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What do we know about "Old Fourlegs"?
As a living fossil, the coelacanth has become a celebrity fish. Dr
Phil Heemstra is an ichthyologist with the South African Institute for
Aquatic Biodiversity. He is one of the fortunate few who has had
first-hand experience of a living coelacanth. Here he shares information
on some of its less - publicized secrets…such as Latimeria's
head-stands.
Dr Heemstra will be one of the key scientists involved in the Coelacanth
Programme.
 Coelacanths have not changed much over the past 380 million
years. The skeleton of Macropoma lewesiensis, which is known from
the upper Cretaceous, is virtually identical to that of the coelacanths
caught off Sodwana Bay, Latimeria chalumnae, and differs little
from the skeleton of most Devonian coelacanths. (Enter the image above to
see skeletons of a living coelacanth and that of a fossil). Thanks to the
efforts of JLB and Margaret Smith, Marjorie Courtenay-Latimer, French
anatomists, and (since 1987) Hans Fricke and his co-workers, Jürgen
Schauer and Karen Hissmann (who were the first to observe and film
coelacanths in their natural habitat at Grand Comoro Island), we have
learned much about the anatomy, biology and ecology of this fascinating
fish.
Behaviour
Latimeria are nocturnal fish, and during the day, they are
usually found in depths of 120-250 m, where they congregate in caves, with
as many as 14 fish crowded together in a single cave. By resting in caves,
they save energy and are also less vulnerable to large predators
(deep-water sharks). Each coelacanth has its own distinctive pattern of
white markings, and this allows recognition of individuals and tracking of
their movements. Although several individuals occupy overlapping home
ranges, no aggressive encounters between coelacanths have been observed. A
single fish may frequent several caves within its home range, and three
individuals were sighted within the same home range over a period of two
years. After sunset, the fish leave their caves and drift slowly , 1-3
metres off the bottom, presumably looking for food. On these nightly
hunting forays, Latimeria may travel as much as 8 km; and before dawn they
shelter in the nearest cave.
Swimming
While searching for prey, or moving from one cave to another, Latimeria
appear to move in slow motion, either drifting passively with the current
and using the flexible pectoral and pelvic fins to adjust their position,
or slowly swimming by a synchronous sculling movement of the second dorsal
and anal fins. In slow forward swimming, the left pectoral and right
pelvic fins move forward, while the right pectoral and left pelvic fins
are pulled backward. This tandem movement of alternate paired fins
resembles the movement of the fore- and hindlimbs of a tetrapod walking on
land. Contrary to JLB's name "Old Fourlegs" and his idea that the
coelacanth stalked its prey "by crawling quietly along gullies and
channels", Latimeria do not use their lobed fins for walking on the
bottom, and even when they are resting in caves their fins usually do not
touch the walls or bottom of the cave. Like most slow-moving fishes, the
coelacanth can make a sudden lunge or fast start by means of a quick sweep
of its massive caudal fin.
Hunting for Food
During their nightly foraging swims, Latimeria were often seen
to perform head-stands, with the body in a vertical position, the head
near the bottom and the tail fin curved perpendicular to the body. They
then held this position for two or three minutes at a time. This curious
behaviour may be used when Latimeria are scanning the bottom for prey. The
large, sensory organ in the snout (known as the "rostral organ") is
thought to function as an electoreceptive organ, in the same manner as the
ampullae of Lorenzini, which assist sharks to find buried prey animals.
Latimeria feed mainly on small fishes that occur in their deep demersal
habitat. In addition to a swell shark and a skate, various bony fishes (a
synaphobranchid eel, a deep-water cardinalfish, etc.) and a cuttlefish
have also been found in their stomachs. Judging from their sluggish
lifestyle and the relatively small surface area of the gills, Latimeria
have one of the lowest metabolic rates of all fishes; consequently, their
food (energy) requirements are relatively low for such a big fish. This
energy-saving lifestyle is an advantage in their specialised habitat on
the relatively barren volcanic slopes of the Comoros where prey is
scarce.
Reproduction
Despite the lack of an obvious copulatory organ, Latimeria are
"ovoviviparous", which means that the eggs are fertilised internally, and
the pups (fetuses) are retained within the mother until they have grown
large enough (36-38 cm) to fend for themselves. The enormous eggs (9 cm in
diameter and over 325 g in weight) are the largest eggs known for fishes,
and the huge yolk supplies all of the nutrients necessary for the growth
of the embryo. Contrary to previous speculation, recent information
confirms that Latimeria pups do not practise "embryonic cannibalism" or
feed on eggs while in the uterus. In August 1991 a 179 cm, 98 kg, pregnant
coelacanth was caught by a trawler off Pebane in northern Mozambique. This
specimen was given to the Natural History Museum in Maputo, where it was
dissected by the Director, Dr Augusto Cabral, who found that it contained
26 near-term pups, 31-36 cm in length. In view of the large size and
advanced development of the pups from the Mozambique female, the size at
birth for Latimeria is probably about 35-38 cm, and this agrees with the
age estimate of 6 months assigned to a 43 cm juvenile that was caught on
hook and line. The gestation period for Latimeria has been estimated at
about 13 months.
