Coelacanths and their living examples the Latimeria
Name:
Coelacanth
(Hollow spine).
Phonetic: See-la-canth.
Named By: Louis Agassiz - 1839.
Classification: Chordata, Sarcopterygii,
Actinistia, Coelacanthimorpha, Coelacanthiformes.
Diet: Piscivore.
Size: Up to around 180 centimetres long and
around 80 kg. Sometimes credited at being up to 200
centimetres long.
Known locations: Fossils known from across the
world. Living specimens have been found off the coast of East Africa
and Indonesia.
Time period: Devonian through to present day.
Fossil representation: Multiple fossil specimens.
Coelacanths continue to occasionally be caught by fishermen.
Coelacanths
are one of the best known animals that today are loosely termed
‘living fossils’. This is because while the first fossils were
known as early as 1839, a living specimen was actually caught off
the coast of South Africa in 1938. Of course this discovery both
shocked and surprised the paleontological community, and triggered a
rethink in evolutionary theory regarding the ancestry of land
vertebrates. However before we look at the living members lets go
back to the fossils so that we can get a more complete picture about
these fish.
Coelacanth fossils
The
first coelacanth fossil was named by Louis Agassiz in 1839, a man
who specialised in studying and naming fossils of fish. In describing
the fossil Agassiz made special note of the hollow rays of the caudal
fin, giving rise to the name coelacanth. However it is important to
remember that today coelacanths are actually treated as a group of fish
rather than a specific genus, but all of the known genera share the
same approximate form and proportions. Coelacanth fossils do however
reveal fish that are a variety of different sizes, from several
centimetres to much larger.
Coelacanths
are members of the Sarcopterygii class of vertebrates. This means
that as a group they are closely related to early tetrapods as well as
lungfish, another group of primitive fish that have managed to
survive into present times. Lungfish are quite similar in form to
coelacanths, but are known to have the ability to breathe air when
out of the water, something that coelacanths as a group are believed
to be incapable of doing.
Fossils
of coelacanths revealed a fish that was not as well adapted for
swimming as other forms, and actually had powerfully built fins that
seemed to be quite muscled. This led to early palaeontologists
considering that coelacanths may be ‘missing links’.
The placement of coelacanths in
evolutionary theory
Evolution
theory really came into the spotlight with the publication of Charles
Darwin’s ‘On the Origin of Species’ in 1859 (however there were
earlier similar theories before this). This new theory of evolution
was controversial because rather than having creatures just appearing
upon the landscape by means of a divine creator, it stated that
animals evolved from one another by a process of natural selection
where successful animals adapt and change to meet new challenges.
This led to study on where life could begin and a scenario was rapidly
constructed where life appeared in the oceans first due to conditions
being more favourable to the creation of life.
The
problem though is that there are creatures that live on the land, and
in terms of evolution they must have a common set of ancestors that
once lived in the ocean. This means that at some point a creature
must have developed both the ability and need to leave the water and
climb onto land. Here the early supporters of evolution began to look
at lobe finned fish, but gave particular attention to coelacanths.
As mentioned above these were fish poorly adapted for fast swimming,
but had limbs that could seemingly support the weight of the body on
land when out of the water. The reasons for a fish leaving the water
were explained as everything from predator evasion to leaving one body
of water to cross land to reach another body of water to either spawn
or spread out into new waters.
The
first problem was pointed out soon after this proposal in that at the
time no transitional form that showed the change from coelacanths to
the early amphibians existed. However this gap was easily enough
explained by supporters who said that just because the transitional
forms had not yet been found, it does not mean that they did not
exist. The biggest blow for the theory would come in 1938 however
when the unthinkable happened; a living coelacanth was caught by
fisherman off the coast of South Africa. This was not a fluke
occurrence either and it was soon established that there were several
surviving populations of them thriving in the waters of the Indian
Ocean.
The
discovery of living coelacanths revealed one key thing; coelacanths
spend their entire lives in the water and never walk on land. Critics
of evolution jumped up and took this discovery as proof that this part
of the theory and even the whole concept of evolution was wrong and
that they were right, but the patience of time and hindsight has not
quelled the theory but reinforced it. The inclusion of coelacanths
into this niche of evolutionary theory was a mistake, but one that
was made by people who were reconstructing a theory upon a very
incomplete record of fossils.
With
successive discoveries, more and more forms have been found which
show more lobe finned fish with strong fins (now widely regarded as
being for the purposes of pushing through dense growths of underwater
weeds) and earlier amphibian forms that show terrestrial developments
but still an aquatic life. The discovery that has made the most
headlines however is Tiktaalik,
a creature that seems to be split
between being a fish and an amphibian. The body of evidence to
support the theory of evolution grows with each passing year, but
today coelacanths are only a footnote that marks a temporary
misdirection.
