9. CERION NANUS (MAYNARD) (MOLLUSCA : CERIONIDAE ) ON LITTLE C A Y b W
M.V. Hounsome and R.R. Askew
C e r i o n nanus (Maynard) was c o l l e c t e d on L i t t l e Cayman by C h a r l e s Johnson Maynard i n t h e w i n t e r of 1888 and d e s c r i b e d t h e f o l l o w i n g y e a r
(Maynard 1 8 8 9 ) . F i n d i n g s h e l l s i n o n l y a v e r y s m a l l a r e a (6 y a r d s by 20 y a r d s ) lie c o n s i d e r e d 'izhat t h i s s p e c i e s h a s t h e most r e s t r i c t e d r a n g e of any animal w i t h which I am a c q u a i n t e d " I n 1930 P a u l B a r ~ s c h v i s i t e d MayaardBs l o c a l i ~ y and ' g a t h e r e d two 8 o z . b o t t l e s f u l l ' of s h e l l s ( B a r t s c h 1 9 3 1 ) . T h i s summarises t h e r e p o r t e d h i s t o r y o f C . n a n u s , a more d e t a i l e d a c c o u n t b e i n g p r o v i d e d by Clench ( 1 9 6 4 ) .
S p e c i e s of C e r i o n a r e o f z o o g e o g r a p h i c a l i n t e r e s t because o f t h e i r g e n e r a l l y r e s t r i c t e d d i s t r i b u t i o n s . C . m a r t i n i a n u r n ( K i i s t e r ) i s known o n l y from Grand Cayman and C . p a n n o s u m (Maynard) o n l y from Cayman Brac and L i t t l e Cayman. These two s p e c i e s d i f f e r from C . nanus i n b e i n g c l o s e l y a l l i e d t o C u b a n s p e c i e s ; ' C . n a n u s does n o t a p p e a r t o be c l o s e l y r e l a t e d t o t h e o t h e r s p e c i e s of C e r i o n on t h e Cayman I s l a n d s o r t o any C e r i o n from t h e s o u t h c o a s t o f Cuba' (Clench 1 9 6 4 ) . L i t t l e Cayman i s t h e o n l y i s l a n d of t h e group t o s u p p o r t more t h a n one C e r i o n s p e c i e s ,
C e r i o n nanus l o c a l i t i e s
C . n a n u s was l o c a t e d a t t h r e e s i t e s c l o s e t o e a c h o t h e r d u r i n g t h e 1975 e x p e d i t i o n ( P l a t e s 5 2 and 5 3 ) . Two s i t e s a r e a d j a c e n t t o t h e North Shore t r a c k r u n n i n g from t h e w e s t e r n end of Blossom V i l l a g e i n a n o r t h e r l y and t h e n n o r t h - w e s t e r l y d i r e c t i o n t o S p o t Bay, and t h e t h i r d s i t e l i e s a l o n g a t r a c k r u n n i n g w e s t towards West End P o i n t from t h e n o r t h s i d e of t h e a i r - s t r i p .
C . nanus was f i r s t found on 25th J u l y a t t h e p l a c e , h e r e
d e s i g n a t e d s i t e A ( c e n t r e a t NM 9 6 0 7 4 4 ) , from which t h e t y p e m a t e r i a l d e s c r i b e d by Maynard was c o l l e c t e d . No l i v i n g C . nanus was found a l t h o u g h empty s h e l l s were s p r e a d o v e r a r o u g h l y c i r c u l a r a r e a , b i s e c t e d by t h e t r a c k , o f a b o u t 35,000 sq.m. T h i s i s a much l a r g e r a r e a t h a n t h e 120 sq.m. r e p o r t e d by Maynard. On 2nd August, l i v i n g s n a i l s were found a t s i t e B ( c e n t r e NM 958746) a b o u t 350 m. NNW o f t h e
A t o l l 9esearch B u l l e t i n No. 2 4 1 : 91-96, 1980.