Unusual Anatomy of Latimeria
Latimeria differs from most other living fishes by a number of
features, some of which are characteristic for coelacanths generally. The
most obvious external feature that readily identifies a coelacanth are the
fins. The lanceolate tail fin with the many-rayed upper and lower margins
and central little supplementary fin projecting beyond the fin margin
distinguishes Latimeria from all other living fishes.
The fleshy, stalk-like pectoral and pelvic fins and similar fleshy
second dorsal and anal fins are also unlike any other marine fishes. The
paired fins of the Australian lungfish, Neoceratodus forsteri (another
living fossil), are similar to those of Latimeria. The skeletal support
for the fan-like first dorsal fin of Latimeria is a single bony plate, and
the skeletons of the fleshy second dorsal and anal fins are identical to
each other. In place of the bony vertebral column of most adult fishes,
coelacanths have a large, thick tube of cartilage, called the 'notochord'.
In the early development of most fishes, the notochord of the embryo or
larva is gradually replaced by the bony (or calcificed) centra of the
vertebral column. But in coelacanths, lungfishes and some primitive
sharks, the transformation of notochord into a segmented bony (or
calcified) vertebral column does not take place. In coelacanths, the
hollow notochord is filled with oil and provides a strong, yet flexible
support for the spinal cord.
The cranium is divided transversely by the intracranial joint, which is
better developed in coelacanths than in other lobefin fishes. The moveable
front part of the cranium provides a larger gape when the mouth is opened,
and this may be advantageous in feeding. Coelacanths have a large median
'rostral organ' in the snout, with three tubes leading to the exterior
surface on each side of the snout. The nerve pathways that enervate the
rostral organ indicate that it may function as an electroreceptive organ.
In adult Latimeria, the brain is simple (not much convoluted), occupies
less than 2% of the cranial cavity, and is confined to the rear part of
the skull. In the late-term foetus, the brain fills the cranial cavity; it
appears therefore, that soon after birth, the brain stops growing, while
the head, body, fins (and everything else) continue to grow. This unusual
configuration of a tiny brain situated at the rear of an enormous brain
cavity is also seen in the sixgill stingray, Hexatrygon bickelli Heemstra
and Smith, 1980. As suggested for the sixgill stingray, a small brain
placed at the rear of the cranium may cause less electrical interference
with the electroreceptive sensors in the snout.
The jaws of coelacanths are unique. The lower jaw is attached by double
(tandem) joints on each side of the head. The upper jaw is absent or
reduced to a small median rostropremaxilla bone, and there is no maxillary
bone. The toothed bones at the front of the palate serve as a functional
upper jaw. The heart appears to be the most primitive of all adult
vertebrates, with the auricle, ventricle and conus arteriosus arranged in
straight line, rather than being doubled over one another. Unlike most
bony fishes, which have a gas-filled swimbladder that provides buoyancy,
the swimbladder of Latimeria is filled with fat. In addition to the
fat-filled swimbladder (which also provides buoyancy, as fat is less dense
than seawater), Latimeria also have a lot of oil in the liver, muscle
tissue and in cells under the skin. In most fossil coelacanths, the
swimbladder appears to be ossified (bony) and, consequently, these fishes
were probably confined to shallow water. Latimeria have no branchiostegal
rays supporting the opercular membranes.
Where Coelacanths Live
 In 1995 a coelacanth was caught in a gill net set in 150 m
off the southwest coast of Madagascar. And in 1998, two coelacanths were
caught at Sulawesi in the Indonesian Archipelago. Although it seems likely
that the South African, Mozambique, Madagascar and Comoran specimens are
all Latimeria chalumnae; a DNA comparison indicated that the recently
discovered Indonesian coelacanths are a separate species
We still have much to Learn
Because of the difficulty of sampling and observing fishes in rough
bottom habitats below depths of 50 m, the fish fauna of these areas is
poorly known. This discovery of coelacanths at a depth of 104 m at Sodwana
Bay, a popular dive site in South Africa, emphasises how little we know
about life in the oceans and the need for further exploration and survey
work to understand the fish diversity of southern Africa and the Western
Indian Ocean.
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