Latimeria -
The living
coelacanths
Because
coelacanth fossils seemed to disappear from deposits at the end of the
Mesozoic, popular opinion suggested that they died at the same time
as the dinosaurs. Then in 1938 Hendrik Goosen, a trawler man
who captained the Neriene returned to the port of East London, South
Africa with a strange looking fish that was unlike anything he had ever
seen. He contacted Marjorie Courtenay-Latimer of the museum of East
London to see if she was interested in it, however the fish did not
make it into port until many hours after it was caught and the skin had
already turned a dark grey colour instead of the blue it had been when
caught. Courtenay-Latimer could not identify the fish either but time
was running out and the only option she had to preserve it was to have
it ‘stuffed’ by a taxidermist. Soon after James Leonard Brierley
Smith returned from a Christmas break and upon first glance immediately
recognised the fish as a coelacanth and chose the new genus name of
Latimeria in honour of Coutenay-Latimer. The full
name of the species
is Latimeria chalumnae (today also called the
West Indian Ocean
coelacanth), after the region it was caught in.
Because
coelacanths were first identified from fossils the new specimen became
known as a ‘living fossil’, a phrase that has since come into
popular use to describe other creatures that have been around for
considerable time. Not all were convinced at the validity of the new
specimen however because the bones and gills were not kept after the
taxidermy, leading to some speculating that the fish had been
modelled to look like a coelacanth rather than actually be one. In
1952 supporters got the evidence they needed to silence these
doubters when a second specimen was caught although at the time this
was named Malania anjouanae in honour of Daniel
François Malan, the
then prime minister of South Africa who was instrumental in the
specimens preservation.
Further
specimens continued to caught and observed throughout the twentieth
century, but in 1997 another exciting find, a new species of
Latimeria was discovered in Indonesia. Found for
sale at a fish
market by Arnaz and Mark Erdmann, this new specimen was named
Latimeria menadoensis (Indonesian coelacanth)
and differed from the
first species by having brown skin instead of blue. Although not
deliberately fished for like some commercial species, Latimeria
continue to be caught with the specimens usually being reserved for
collectors and scientific institutions.
As
one might expect, Latimeria are very primitive in
physical form,
with perhaps their most archaic feature being the presence of a
notochord (a pressurised tube) rather than a developed spine.
Most fish today have a spine composed of lots of vertebra (even
sharks have vertebrae made from ossified cartilage). Latimeria
also
have tube shaped hearts that are different to modern fish genera.
Latimeria are also not thought to be great problem
solvers since the
actual brain only takes up 1.5% of the internal volume of the brain
case where it is housed (the remainder is filled with fat).
Latimeria
are deep water fish that are more active at night. During the day
they retreat to the shelter of caves and crevices of submerged rock
formations. Populations of Latimeria are known from across the Indian
Ocean where such submerged habitats occur, with one such location
being near the Comoros Islands that lie between Northern Mozambique and
Northern Madagascar (Latimeria chalumnae is also
sometimes more
commonly known as the Comoro coelacanth). By night they leave their
retreats and hunt for other marine creatures such as fish which are
themselves sheltering for the night. Latimeria
have large eyes that
are adapted to pick-up blue wavelengths of light (blue is one of the
few wavelengths that penetrates into deep water) and is probably
one of the reasons why they shelter in dark areas during the day so
that they are not overwhelmed by the increased light). However
researchers believe that Latimeria may also have an
electro-receptive
ability similar to how sharks hunt that allows them to zero in on
hidden prey in the dark.
Latimeria
come in both bright blue and brown forms with a speckling of lighter
spots across the body. However this colouring is only seen in living
or recently caught specimens and after death and possibly exposure to
the air the colour quickly fades to a dark grey. The spots on a
Latimeria seem to have a completely random spread
that can differ
greatly amongst individuals, which has led to speculation that
Latimeria may be able to recognise one another,
especially when they
congregate in submerged caves. Some scientists have also speculated
that Latimeria may be able to communicate with each
other via
electro-receptive pulses. Although social enough to gather together
during the day, Latimeria have been seen to shy
away from physical
contact with one another. Latimeria also seem to
have a shy
disposition and shy away from anything they perceive to be a potential
predator.
