Sea Shells are calcium carbonate skeletons constructed by soft-bodied invertebrates called molluscs. The name mollusc is derived from the Latin word mollis, which means soft.
The basic molluscan animal has a soft, fleshy, non-segmented, contractile body, unsupported by bones. The mollusc's shell helps to protect the soft animal from predators. Although the majority of molluscs are born with shells, not all retain them throughout their lives. The nudibranchs and the octopus are two such examples and they have other methods of defence.
An assortment of sea shell species collected by scuba diving in the Coral Sea. Once a collector becomes familiar with the molluscs habitat and behaviour it is very easy to find hundreds of species and even denude an area of some species.
(Photo Neville Coleman)
Over two centuries ago Australia and the Indo - Pacific was discovered by seafaring Europeans who faced the unknown of their time and accepted the challenge of the sea. The frontier of our time is the same sea and its challenge has by no means diminished with the passage of time, for it is beneath the sea that we face an inheritance of ignorance. But before one ventures beneath the sea there are inherent fears to be eliminated, new technologies to be learned, new skills to be attained, and new philosophies to be acquired.
To glean even the tiniest morsel of information from the sea takes effort, time, patience, and skill. Perseverance is necessary before naturalists and scientists can build up a detailed picture of the smallest element of subtidal marine life in its own environment.
Shell ornaments and necklaces on show at a local shell market in Fiji. Sustenance collecting for food or, for sale to tourists, is simply extending what has been traded throughout the Indo - Pacific for thousands of years.
(Photo Neville Coleman)
During the earlier stages of Australian scientific endeavours it was only possible to study the various morphological features of animals in a state of preservation. Due to the fact that most members of the phylum mollusca shells (which encompasses all mollusca) have an easily preserved external skeleton this division drew a great deal of attention, and the sciences of conchology (the study of shells) and malacology (the study of molluscan biology and ecology) have flourished.
Professional Marine Scientists participating in a Northern Territory Museum Marine Biology Workshop at Darwin, Northern Territory. One of the principal subjects was based on malacology, the study of living molluscs.
(Photo Neville Coleman)
Although the primary object of many collectors is the acquisition of beautiful or rare shells, there are as many naturalists who are concerned with building up knowledge. Learning about shells is far more exciting for each discovery is a product of the naturalist's own endeavours. It matters little if the species is small or seems insignificant in the eyes of others ignorant of true value. What does matter is that somebody somewhere is concerned enough to contribute time, effort and interest in seeking out the many secrets of our living molluscs. Today, there are so many ways in which an interested person can join in the adventurous investigation of shell natural history.
With well executed balance and poise, this underwater photographer is hovering just above the coral recording living sea shells eating the coral.
( Heron Island , Great Barrier Reef)
(Photo Neville Coleman)
An enthusiast can keep log books or a diary and enter all information obtained and observations made on field trips. Molluscs can be observed in rock pools with the aid of a look-box or face mask. Snorkelling, snorkel diving, scuba diving or hookah diving can open up the vistas of the sub-littoral world. At home a small sea water aquarium, or a terrarium for land snails can be set up and stocked.
The taking of photographs underwater, at low tide, or in an aquarium can bring an entirely new aspect to the art of observation. A picture on a screen can bring back far more memories than a cleaned or pickled specimen can invoke, and you will be surprised at just how much easier learning becomes each time the mollusc is seen alive.
What is a Sea Shell?
This Lightning Volute shell Ericusa fulgetrum is an excellent example of a mollusc! It shows the both the protective shell and the soft - bodied animal to good effect.
( 15 metres, Sir joseph Banks Group, Port lincoln, South Australia)
(Photo Neville Coleman)
Shells are calcium carbonate skeletons constructed by soft-bodied invertebrates called molluscs. The name mollusc is derived from the Latin word mollis, which means soft. The basic molluscan animal has a soft, fleshy, non-segmented, contractile body, unsupported by bones. The mollusc's shell helps to protect the soft animal from predators. Although the majority of molluscs are born with shells, not all retain them throughout their lives. The nudibranchs and the octopus are two such examples and they have other methods of defence.
