"Animals with metallic teeth"

All about chitons (Phylum: Mollusca; Class: Polyplacophora)

by Bevan Pank

It was a typically warm, windless morning in March and the clear waters of False Bay were calm. Children from all over South Africa had gathered on the pristine beach at Buffels Bay near Cape Point in the National Park. Led by two conservation officials, they had come to put classroom theory on the marine aspect into practice. Among their lessons was a "treasure hunt" in the rock-pools, whereby each team was given several cards with clues in poetic form to identify a particular animal. One of the most intriguing was as follows:

I am one of the oldest sea creatures,
Plates on my body are large features.
I sometimes have tufts on my back;
My colour is grey, quite often black.

Origin

The facilitator had described the chiton, a marine invertebrate so strange that it even has some teeth capped with magnetite - an iron oxide! Scientists believe that some 500 million years ago during the late Cambrian Period, it diverged from the main line of evolution. This is because it is very different from other molluscs in the structure of its shell, presence of multiple gills and yet lack of sensory tentacles. Furthermore, its poorly developed head is so concealed beneath the shell as to suggest blindness.

Sensors

In fact, at the very least, chitons have numerous eyes of sorts known as "esthetes". These are vertical canals with capped sensory terminals on the shell. One foreign species - Lepidochitona cinerea - has been found to have 1750 per square millimetre of its shell surface! Best sighted among the approximately 500 recorded species are some members of the family Chitonidae. These have photo-receptive nerve endings, pigmented layers and tiny lenses.

Shell structure

Chitons are zoologically classified as Polyplacophora, which can be loosely translated as "bearer of many plates". Actually, their shell consists of eight arched, overlapping plates or valves in an ovoid configuration to minimize water-drag and are articulated. They not only protect the flattened body of this slow-moving 10-100 mm herbivore, but also enable adherence with its large muscular foot to an irregular rock surface. Furthermore, they allow it to roll up into a ball in the unlikely event of dislodgment.

Adherence

Just as two glass sheets with a film of water between them are difficult to separate, so a secreted mucus between the animal’s foot and the rock face provides adhesion. It is the same with limpets, whereby an applied force of over 100 kg would be necessary to dislodge a large species. Chitons go one better, by tightly clamping their outer margin to the rock face and then raising the inner margin to create a vacuum seal.

Girdle and habitat

Besides giving rise to the shell, a fold of skin or "mantle" covering the visceral mass also overlaps to form a broad and tough flexible girdle. This is usually armed with bristles, scales or spines, which help to identify the different species. One often found with a scaly girdle is the colourful 30-40 mm tulip chiton (Chiton tulipa). Like most low-shore inhabitants, it lives under stones. Some species also live in crevices, but one even occurs on exposed high-shore rocks. This is the 30-45 mm spiny chiton (Acanthochiton garnoti), which is quite common in the Cape.

Feeding

Chitons usually emerge at night during high tides, to feed on minute algae and other encrusting organisms present on rocks. Before and even during feeding at times, they test the substratum with an extensible sensory structure called the "subradular" organ. They graze by rasping a long tongue-like ribbon with numerous transverse rows of 17 renewable teeth termed the "radula", which is projected from the mouth. The lateral teeth are capped with hard magnetite and self-sharpen as their softer trailing-edges wear. They have also been found to be magnetic in some species. Although not yet proven, some scientists suspect that the creature uses this property as a compass to align it with magnetic north. So perhaps people living in the Northern Province had better watch out!

Digestion

To process their difficult to digest vegetable diet, chitons have a very long coiled gut. It is also used as temporary storage for waste products compacted in mucus. To avoid accumulation under the mantle, these pellets are only flushed away when tides rise over the animal. Before particles of food can reach the stomach, they are mixed with amylase, which is an enzyme produced by a gland to help convert starch into sugar. In the stomach, they are further mixed with yet another enzyme from a large digestive gland to break down any protein present. This intracellular method of digestion is a problem for molluscs that feed on large prey, because they need to sort food according to size before it can be digested.

Oxygen uptake

Gills are arranged in a linear series within a groove on each side of the body between the foot and mantle edge. Depending on the species and their size, numbers vary from six to no less than 88 pairs. Towards the front, both sides of the mantle margins are raised to form inhalant openings. After passing over the gills, water is expelled from two exhalant siphons at the back.

