"Animals with metallic teeth"
All about chitons (Phylum:
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.
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
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.
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.
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.
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!
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.
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.
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.
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
Identification of southern African
For those readers wishing to locate and
identify various chiton species, the following guide will hopefully be of some
SPECIES WITH SCALY GIRDLES:
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 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.
SPECIES WITH HAIRY GIRDLES:
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.
Branch, G. and Branch, M.
(1981). The Living Shores of Southern Africa. C. Struik Publishers,
Cape Town. Pp 211-213.
Branch, G., Griffiths,
Branch, M. and Beckley, L. (1994). Two Oceans, a Guide to the Marine Life
of Southern Africa. David Philip Publishers, Cape Town. pp 128-130.