The shell is secreted by an organ common to all molluscs called a mantle. The mantle is a membranous envelope of tissue which generally encloses the mollusc's body. On the surface edge of the mantle are cells which secrete various substances such a conchiolin and calcium carbonate in the form of calcite and aragonite. Conchiolin is a proteinous substance characteristic of molluscs and forms the basic framework on which minute shell-building crystals are laid. Conchiolin is also the main ingredient used in the formation of the periostracum - the brown, horny, skin-like, outer covering on many shells.
A Milk - spot Cowry Lyncina vitellus has its mantle half extended over its shell. Most identification of cowry shells can only be accomplished by sight of the shell. Although I have been recording living sea shells for many years, only a few dozen can be identified from the mantle pattern.
( 3 metres, Tryon Island, Great Barrier Reef)
(Photo Neville Coleman)
The outer layers of shell are comprised of calcite - prism-like six-sided crystals of calcium carbonate laid down by the outer lip of the mantle edge. The inner mantle secretes a more porcellaneous material comprised of compact, low-profile crystals of calcium carbonate known as aragonite. If these layers are deposited in such a way that a pearly lustre is produced, this shelly material is called nacre, or mother of pearl. Between the two shell-secreting lips of the mantle is another lobe, but its functions are thought to be mainly sensory.
Substances that form part of shell secretions are transported throughout the body by the blood and by special mobile cells which roam at will. These amoebocytes are able to secrete calcium carbonate anywhere within the body cavity of the shell. In this way structural damage can be repaired internally. Abalone can actually repair and block up holes drilled in the top of their shells by predacious thaids, covering the opening by the formation of an inverted shelly blister. This is much the same method used when natural pearl blisters are formed in pearl shells.
The Map Cowry Leporiccypraea mappa is easy to identify whether its mantleis expanded or not, as it has a transparent mantle through which its very distinctive pattern can be recognised. ( 9 metres, Tryon island, Great Barrier Reef at night)
(Photo Neville Coleman)
Lots of animals attack shells, including sponges, worms, other molluscs, fish and crustaceans, and still more are damaged by storms and trawling nets, but as long as the mantle or the body has not been permanently damaged there is every chance that repairs can be made.
On the outside of the shell, repairs can only be carried out in those areas which can be reached by the mantle edge, which is why many older univalves have unrepaired damage to earlier healed breaks or varices.
Molluscs are suited to fit into the many different environments they occupy. Some species are very specialised and it is often possible to predict the habitat of the animal by examining the external features of the shell. Regardless of how complex or varied the shell of a mollusc may be, it must not be assumed that these shapes are curious freaks, or whims of nature; for every form and shape there is a reason.
How Shells Grow
Because there is no set pattern of growth, the age of a shell is not easy to estimate. It is known that most shells take between one and six years to mature, but in some cases the animal may be sexually mature before its shell has reached adult form. As the animal within the shell grows, so the shell must be added to. This is usually not a continuous process and at times the animals have rest periods in adding to the shell. These rest periods can be easily recognised on univalves in the form of growth lines, or ridges called varices.
The growth of this Venus Comb Murex Shell Murex pecten can be followed by tracing its previous perods of growth. These can be determined by the three sets of varicies visible. Each extended growing edge is lined with spines and thickened for strength, before the next growing period is undertaken, until the mollusc is adult.( 20 metres, Milne bay, Papua New Guinea at night)
(Photo Neville Coleman)
The reasons for these rest periods are not known in most cases, but they may be connected with restoring used calcium deposits, with food requirements, with environmental factors, or with unknown genetic responses. These rest periods are reflected in bivalve shells by fine concentric lines following the general shape of the shell. In univalves the lines run parallel to the aperture, as the body whorl is extended to meet the requirements of the growing animal.
If the mollusc has been experiencing a good season with lots of feed and no adverse conditions, the shell growth over that period may be extensive. However, during lean seasons only small amounts of shell may be added. Cowries and most other molluscs may stop growing in size after they are sexually mature, but they continue to add to the thickness of the shell for some time after.
It also seems evident that many molluscs grow throughout their entire lives. Experts have expounded a belief that the giant clam Tridacna gigas may live for a hundred years.