c e n t r e of s i t e A . S n a i l s o c c u r r e d up t o a b o u t t e n m e t r e s on e i t h e r s i d e of t h e t r a c k f o r a d i s t a n c e o f a b o u t t h i r t y m e t r e s . The t h i r d C . n a n u s s i t e , a t which a g a i n o n l y empty s h e l l s were f o u n d , e x t e n d e d up t o two m e t r e s from t h e t r a c k ( c e n t r e NM 940743) f o r a d i s t a n c e of a b o u t 490 m. Specimens a r e d e p o s i t e d i n t h e B r i t i s h Museum ( N a t u r a l H i s t o r y ) ( a c c e s s i o n number 2265) and i n t h e Manchester Museum
( a c c e s s i o n number 1 0 2 4 ) . The specimens f i g u r e d i n P l a t e 54 a r e i n t h e Manchester Museum ( r e g . no. EE6001)
.
T o p o g r a p h i c a l l y t h e t h r e e s i t e s a r e s i m i l a r and o f r a t h e r
d i s t i n c t i v e f o r m a t i o n . The c o r a l l i m e s t o n e i s f l a t , r e l a t i v e l y smooth, and composed o f l a r g e s l a b s w i t h a c c u m u l a t i o n s of sandy s o i l i n
d e p r e s s i o n s and c r a c k s . The p l a n t c o v e r i s s p a r s e and a r e a s s e v e r a l s q u a r e m e t r e s i n e x t e n t a r e d e v o i d o f s h r u b s e x c e e d i n g a b o u t one m e t r e i n h e i g h t . I n t h i s o p e n , sun-baked, d r y t e r r a i n t h e most
c o n s p i c u o u s p l a n t i s E r r o l v u l u s a r b u s c u l a P o i r . ( C o n v o l v u l a c e a e ) , t h e c r a b bush, undoubtedly t h e ' h e a t h - l i k e p l a n t - e f e r r e d t o by Maynard on which he found C . n a n u s . L i v i n g C . n a n u s a t s i t e B were found o n l y on E v o l v u l u s and t h e l a r g e s t a c c u m u l a t i o n s of empty s h e l l s a t s i t e A were
u s u a l l y b e n e a t h E v o l v u l u s ,
Only one o t h e r a r e a t h a t b o r e a r e s e r i l a n c e , a t l e a s t s u p e r f i c i a l l y , t o t h e t h r e e C . n a n u s s i t e s was found on L i t t l e Cayman. T h i s i s a
s m a l l p l a t e a u on t h e n o r t h s l o p e o f Sparrowhawk H i l l (PM 0 0 2 7 8 1 ) , b u t no t r a c e of t h e s n a i l was found h e r e .
E . a r b u s c u l a i s a p p a r e n t l y c o n f i n e d t o L i t t l e Cayman i n -the
Cayman I s l a n d s , b u t i t grows a l s o on Cuba, H i s p a n i o l a , J a m a i c a and t h e Bahamas. C . n a n u s i s v e r y s t r o n g l y , p e r h a p s e x c l u s i v e l y , a s s o c i a t e d w i t h t h i s p l a n t , a l t h o u g h i t p r o b a b l y f e e d s upon e p i p h y t i c a l g a e r a t h e r t h a n on E v o l v u l u s i t s e l f . C . p a n n o s u m i s a l s o f r e q u e n t l y found on E v o l v u l u s b u t i t i s p l e n t i f u l on o t h e r p l a n t s a s w e l l . C . p a n n o s u m i s w i d e s p r e a d on L i t t l e Cayman, u n l i k e C . n a n u s , and was found a t most p l a c e s v i s i t e d and i n p a r t i c u l a r abundance on t h e c o a s t a l s t r i p .
