The Amazing World of Molluscs - and their Decline World Wide

Hermit crabs are a common sight in Cayman
particularly along the Mastic Trail

A marine Flamingo Tongue shell on coral

by Lilian Hayball-Clarke

Tuesday,  January 10, 2006

In this decade of declining delicacies, in or out of the shell, molluscs have provided highly nutritional food from the sea and the land. Many land snails feed on vegetation, including our crops, biting holes in the leaves that reduce photosynthesis and therefore the crop-yield for the farmer.

It is true that plant grazers like conch and whelks have always been considered gustatory delights, due to their sweet-tasting flesh. ‘Oysters are amorous’ which suggests they convey powers beyond their taste or nutrition. 

However, although protected on the Cayman Islands, molluscs are declining worldwide, due to increased demand to satisfy our insatiable appetites. Unfortunately, the molluscs cannot keep up in reproductive terms with our collection and consumption levels, especially since human population figures are rising so fast. 

Did you know that a third of the most thoroughly studied animals here on Cayman are snails? As many as 30 out of 48 different species of land snails are unique to Cayman, and over 509 different marine snails have been recorded here.

Some snails are totally at home on dry land, others cling to sponges and coral reefs or roam the depths as top carnivores, others love rushing fresh-water streams. 

Marine molluscs of the Cayman Islands were little studied until the early 1950’s, when the Philadelphia Academy of Natural Sciences expeditions to Grand Cayman Islands began.

They collected seashells extensively, and in 1958 produced a classic monograph on shelled animals. From 1968 on, a network of local collectors and scientists, including SCUBA divers, consulted experts, and then added more than 100 mollusc species names to the book ‘The Natural History and Biogeography of the Cayman Islands’ which actually came out in 1994. 

The work continues under the auspices of the Cayman Islands’ Department of Environment which sets limits on collection of molluscs during their breeding season.

The Cayman Islands continues to come under close scientific scrutiny: shell surveys have already been made beyond the 18m depth at Bloody Bay Wall off Little Cayman to at least 15m depth off George Town, Grand Cayman. 

The vast majority of recorded shells were micro-molluscs, only visible under the microscope after sifting through sand, coral rubble and silt. 

We still tend to hunt, collect and eat the larger of the mollusc species, most of which are edible and yield the most meat. Older, larger molluskcs also represent the reproductive wisdom of the sea, and their loss is a disaster for ocean food chains and webs alike, as well as sustainability of the food industry. 

How can molluscs protect themselves against such an onslaught on their dwindling numbers? Few have developed protection against attack, living benignly. They are sheltered and fed by algae and corals on the reef and when burrowing in sand, except for the poisonous cone-shell species, which, like the helmet-shell, feeds on other mollusc species. 

It is unwise to make a collection of dead shells, since hermit-crabs are constantly on the lookout for a bigger shell ‘home’ for their growing bodies. Hermit or soldier crabs inhabit all parts of Cayman, and can most readily be seen along the Mastic trail, where two or three may be fighting each other off, competing to occupy an empty mollusc shell like a top-shell laying nearby.

Eminently edible molluscs are reckoned to be among the most intelligent and beautiful invertebrates found in the ocean. Consider spectacular, spiral, decorated sea-shells, permanent home to each animal, which, after collection, the animal well-dead, we use to decorate our gardens. 

Then weep for the double-shelled oyster, holding its shell tightly closed against all predators except those of us armed with sharpened knives. In the ocean, watch the stroboscopic squid do sexy line-dancing in the shallows to attract a mate. Beaked, eight armed suckered octopuses flair blue in the night divers’ torchlight to frighten us away. Shell-free, armless and harmless, brightly or dull-coloured sea and land slugs remain camouflaged amongst the seaweeds and corals within our gaze.
Many molluscs swim and dance through the ocean; others burrow, crawl or jump like clams, while chitons, winkles and whelks graze algae so slowly that they seem fixed, immobile and not worthy of interest, or so these animals hope.

This behaviour is a finely tuned survival tactic, honed over time, without which species die. However, this behaviour stands up poorly in the face of clever remote dredging and netting technology now used for catching shell-fish more and more efficiently.

The larger Cephalopods [meaning head attached to a foot] are amongst the most formidable molluscs in size and behaviour, the most likely to outwit us: squid and octopuses are highly intelligent molluscs - speedy top carnivores of the oceans, much feared in whale hunting times. 

You may have read of the massive 6 to 20 foot-long squid called Architeuthis dux [see New Scientist.com, ‘The gruesome eating habits of the giant squid’, 30 July 2005] been brought up whole or in part from great ocean depths where they hunt, feed and mate, once beyond our reach. 

No more - even their lives are threatened with continued negative interference in ocean food-chains and webs. 

Snails too small to eat remain free to feed on algal covered mud or tiny microscopic coral polyps.

This Flamingo-tongue shell shows its delicate skin or mantle, decorated with orange circles, extended out over the shell, absorbing oxygen from the seawater. 

Disturbance will cause the animal to retract the mantle and the shell will again appear plain. Mollusc shells and bodies exhibit a wild variety of shape fashioned through natural selection to survive the sands of time, unable now to avoid our relentless searching, made ever more efficient with technological advance.

Mating takes place seasonally and some species are hermaphrodite, exchanging packets of sperm with each other to fertilise the eggs kept internally until laid. Eggs are laid singly or in slimebands, attached to rocks or weeds for camouflage. Little research has been done on parental care and the hatchlings are thought to be on their own in the survival stakes. 

Huge slimy masses containing millions of eggs are laid to ensure that a small percentage survive to adulthood, because there are other predators besides ourselves, helping themselves to a ready meal, considered by many delicious raw or cooked.

The world population of the rarest land snail in the world, Cerion nanus, the Little Cayman Pulmonate, only a centimeter or so long, is at risk since, in all the world, it is only found at the western end of Little Cayman feeding on a single species of plant not found anywhere else. 

Over-hunting and loss of habitat are the two major causes of declining mollusc numbers - we can help by limiting our consumption, never over-hunting, respecting closed seasons when they are breeding, as announced by the Department of Environment, and learning more about these amazing, intelligent, fascinating animals.