This adult Bubble Stromb Strombus bulla has grown from the immature 'roller' stage ( with a thin, non - lipped edge) and extended and thickened its lip to be a fully formed representative of its species.
( 20metres, Picker's Gill Sand Cay, Mosman Queensland at night)
(Photo Neville Coleman)
In some families of univalves, particularly strombs and tritons, the thickened lip on the aperture represents the end of a new growth period. The muricids also do this, but they often re-absorb part of the original lip before beginning on the new growth. These old lips are also known as varices and are often used to determine the age of some species.
Muricid shells have very definite, well-developed varices. When a shell is found that is in a stage of rapid growth it is usually referred to as juvenile. This, however, is not always correct: adult shells also grow new varices and a more accurate term is intervarical.
Growth Studies
Denuded murex
Chicoreus denudatus
(Perry, 1811)
Colour variations of the Denuded Murex Shell Chicoreus denudatus, showing dorsal and ventral aspects, operculum present. Both these specimens can be identified as fully adult specimens with very well developed variceal frills. These are two specimens from my growth studies, selected and tagged at intervariceal stages, that grew up in my controlled habitat, inside cages placed on the bottom on the reefs they generally inhabited.
( 5 metres, from Obelisk bay, Sydney Harbour New South Wales)
(Photo Neville Coleman)
In the years 1965 to 1968 the author conducted the first subtidal studies into determining the age, structure, and growth patterns of communities of the denuded murex Chicoreus denudatus. As this type of work had never been attempted before in Australia there were no guidelines to follow, nor were there people to offer advice. Each preliminary step was exploratory and could only be conducted at the mercy of the elements.
The area selected for the growth studies was Obelisk Bay, directly opposite Sydney Heads in Sydney Harbour. The offshore reef system here provides excellent environmental conditions and all isolated colonies were full of healthy fast-growing individuals. The reefs are low profile and are surrounded by sandy sea floor at a depth of approximately five to seven metres. The habitat is sheltered beneath large stands of the kelp Ecklonia radiata, and the rock surfaces are covered with a dense matting of the hairy mussel Trichomya hirsuta. This bivalve provided the main food source for Chicoreus denudatus within the limits of the study area.
The first attempts at measuring and tagging the shells in their natural habitat did not prove to be as simple in practice as it seemed in theory. The habitat adjacent to the opening of Sydney Heads was constantly subjected to ground swells and there was an average visibility of two metres. The perpetually swaying, all-enveloping, multi-fringed tangle of rubbery kelp canopy in six metres of murky water might be a healthy habitat for the murex, but it is anything but this for the diver.
To swim through swaying kelp beds you have to wait for each alternate swell to orientate the kelp upright to allow passage through. Once a shell is located you stop and hang on to the tough plastic-like kelp stalks to stay at that position. The interesting part comes when you have to let go of the kelp in order to measure the length, breadth, and new lip growth of the murex shell, and then write it down on a slate, tag the shell, and replace it.
By the time this simple little chore is completed you have rolled over several times, dropped the shell, broken the pencil, and been bounced on your head across the bottom, ending up with a mask full of water, metres away from your original location. Tangled up in a mass of brown seaweed and completely disorientated you realise with a flash of insight the reason why this field of study had remained uninvestigated for so long.
After serious re-evaluation of these procedures I collected a number of shells and took them back to the shore where they were photographed, measured and tagged before being returned to their respective reefs with the aid of a compass-plotted recognition system. This method proved more satisfactory than the first, but was not entirely successful as later, it took several weeks for me to find the tagged shells again to make further measurements. Experiments with small cages were also unsuccessful, as the curiosity of the human species had not been taken into consideration and cages, generally, ended up above the high water mark minus their shells.
Eighteen months later I had not really accumulated much data, so a cage was constructed which was so heavy a crane would have been needed to lift it off the bottom. It was towed out beneath empty oil drums and lowered into position on to the reef some 100 metres off shore.
Mussel clumps of rock were positioned inside the cage to reproduce a habitat similar to that of the natural reef. Once this was accomplished 20 shells at various stages of intervarical growth were collected and taken ashore for measuring.