Numbers of C e r i o n nanus and C . w a n n o s u m a t s i t e s A and B
Empty s h e l l s o f C . n a n u s a n d C . p a n n o s u m were c o l l e c t e d a t s i t e s A and B from 0 . 2 5 sq.m. q u a d r a t s . Twelve q u a d r a t s were sampled a t s i t e A and e i g h t a t s i t e B. A t e a c h s i t e , h a l f o f t h e q u a d r a t s
encompassed E v o l v u l u s p l a n t s and t h e r e m a i n i n g q u a d r a t s were p l a c e d between p l a n t s . A l l empty s h e l l s l y i n g w i t h i n t h e q u a d r a t s were c o l l e c t e d and any s o i l p r e s e n t was s i f t e d f o r b u r i e d s h e l l s . Numbers o f f u l l y grown and p a r t l y grown s h e l l s were c o u n t e d ( t a b l e 9 ) .
A t s i t e A , numbers o f l i v i n g a d u l t and immature C . p a n n o s u m on 116 E v o l v u l u s p l a n t s were c o u n t e d . A t s i t e B, numbers of l i v i n g s n a i l s o f b o t h C e r i o n s p e c i e s on a l l t h e E v o l v u l u s p r e s e n t , t o t a l l i n g 157 p l a n t s , were c o u n t e d . These numbers a r e shown i n t a b l e 9.
The d a t a i n t a b l e 9 can be i n t e r p r e t e d i n a number of ways. I f i t i s assumed t h a t empty s h e l l s of t h e two s p e c i e s p e r s i s t f o r a b o u t t h e same l e n g t h o f time and t h a t n e i t h e r s p e c i e s i s p r e f e r e n t i a l l y removed from t h e s i t e , t h e c o m p a r a t i v e l y low numbers o f empty C . n a n u s s h e l l s a t s i t e A s u g g e s t t h a t i t h a s been e x t i n c t t h e r e f o r some time and/or i t was always l e s s numerous t h a n C . p a n n o s u m . A t s i t e B , empty C . n a n u s s h e l l s occur a t an i n s i g n i f i c a n t l y lower d e n s i t y t h a n a t s i t e A , b u t t h e y comprise a h i g h e r p r o p o r t i o n of t h e t o t a l number of C e r i o n s h e l l s . A t s i t e B , f u l l y grown s h e l l s of C . n a n u s and C . p a n n o s u m a r e p r e s e n t i n e q u a l numbers, b u t a t s i t e A f u l l y developed C . p a n n o s u m s h e l l s c l e a r l y outnumber t h o s e of C . n a n u s . T h i s might i n d i c a t e a r e l a t i v e l y r e c e n t c o l o n i s a t i o n of s i t e B by C . p a n n o s u m .
Numbers of l i v i n g s n a i l s a t t h e two s i t e s f o l l o w t h e p a t t e r n of t h e d e n s i t i e s of empty s h e l l s , e x c e p t f o r t h e absence o f l i v i n g C . n a n u s from s i t e A , A t s i t e A , numbers of C . p a n n o s u m p e r E v o l v u l u s p l a n t a r e v e r y much h i g h e r t h a n a t s i t e B ( 8 . 8 compared w i t h 2 . 0 ) , c o n s i s t e n t w i t h t h e view t h a t s i t e B i s a newly c o l o n i s e d , W d e r - s a t u r a t e d b i t e f o r t h i s s p e c i e s . F u r t h e r , t h e p r o p o r t i o n of j u v e n i l e t o a d u l t
C . p a n n o s u m i s much h i g h e r a t s i t e B (2-6 : 1) t h a n a t s i t e A ( 0 . 9 : 1 ) . A f e a t u r e o f t h e d a t a i n t a b l e 9 i s t h e r e l a t i v e s c a r c i t y of
j u v e n i l e C. n a n u s i n t h e samples. We a r e unable t o f a v o u r any of t h e s e v e r a l p o s s i b l e e x p l a n a t i o n s of t h i s and t h e s i t u a t i o n r e q u i r e s
f u r t h e r i n v e s t i g a t i o n .