Unfortunately, the gills are "bipectinate" with a precariously supporting central axis, from which plate-like lamellae project on either side. They are prone to clogging by silt sticking between their platelets and the mantle wall. For this reason, chitons avoid estuaries and lagoons, occurring only in clear water. In more evolutionary advanced molluscs, gills are "monopectinate" with platelets on one side of the axis. The other side is firmly fixed to the mantle wall, thereby strengthening the gill and preventing particles from blocking it.

Reproduction

Most chitons are dioecious and reproduce by external fertilization. A simple gonad above the gut sheds sperm or several hundred thousand eggs per spawning. A notable exception in southern African waters is Chiton nigrovirescens, which broods its eggs under the girdle. In the open sea, eggs hatch as "trochopore" larvae. Ringed with beating hairs or "cilia" for propulsion and to capture food particles, they have an almost ethereal appearance. Before settling, molluscs usually develop into "veliger" larvae and produce a tiny shell. Chitons bypass this stage, rapidly metamorphose in the plankton and sink to the bottom fully formed except for its still larval eyes.

The future

Scientists have long been puzzled by the ancestry of molluscs and chitons add to the intrigue. Unfortunately, they are not nearly as numerous as the bivalves, limpets, sea snails and others of their kin. Being mainly intertidal, they are highly vulnerable to gill blockage when nearby swimmers churn up the water. On the northern KwaZulu- Natal coast, they were also vulnerable to exploitation for medicinal purposes until the people learnt of their huge potential as a tourist attraction. Much to the excitement of marine biologists from around the world, an unknown species was recently discovered to be endemic to the region. With great pride, the locals learnt of its new name - Chiton Saliharfui. Although only 30 mm in size, it has the beautiful ornamentation of its kind artistically developed over 500 million years. Long may future generations be able to marvel at phylum (major grouping) Mollusca class Polyplacophora - some of the first marine invertebrates on planet Earth!

Identification of southern African species

For those readers wishing to locate and identify various chiton species, the following guide will hopefully be of some assistance.

SPECIES WITH SCALY GIRDLES:

The 10 mm dwarf chiton (Ischnochiton oniscus) with often white valves in various patterns and a narrow girdle with the scales almost smooth, lives in groups on the underside of rocks in sandy pools from False Bay to northern KwaZulu-Natal.

The 10-20 mm ribbed-scale chiton (Ischnochiton bergoti) with brown to off-white pitted valves and ribbed girdle scales, found under stones at low tide from the Cape Peninsula to northern Namibia.

The 10-25 mm brooding chiton (Chiton nigrovirescens) with ridged brownish-black valves and large but smooth coppery girdle scales, usually in groups under stones from Walvis Bay to Cape Agulhas.

The 20-40 mm textile chiton (Ischnochiton textilis) with a ridged pale yellow or grey shell and small oval scales on the girdle, found on the underside of boulders in rock-pools from northern Namibia to East London.

The 30-40 mm tulip chiton (Chiton tulipa), valves pink streaked or flecked with brown patches or zigzags and striped girdle with smooth but large overlapping scales, often discovered under rocks near low tide from Saldanha to the Transkei.

SPECIES WITH VELVETY GIRDLES:

The 20-50 mm broad chiton (Callochiton castaneus), dark-brown to orange with a wide spine-covered girdle of velvety texture and found under stones at low tide from Port Nolloth on the west coast to Cape Agulhas.

The 25-50 mm black chiton (Onithochiton literatus), dark-brown with wavy lines and broad velvety girdle with minute embedded rod-shaped calcareous "spicules", common on wave-washed rocks and the margins of rock-pools from the Transkei to northern KwaZulu-Natal.

The 30-45 mm spiny chiton (Acanthochiton garnoti), with brown valves partially buried by a wide girdle covered with nine pairs of tufted glassy spines and seen on high shore rocks from St. Helena Bay to Port St. Johns.

SPECIES WITH HAIRY GIRDLES:

The 40-50 mm orange hairy chiton (Chaetopleura pertusa), orange with beaded ribs and wide pink girdle bearing bristles and short hairs, ranging from Saldanha Bay to northern KwaZulu-Natal.

The 40-70 mm hairy chiton (Chaetopleura papilio), smooth shiny valves with alternate dark and light brown stripes and a wide brown girdle sparsely covered with black bristles, seen under rocks in low-tide pools from Lüderitz to Table Bay.

The 70-100 mm giant chiton or armadillo (Dinoplax gigas), steeply arched grey or brown valves and a brown girdle dotted with tufts of short hair, usually partially or fully buried in sand on rocky reefs from False Bay to the Transkei. Replaced by related species - Dinoplax validifossus - with uniformed instead of tufted girdle hairs in northern Transkei and KwaZulu-Natal.