Small plastic tags were attached around the spire of the shells with telephone wire. The shell length, new lip growth, and any notable characteristics were listed beside the tag number in a catalogue, and the tagged shells were then placed in the underwater cage.
During the following months measurements were repeated at intervals, depending on the clemency of the ocean, and tables for monthly (28-day period) growth were drawn up.
Shell Growth Results
It was concluded that Chicoreus denudatus was capable of producing new shell growth at the rate of 1.2 mm in 24 hours, as it took an average time of 58 days to complete new shell growth from varix to varix.
Almost all shells building or adding extensions to body whorls were found buried aperture down in mud, with any new growth out of sight. They stayed this way until the new shell growth was completed to the fullest extremity of the varix. No intervarical shells were found feeding. Growth patterns varied somewhat but only 20 per cent of shells observed filled in the apertural lip with solid shell, denoting a complete rest period. All others began a new shell growth while the outer apertural lip was still hollow (concave).
The mantle completes the new shell growth right to the scalloped edges of the varice and then fills in the gaps with smaller scalloping from the inside, adding to the strength of the new formed varix edge. It then produces a coating of thin translucent shell which is deposited along the inner edge of the scalloped varix edge. This performance is repeated several times until the individual scallops resemble small sugar scoops. From this point the apertural teeth are formed, after which new shell growth is begun from the inside, or extensions are stopped and the thin hollow (concave) lip is filled in with successive layers of shell to form what is generally recognised to be a fully adult shell.
Intervarical Nodules
In the past, some taxonomists placed emphasis on the numbers of intervarical nodules in determining species and sub-species.
Over a period of some eight years the author has had the opportunity to study many hundreds of forms of Chicoreus denudatus from dozens of localities and depth ranges. There seems little evidence to support this emphasis as no definite pattern could be found.
Shells having one large intervarical nodule on the body whorl, when traced back towards the spire may have two intervarical nodules on the intermediate whorls. Correspondingly, some shells have alternate numbers of nodules as one, two, one, two, etc. There are even shells with three intervarical nodules and all forms can be found within the one small isolated breeding colony.
It is therefore the opinion of the author that the number of intervarical nodules is not a reliable characteristic and has little significance in the determination of species or sub-species within the complex of Chicoreus denudatus.
The Mollusc
One of the first images of the living animal of the Red - mouthed frog Shell Tutufa bufo which shows off the strong muscular foot upon which it crawls along the bottom and the sensory tentacles with rudimentary eyes with which it can detect light and dark.
( 30 metres, off Shark Point, Clovelly New South Wales)
(Photo Neville Coleman)
Molluscan animals have no legs; their locomotion is provided by a muscular organ called a foot. On some gastropods (univalves) such as marine snails, this foot exceeds the length of the shell and may be brightly coloured. Within the various classes of molluscs, the foot is modified to suit each mode of living. Most univalves have a powerful, well-developed foot for crawling, while a bivalve's foot is usually more like a shovel and suited to a burrowing existence.
Whereas many bivalves that live in the sand have white, non descript shells, the Camp Pittar Venus Shell Lioconcha castrensis has a very uniquely patterned shel which make it easy to identify. This species id quite common from low tide to 20 metres in the waters off Queensland, the Great Barrier Reef and north Western Australia.
(Photo Neville Coleman)
Bivalves form the only class of molluscs which does not have some form of head. Many of their sense organs are concentrated either at the end of their siphons (as in burrowing bivalves) or in fringing tentacles along the mantle edge (as in scallops, or file shells).
With one of the most startling bivalve animals, this Flashing File shell Ctenoides ales is able to create the impression of a 'flash' by rippling the white edge of its mantle.
( 9 metres, Uepi Island, Solomon Islands)
(Photo Neville Coleman)
Univalves utilise head tentacles, eyes, an extensible siphon, or combinations of all three to feel or smell their way along.
Chitons have up to several thousand sensory spots on the backs of their shells. These spots act as eyes and enable the chiton to respond to darkness and light. Most chitons are nocturnal and during the day hide beneath rocks and stones. Some bivalves like scallops and spondylids have small elaborate `eyes' situated at the end of tentacles which protrude from the shell gape. These `eyes' are only light detectors and have no power to distinguish shapes. Strombs and cuttles have large, highly developed eyes and more acute vision.
