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ABSTRACT
Experiments on t h e responses to osmotic stress and d e s i c c a t i o n , a r e r e p o r t e d f o r several species of New Zealand littoral chitons. Qualitative and quantitative studies
on t h e ecology of t h e common Auckland species have been undertaken, with special refe- rence to microhabitat conditions. The micro-anatomy of t h e dorsal sensory organs has been studied, and r e l a t e d t o t h e comparative responses to l i g h t s t i m u l i , and experiments involving i n t e r f e r e n c e with t h e s e organs.
Sypharochiton p e l l i s e r p e n t i s was found t o be an osmo-conformer, showing only limited volume regulation but a wide tolerance t o s a l i n i t y f l u c t u a t i o n s . Size of the animal had a negligible effect on the osmotic response, and i t i s l i k e l y t h a t o t h e r c h i t o n s are osmo-conformers b u t with l e s s tolerance of o s m t i c stress. Animals of this species were b e t t e r able t o resist t h e e f f e c t s of desiccation than several o t h e r chitans.
T h i s resulted from a k i n e t i c behaviour p a t t e r n i n response t o low h u m i d i t i e s causing aggregation i n t h e laboratory, a d a w e l l developed t o l e r a n c e of extrene water l o s s , Sypharochiton p e l l i s e r p e n t i s c o l o n i s e s a wide r a g e of microhabitats on r o c k surfaces, some exposed t o wide s a l i n i t y changes (13. ~ 8 ~ / o a t o 45 .80°/oo~) between tides, and o t h e r s exposed t o drying. Population s i z e s t r u c t u r e s i n d i c a t e t h a t exposure t o desie- c a t i o n i s l i k e l y t o be more important than osmotic stress i n c o n t r o l l i n g populations, and microhabitat more important than t i d a l level. The other chitons studied mere
restricted t o pools, beneath boulders o r t o wave-splashed situations which did n o t dry.
me very fast-moving c h i t o n , Ischnochiton maorianus exhibited photo- negative o r i e n t a t i o n and t h e ~shctdow response', t h e f u l l range of reactions t o light stimuli y e t r o p o r t e d i n c h i t o n s , It possesses only t h e megalaesthete type of d o r s e l sensory receptor. Experiments showed masking or abrading these organs t o a f f e c t t h e rate of r e s p o n s e , and i t is concluded t h a t a t l e a s t one of t h e i r f u n c t i o n s i s l i g h t reception.
h i t h o c h i t o n neglectus, a species possessing w e l l developed extra-pigmental o c e l l i , d i d n o t show such c l e a r responses t o iight. Problems of t h c structure and function of these o r e n s m e discussed.
