a t e t h e g r e a t e r e f f e c t o f d e s i c c a t i o n a t h i g h e r l e v e l s . T h i s c a n t o some e x t e n t b e a c h i e v e d by r e d u c i n g body s i z e , o r by a d o p t i n g a s e m i - c r y p t i c mode of l i f e a s s e e n i n most h i g h t i d e l i t t o r i n i d s o r by d e v e l o p i n g a c l o s e t e x t u r e d s h e l l t h a t w i l l r e s i s t w a t e r l o s s a s found i n T e c t a r i u s r n u r i c a t u s .
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T e g u l a e x c a m t i : ( Lrxi!, ) T e g u l a f a s c i a t z (Ron!) Neri t a p e l o r o n La (L, ) Neri t a t e s s e l l a t a C;melin Neri t a v e r s i c o l o r Gnle1.i.n P u p e r i t a p u p a (L
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L i t t o r i n a z i c z a c ( G w 1 i . n ) L i t t o r i n a 1 i r ~ c : a t a d ' l l r b i g n y L i t t o r i n a l i n e o l a t a d Y O r b i g n y N o d i l i t t o r i n z t u b e r c u l a t a (Menke) E c h i n i n u s n o d u l o s u s ( p f r , )
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T e c t a r i u s m u r i c a t u s o c c u p i e s t h e h i g h e s t p o s i t i o n o f t h e t r u l y marine g a s t r o p o d s a l o n g t h e s t r a n d l i n e and i t i s i n t e r e s t i n g t o n o t e
t h a t t h e mean s i z e d i d n o t v a r y s i g n i f i c a n t l y a t d i f f e r e n t s i t e s around t h e i s l a n d r e g a r d l e s s o f t h e d e g r e e o f e x p o s u r e . However what d i d change was t h e h e i g h t above s e a l e v e l which was h i g h e s t on t h e e a s t e r n most exposed end o f L i t t l e Cayman. Along t h i s storm beach i s an
e x t e n s i v e s t r a n d l i n e o f washed up t i m b e r and i t i s on t h i s t i m b e r t h a t t h e d e n s e s t numbers o f T . m u r i c a t u s were found a t a h e i g h t t h a t c o u l d o n l y have been w e t t e d by t h e o c c a s i o n a l s p r a y from rough s e a s d u r i n g s t o r m s . I n t h i s p o s i t i o n n o t o n l y must T . m u r i c a t u s be t o l e r a n t t o t h e d e s i c c a t i n g e f f e c t o f t h e sun and wind a s h a s been shown by Rosewater ( 1 9 6 3 ) , b u t i t must a l s o be a b l e t o t o l e r a t e t h e o s m o t i c s t r e s s when w e t t e d d u r i n g r a i n .
The d i f f e r e n c e i n s p e c i e s composition a t any one s i t e can be e x p l a i n e d i n t e r m s o f p h y s i o l o g i c a l t o l e r a n c e , b u t it i s p r o b a b l e t h a t o t h e r f a c t o r s a r e i n v o l v e d . P a i n e (1966) d i s c u s s e s t h e p o s s i b l e i n f l u e n c e o f p r e d a t o r y s p e c i e s and how i n an a r e a where s p a c e i s a t a premium, s u c h a s t h e l i t t o r a l zone, t h e n a r e d u c t i o n o f p r e d a t o r s may a l s o c r e a t e a l e s s d i v e r s e system, The s i t u a t i o n c a n n o t be e a s i l y s u m a r i s e d i n a dynamic environment l i k e t h e l i t t o r a l r e g i o n where a t h i g h w a t e r , b e n t h i c s p e c i e s may be s u b j e c t t o p r e d a t i o n by a r a n g e o f s u b l i t t o r a l s p e c i e s , such a s f i s h , and a t low w a t e r a v i a n o r mammalian p r e d a t o r s may become s i g n i f i c a n t .
I n t h e p r e s e n t work t h e p r e s e n c e of c a r n i v o r o u s g a s t r o p o d s d o e s n o t a p p e a r t o have any o b v i o u s e f f e c t on t h e molluscan d i v e r s i t y , b u t it i s p o s s i b l e a more e x t e n s i v e s u r v e y would b e n e c e s s a r y t o be c e r t a i n .
While t h e r e i s a u s e f u l l i t e r a t u r e on t h e l i t t o r a l f a u n a s o f t h e Caribbean r a t h e r few a c t u a l l y mention t h e s i g n i f i c a n c e o f r o c k p o o l s i n t h i s r e g i o n . North (1954) comments on t h e e r o s i o n o f p o o l s a s a r e s u l t of wave a c t i o n , b u t adds l i t t l e t o t h e u n d e r s t a n d i n g o f t h e s e i m p o r t a n t l i t t o r a l h a b i t a t s . Houbrick (1968) r e c o g n i s e s t h e i r importance i n t h e c o a s t a l zone o f Costa R i c a and shows how c e r t a i n m o l l u s c s t e n d t o p r e f e r p o o l s t o o t h e r l i t t o r a l a r e a s and Abbott (1958) comments t h a t i n t h e upper i r o n s h o r e o f Grand Cayman, P u p e r i t a pupa and L i t t o r i n a m e s p i l l u r n were most common i n " s p a s h p o o l s " . The West End r o c k s on L i t t l e Cayman c o n t a i n e d s i m i l a r " s p a s h p o o l s " where t h e h e r m i t c r a b , C l i b a n a r i u s
t r i c o l o r became v e r y a b u n d a n t , a s d i d t h e goby B a t h y g o b i u s s o p o r a t o r . Around L i t t l e Cayman o t h e r molluscan s p e c i e s were found i n r o c k p o o l s i n c l u d i n g L i t i o p a m e l a n o s t o m a , T e g u l a e x c a v a t a , E c h i n i n u s n o d u l o s u s , N o d i l i t t o r i n a t u b e r c u l a t a , L i t t o r i n a z i c z a c , Acmaea a n t i l l a r u m and t h e n e r i t e s , N e r i t a versicolor, N . t e s s e l l a t a and N . p e l o r o n t a . I n low w a t e r p o o l s t h e e c h i n o i d s E c h i n o m e t r a l u c u n t e r and Diadema a n t i l l a r u m were p r e s e n t ; t h e former b e i n g r e c o r d e d i n p o o l s o f t h e i n s h o r e rocky a r e a s o f F l o r i d a Keys by McPherson ( 1 9 6 9 ) .
A p a r t from t h e molluscan c o l l e c t i o n s by S a l i s b u r y (1953) and Abbott (1958) v e r y l i t t l e h a s been done t o t h e l i t t o r a l f a u n a o f t h e Cayman I s l a n d s and n o t h i n g h a s been p u b l i s h e d s p e c i f i c t o L i t t l e Cayman.
The n e g l e c t o f t h e l i t t o r a l r e g i o n i s n o t j u s t i f i e d a s it o f f e r s a v a r i e d and d i v e r s e h a b i t a t i n which many s p e c i e s l i v e . Around t h e