Feeding Habits
First photographs ever published of the hunting strategy of the Southern Volute Shell Melo miltonis.
The mollusc visits rubble reef areas and captures univalve prey. It then wraps the prey in a folded pouch at the rear of the foot and carries it there as it travels back to its sandy home habitat whre it buries, smothers its prey and when it is dead consumes the flesh and discards the shell and operculum.
( 3 metres, Houtman Abrolhos, Western Australia)
(Photo Neville Coleman)
In this image, the Southern baler Shell has been disturbed to prove that the lump in the rear of the foot was actually a reef turban shell that had been captured and carried along. Although the baler shell animal has retracted into tis shell , it has not relinquished its deadly hold on the turban shell and still retains a grip by plugging the turban's aperture and not allowing it to breathe.
( 3 metres, Houtman Abrolhos, Western Australia)
(Photo Neville Coleman)
Molluscs feed in various ways. The bivalves are mostly filter feeders and consume microscopic organisms strained from the water as it passes through the animal's gills. The food progresses to the stomach, where digestive enzymes assist in breaking down the particles. There is no differentiation made between animal or plant food but the gills and mouth palps reject larger particles.
This method of feeding often results in the ingestion of noxious substances from contaminated water. These noxious substances can be derived from plankton `blooms' known as the `red tide' or by human pollution.
All bivalves can become toxic from the effects of red tide. Those that affect humans are most commercial species such as the Pacific oyster Crassostraea gigas.
( low tide Leigh, New Zealand)
(Photo Neville Coleman)
The planktonic dinoflagellates which form the `red tide' contain toxins, and bivalves feeding in waters which have high concentrations of these organisms accumulate these poisons. Anybody eating the affected bivalves may become partially or completely paralysed and this condition often results in death.
The toxin is not in any way reduced in potency by cooking. Bivalves from areas adjacent to effluent or factory waste discharge systems also accumulate noxious substances and bacteria which can poison humans and cause disease. (See Dangerous Sea Creatures by the author for more details).
Collecting Mud Ark Shells for food at Botany Bay New South Wales. This practise is only safe as long as there are no pollutants washing over the mudflats and there are no presense of Red Tide blooms.
(Photo Neville Coleman)
The largest bivalve in the world is the giant clam Tridacna gigas, which may grow to 1.5 metres in length and weigh over 200 kilograms. Although the giant clam is basically a ciliary feeder, it also possesses another food source within its own tissues. Vast numbers of small unicellular plants grown in the clam's mantle. These symbiotic algae are called zooxanthellae and they occur in huge numbers. The clam is thought to be able to `crop' quantities of these zooxanthellae and pass them through to the digestive organs.
The Radula
All classes of shells, with the exception of bivalves possess a very distinctive feeding mechanism called a radula. Although the radula may be modified in some species or be degenerate in others, it is basically a tongue-like ribbon covered with small, hooked, chitinous teeth arranged in groups. The radula is carried on a muscular pad and can be thrust out through the mouth to rasp off food particles; it can then be retracted, bringing the minute shreds of food back to be swallowed.
Chitons rasp algae from the rocks and are mostly herbivores. The univalves feed in a number of different ways and utilise many animals and plants for food. Abalone, limpets, turban shells, and strombs all feed on algae. The pelagic purple snail Janthina feeds on planktonic cnidarians, while squids and cuttles include fish in their diets. Some tritons feed on ascidians, as do the small Trivia cowries.
There are cowries and nudibranchs which feed entirely on sponges, while helmet shells eat sea urchins. Cones have an extremely modified radula and the teeth resemble small harpoons. These harpoons and the associated nerve poison enable cones to kill and eat small fish, worms, and molluscs.
The vermetid worm shell snares its food by emitting strands of mucus which entrap minute animals. Wentletraps suck the juices from anemones and corals, and octopuses feed on crabs and other molluscs.
(12 metres, Loloata Island Papua New Guinea)
(Photo Neville Coleman)
Respiration
Respiration in molluscs is carried out by gills which absorb oxygen from the water. In gastropods (univalves) these gills can be situated in the mantle cavity, or in nudibranchs may project from the animal's back.