The numbers of E v o l v u l u s p l a n t s w i t h and w i t h o u t s n a i l s of e i t h e r s p e c i e s a t b o t h s i t e s a r e given i n t a b l e 10. S i g n i f i c a n t l y more p l a n t s a t s i t e A were occupied by s n a i l s
(x2
= 2 1 . 1 , P<0.001),
adding weight t o t h e s u p p o s i t i o n t h a t s i t e A i s more s n a i l ' s a t u r a t e d ' t h a n s i t e B.C o n s i d e r i n g t h e d i s t r i b u t i o n of l i v i n g s n a i l s of t h e two s p e c i e s on a p r e s e n c e o r absence b a s i s o v e r a l l t h e p l a n t s a t s i t e B ( t a b l e 11)
,
more E v o l v u l u s a r e w i t h o u t s n a i l s t h a n would be e x p e c t e d on a random d i s t r i b u t i o n of each s n a i l s p e c i e s . There i s , t h e r e f o r e , a tendency f o r t h e two s p e c i e s t o favour t h e same E v o l v u l u s p l a n t s
(x2
= 9 . 4 , P<0.01, C o l e ' s C o e f f i c i e n t o f A s s o c i a t i o n = 0 . 5 3 ) .Discussion
The o c c u r r e n c e of two s p e c i e s o f C e r i o n on L i t t l e Cayman, a s m a l l i s l a n d , i s unusual. The l a r g e r C . p a n n o s u m i s c l e a r l y t h e dominant s p e c i e s , b e i n g widespread on t h e i s l a n d and occupying a v a r i e t y o f h a b i t a t s . C. n a n u s i s e x c e e d i n g l y l o c a l i s e d and a p p e a r s t o be dependant upon E v o l v u l u s , a l t h o u g h i t i s by no means found wherever t h i s p l a n t grows. The c o - e x i s t e n c e o f t h e two C e r i o n s p e c i e s a t s i t e B, and t h e i r tendency t o o c c u r on t h e same E v o l v u l u s p l a n t s h e r e , might s u g g e s t t h a t i n t e r s p e c i f i c c o m p e t i t i o n i s somehow reduced o r avoided.
On t h e o t h e r hand, i n t h e above a c c o u n t o f t h e s i t u a t i o n i n 1975, we imply t h a t t h e two s p e c i e s a r e unable t o c o - e x i s t f o r a prolonged p e r i o d , C . p a n n o s u m u l t i m a t e l y r e p l a c i n g C . nanus. Such a replacement h a s c e r t a i n l y o c c u r r e d a t s i t e A , C . n a n u s having
T a b l e 8. Numbers o f l i v i n g a d u l t and j u v e n i l e C e r i o n n a n u s and C . p a n n o s u m found on E v o l v u l u s p l a n t s , and t h e numbers o f empty s h e l l s of t h e two s p e c i e s p e r 0 . 2 5 s q - m . , a t s i t e s A and B .
C e r i o n n a n u s C e r i o n p a n n o s u m a d u l t s j u v e n i l e s a d u l t s j u v e n i l e s L i v i n g s n a i l s
S i t e A (116 p l a n t s ) 0 0 547 46 8
S i t e B (157 p l a n t s ) 7 8 1 0 8 8 226
Mean numbers o f ernpty s h e l l s p e r 0 . 2 5 sq.m. ( f s , e . ) S i t e il
S i t e B
T a b l e 9 . Numbers of E v o l v u l u s p l a n t s w i t h and w i t h o u t C c r i o r r o f e i t h e r s ~ e c i e s a t s i t e s A and B.
w i t h C e r i o n w i t h o u t C e r i o n number of p l a n t s
S i t e A 1 0 8 8 116
S i t e B 111 4 6 1 5 7
T a b l e 10. Numbers of E v o l v u l u s p l a n t s c a r r y i n g l i v e C e r i o n of one o r b o t h s p e c i e s a t s i t e B. The e x p e c t e d f i g u r e s assume a random d i s t r i b u t i o n of e a c h s p e c i e s .