For
such a large fish, Latimeria has proportionately
small fins and a
tail fin that is split into three parts called ‘lobes’. These
fins are supported by hollow spines that were Agassiz’s original
inspiration for the name coelacanth when he first looked at the
fossils. Earlier studies depicted the coelacanth as a fish that was
not as well adapted to swimming as other fish, but the living
Latimeria has revealed a very special method of
locomotion. Although
Latimeria are capable of propelling themselves by
their tail, they
seem to prefer drifting upon currents and using their fins just to
steer and orient themselves as they drift. This is a highly energy
efficient form of locomotion and while Latimeria
may not be the fastest
fish in the ocean, it seems to be a successful survival tactic that
has served them for several hundred million years. Latimeria
have
also been seen to swim belly up and even do ‘head stands’ on the
ocean floor as they drift with no ill effects being noted by the
observers. Latimeria seem to be all about
conserving energy as the
deeper waters that they prefer to live in never get as warm as the
surface layers that are warmed by the sun. In an ectothermic
(cold-blooded) organism like a fish, this reduces the metabolic
rate which means that they can survive upon less food. Also staying
in caves means that coelacanths can stay out of the currents that use
at night time so that they can just float in the water without
expending energy swimming to maintain position.
Because
Latimeria are adapted to live in deeper high
pressure waters they do
not do well in aquariums where the water cannot be maintained at the
same pressures as the water at the depths that they inhabit. Like
many other deep water organisms that have been transferred to an
aquarium for study, Latimeria typically do not
live for more than a
few hours.
Latimeria
are not considered to be under direct threat from commercial fishing
because they are considered to be a very poor food source. This is
because Latimeria flesh has a much higher
concentration of oils,
wax, and urea amongst other compounds that are indigestible by
humans. As such these compounds pool and collect inside the digestive
system until they are eventually passed out in liquid form. Fisherman
who catch a Latimeria today usually only keep it
for sale to a
collector or scientific body that might be interested in studying the
specimen, since Latimeria have gained a
reputation amongst consumers
as a fish to avoid. It is uncertain if this cocktail of oils and
waxes helps to prevent them being attacked by predators, but some
Latimeria have been observed with shark bites to
their bodies.
However the fact that these fish survived to be seen by people may
indicate that sharks also find them distasteful since sharks will
sometimes ‘taste’ what they think is food by biting or mouthing at
it. Also if Latimeria do hunt and/or communicate
with each other by
elctro-receptive pulse then these signals might confuse a sharks own
electro-receptive system into thinking the Latimeria
is a fish in
distress and as such a viable target, leading the shark to a mistaken
attack.
However
deep sea trawling may still be having an indirect impact upon Latimeria
since this is an indiscriminate method of fishing that drags up all
kinds of fish and other animals, including those that are not desired
like Latimeria. Over use of this method can also
leave extensive
areas of ocean barren of life as the other organisms that make up the
ecosystem are fished out to the point where they disappear. This
upsets the balance and previously stable populations of all creatures,
not just Latimeria, can crumble and collapse to
the point where they
too disappear because the ecosystem that supported them no longer
exists. Because Latimeria are so rare already,
it is difficult to
ascertain specific population numbers and geographic ranges without
them already getting caught in fishing nets. Something that all
people can agree upon however is that it would be incredibly sad if
coelacanths could survive for some four hundred million years to just
be wiped out within a few decades after their rediscovery by mankind by
over fishing.
What else has survived?
The
1938 discovery of a living coelacanth caused people to ponder what
else might have survived the extinction events that mark major declines
in animal species across the globe. Indeed the Permian–Triassic
extinction event was a particularly bad one with an estimated
ninety-six per cent of all marine creatures becoming extinct, yet the
coelacanths survived this, possibly due to their energy efficient
lifestyle which meant they did not need to eat as much as other
creatures that were disappearing. Popular fiction would have us
believe that the great marine reptiles of the Mesozoic like
Plesiosaurus
and Mosasaurus
might have survived in small populations as
well as the giant shark C.
megalodon. However the reality is often
less spectacular than fiction depicts it to be and any surviving
creatures are highly unlikely to be the giant predators of old due to
their survival requirements (i.e. larger or greater amounts of
prey).
In
short you are actually spoilt for choice when it comes down to
identifying creatures that lived millions of years ago and yet are
still represented today. In terms of fish, hagfish and lungfish are
still present in the worlds waters today, as well as sharks such as
the frilled shark (Chlamydoselachus sp.) and
goblin shark
(Mitsukurina owstoni), although there is still
confusion between
this and the shark Scapanorhynchus.
Horseshoe crabs are armoured
arthropods that are roughly as old as coelacanths yet they can still be
seen climbing onto shorelines to spawn. Velvet worms (of the
Onychophora phylum) that have today become quite popular in the pet
trade first entered the fossil record over five hundred million years
ago and are commonly found hunting in the moist undergrowth’s of
tropical forests today. Other kinds of animals such as amphibians
like frogs, and reptiles like lizards, snakes, turtles and
crocodiles also lived when the dinosaurs where roaming the land (in
fact some like the crocodile Deinosuchus
actually attacked them) and
these too have survived in what are roughly the same form for all this
time. Mammals and even birds that were once thought extinct have
occasionally been found to have survived in small populations and it is
realistically only a matter of time before the next ‘lost animal’
is rediscovered by man.
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