The rear mounted external gills of these pair of 'trailing' Tryon's Ribecia nudibranchs Risbecia tryoni are easily seen, whereas most shelled univalves have internal gills and breathe water in through their siphons.
( 10 metres, Milne bay, Papua New Guinea)
(Photo Neville Coleman)
Molluscan animals which are exposed during intertidal lows must retain a certain amount of water within the shell. Bivalves, like oysters and mussels, can clamp their shells together tightly, trapping water within until the tide returns, or in the case of fresh water molluscs, until the next rains come.
A Hairy Triton Shell Cymatium parththenopeum with its operculum withdrawn to protect its animal. The hairy skin ( periostracum) covering the shell is a protective camouflage which allows the mollusc to blend into its rocky reef , muddy rubble habitat.
(Photo Neville Coleman)
Many univalves with an opercula are also able to trap water within their shells. Fresh water molluscs cannot tolerate salt water and salt water forms die in fresh water. When heavy rains or floods dilute the sea water along the coastal areas, the free-moving molluscs depart for deeper water. Those which cannot move, or are dependent on a shallow water host are often wiped out.
Reproduction
The breeding habits of molluscs and their methods of reproduction vary considerably. Many species shed eggs and sperm into the water where, after fertilisation, the eggs develop into small planktonic larvae called veligers. These veligers drift with the currents until development enables them to settle to a demersal (i.e. on or near the bottom) existence.
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A male Hairy Triton Shell Cymatium parthenopeum showing the rather large penis issueing from the rear of the right side of the neck. After impregnation the female builds a circular nest of eggs and sits on them, protecting them till they hatch.
(Photo Neville Coleman)
Other reproduce by internal fertilisation, with the male depositing sperm into the female's mantle cavity by way of a penis. Molluscs with internal fertilisation sometimes lay eggs in capsules attached to the substrate. Depending on the species, these eggs may hatch as veligers or may exhibit direct development where the young crawling juveniles simply leave through the capsule's escape hatch and go on their way. Many of the molluscs which show direct development lay their egg capsules on or near their source of food, thus guaranteeing an immediate supply for the young animals.
The female Spiny Murex Shell Murex acanthostophes lives at 20 metres in muddy sand off Dampier Western Australia and lays an upright egg mass which is anchored to an object buried in the sand. This a different egg laying strategy than others of its genus who often participate in mass laying behaviour.
( Photo: Wally Rowlands)
Mollusc eggs are subject to predation from the moment they are laid. Egg capsules are preyed upon by fish, crabs, worms, and other molluscs. Because the egg-laying behaviour of many molluscs is controlled by innate releasing mechanisms, there is no way the females of some species can anticipate or prevent egg predation. On many occasions I have observed a female mollusc depositing egg capsules while two or three centimetres away another species was gobbling up the eggs as fast as they were laid.
Female Textile Cones Conus textile generally lay their egg capsules on the undersides of dead coral slabs and rocks buried in sand. Cone shells lay very characteristic, flat egg capsules which are easily determined to genus.
(Photo Neville Coleman)
Some female molluscs brood their eggs until hatching takes place. Most tritons, cowries, and octopuses are extremely protective of their eggs and only leave them if unduly harassed. The female tritons construct cellulose nests in which they lay their eggs. If the females are removed or driven away from the egg mass, the fate of the eggs is often sealed for within minutes predators will move in and consume them.
Close up study of a nest of eggs laid by a female Australin Triton Ranella australasia which are laid beneath rocks and also on the roofs of shallow caves. ( Bass Strait, Tasmania June /July)
(Photo Neville Coleman)
Coat-of-mail shells
(Chitons)
The Pretty Chiton Rhyssoplax exoptanda is one of the most attractive species and serves as a good example as to the fantastic patterns and colour there is within this little noticed group.
(15 metres, Fremantle, Western Australia)
(Photo Neville Coleman)
Due to their cryptic natures, flat profiles and generally nondescript appearance very few divers actually see or take notice of chitons. However, when looked at closely these ancient armour-plated molluscs have remarkable colour patterns, with amazing textures and designs.