Numbers of p l a n t s w i t h : o b s e r v e d expected.
no s n a i l s 46 3 7
C , p a n n o s um on1 y 6 1 7 0
C . n a n u s o n l y 9 1 8
b o t h . m e c i e s 4 1 3 2
d i s a p p e a r e d a s a l i v i n g a n i m a l d u r i n g t h e p e r i o d s i n c e Maynard made h i s o r i g i n a l c o l l e c t i o n . Whether o r n o t C . pannosum o c c u r r e d a t s i t e A when Maynard and B a r t s c h c o l l e c t e d t h e r e i s u n f o r t u n a t e l y n o t known.
I f , a s we i n c l i n e t o b e l i e v e , c o n t i n u e d c o - e x i s t e n c e o f t h e two s p e c i e s i s i m p o s s i b l e , C. pannosum might be e x p e c t e d e v e n t u a l l y t o c o m p l e t e l y d i s p l a c e C . n a n u s from L i t t l e Cayman. However, it i s
d i f f i c u l t t o r e c o n c i l e t h i s s i m p l i s t i c view w i t h t h e c o n t i n u e d p r e s e n c e o f C . n a n u s . Why h a s d i s p l a c e m e n t n o t a l r e a d y o c c u r r e d ? C. pannosum h a s been on L i t t l e Cayman f o r a l o n g t i m e , a s shown by t h e s u b - f o s s i l s h e l l s found a t v a r i o u s p l a c e s on t h e i s l a n d . I t i s no r e c e n t c o l o n i s e r . F u r t h e r , t h e r e i s no e v i d e n c e t h a t C. n a n u s was a t one t i m e more w i d e s p r e a d on t h e i s l a n d t h a n it i s t o d a y .
F u r t h e r s p e c u l a t i o n i s u n l i k e l y t o b e p r o f i t a b l e u n t i l more i n f o r m a t i o n i s a v a i l a b l e , b u t t h e s i t u a t i o n i s i n t e r e s t i n g and i t i s t o be hoped t h a t any development on L i t t l e Cayman a v o i d s d i s t u r b a n c e t o t h e s m a l l a r e a where C. n a n u s c o n t i n u e s p r e c a r i o u s l y t o s u r v i v e .
R e f e r e n c e s
B a r t s c h , P. 1931. F u r t h e r e x p l o r a t i o n s f o r M o l l u s c s i n t h e West I n d i e s . E x p l o r a t i o n s and ~ i e l d Work o f the S m i t h s o n i a n I n s t i t u t i o n i n 1 9 3 0 , p u b l . 3111, 91-102.
C l e n c h , W . J . 1964. Land and f r e s h w a t e r Mollusca o f t h e Cayman I s l a n d s , West I n d i e s . O c c a s i o n a l P a p e r s o n M o l l u s k s , Harvard, 2 , 345-380.
Maynard, C . J . 1889. C o n t r i b u t i o n s t o S c i e n c e , 1, 27.
P l a t e 52. The s i t e on t h e t r a n s - i s l a n d t r a c k n o r t of Blossom V i l l a g e where l i v i n g C e r i o n n a n u s was found. The s n a i l s were l i v i n g on s h r u b s of E v o l v u a r b u s c u l a h e r e s e e n growing o n t h e p a t h
:h l u s
P l a t e 53. C e r i o n nanus and young C . pannosum on a s i n g l e p l a n t of E v o l v u l u s a r b u s c u l a . I n s e t : a l i v i specimen of C . n a n u s
P l a t e 5 4 . Six s h e l l s of C. n a n u s with an example of C.
pannosum f o r comparison ( t o t h e same s c a l e ) . Four t y p i c a l s h e l l s a r e shown t o g e t h e r with two examples of extremes i n l e n g t h / h e i g h t r a t i o