Unlike the shells of other molluscs, that of the adult chiton is a multi-valve structure and normally has eight separate plates.
These plates generally overlap each other and are attached to muscles and supported around the outer periphery by a tough, fleshy, leather-like girdle. The girdle is muscular and, combined with the separately moveable shell valves, allows the chiton to curl up into a ball when removed from the substrate.
Chitons are slow-moving and nocturnally active herbivores which are all marine. They venture forth in the hours of darkness from their holes and crevices to search for growths of minute algae upon which they feed. Some forms feed only when covered by water, but others manage quite well under exposed conditions, as long as the rocks are damp. They feed by means of a wide, many-toothed radula which rasps algae from the substrate. This radula is quite long in comparison with that of other molluscs, for the unyielding rocks wear the teeth away and the chiton has to replace them constantly.
The majority of chiton species exist in the littoral and shallow sub-littoral zones of rocky seashores. Because of their low profile shells and excellent adhesive abilities they are able to resist ocean swells and dislodgment by the attacks of their natural predators.
At low tide, under the cover of darkness the Spiny Chiton Acanthopleura spinosa crawls from its homing "hollow" to feed by grazing micro algae from the intertidal rock surfaces. Even though it appears very well protected it does not stay in the open. Once the tide begins to turn, it huries back to the protection of its hollow, by back - tracking its own slime trail.
(Low tide, Mandorah Point, Darwin Northern Territory)
(Photo Neville Coleman)
Movement is accomplished by waves of muscular contractions which pass along the mollusc's foot from one end to the other. From above, the chiton looks to be a fixture, but some species are relatively mobile and show a determination to go in the direction they have chosen. A few deep water chitons are very restricted in habitat.
Whereas a normal mollusc's shell is an inanimate structure that is devoid of any sensitivity, the chiton's shell valves are different. Sensory organs which are capable of light reception are located in pores in the animal's shell. These simple `eyes' are very light sensitive and each one is housed in a minute chamber in the outer layer of the shell valve. These `eyes' were first discovered in 1884 by Professor Moseley, an English scientist, who figured prominently in the Challenger expeditions which were responsible for recording a great deal of data on the Australian marina fauna.
Little work has been done on the reproductive habits of Australian chitons. It seems incredible, but it is in keeping with our approach to Australian marine faunal work in general that significant discoveries made in the early 1900s were not followed up until some half a century later.
Although observations that some temperate and cold water chitons brood their young were made in 1922, it was not until 1971 tha interest was again stirred by a similar discovery in a Tasmanian species by J R Penprase of Hobart, Tasmania, and officially recorded in 1978 by Elizabeth Turner of the Tasmanian Museum and Art Gallery.
At least three species of southern chitons are now known to brood their young and others are suspected of doing so. Ischnochiton (Ovatoplax) mayi, Heterozona subviridis, and Paricoplax crocinum brood their eggs in the mantle cavity on both sides, or alternate sides, of the foot and may carry from 50 to 300 eggs at a time.
In the species I. (O) mayi and H. subviridis brooding continues until the eggs have hatched and the young have metamorphosed into eight-valved juveniles about 0.5 mm in length.
Although this behaviour is only recorded as yet from Tasmania it will be interesting to see if reports from other areas turn up now that it is common knowledge. Chitons are known to reproduce in several ways. The males have no penis and simply release sperm into the water. The presence of the sperm in the water triggers the females to emit their eggs, or else the sperm is taken into their mantle cavities with the inhalant water current of the females. Either way the eggs are fertilised.
Most chitons lay egg strings attached to the substrate. Young chitons are born with a complete, single-valved shell, but this cleaves during development.
Chiton species may all appear to be similar to the casual observer, but to those who wish to know them they are exciting and interesting.
Different species have variously shaped and coloured valves, and even the ornamentation of the girdles are individual to the species. There are plated girdles, scaly girdles, hairy girdles, spiky girdles and many more.
Although chitons occur in most oceans of the word, in tropical and temperate waters, they are particularly abundant in the Australian waters. With over 300 recognised species Australia boasts almost half the known chiton fauna of the world
SPECIES TO BE ADDED AT REGULAR INTERVALS