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Summary | |
C. trilobatum(Grey, 1827) is a species of cryptobranch dorid nudibranch in the specious family Chromodorididae, shell-less slug like animals with bright conspicuous colouring (Karuso). Compared to majority of the other members of the order Nudibranchia, C. trilobatum can reach a relatively large size (Willan & Coleman;Marshall & Willan), approximately 130mm. Even amongst other nudibranch species, C. trilobatum has large variety of colour variations throughout its geographic range, leading to many mis-identifications of individuals, and a plethora of synonyms.
The individuals studied here are from the Moreton Bay region, and are found in abundance in the sandy bottomed reefscape along the wall at Amity Point, North Stradbroke Island as well as on the island's north east rocky reefs (personal observations; community observations).
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Figure 1 |
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Physical Description |
External Morphology | |
Named for it’s ‘t-bar’ shape, C. trilobatum’s most distinguishing
feature is its mantle. Covering the whole anterior dorsal side of the
individual, the mantle extends from the distinctly cephalised head, culminating
in the ‘lure’ like protuberance above the anal gill. It is margined be a thick,
unbroken dark purple line. There are two sets of paired lateral lobes, one
cephalic and one in line with the gills. At the head, there are paired
lamellate rhinophores. As in all dorid nudibranchs, the gills are clustered
circularly around the dorsal anus, with twenty branches featuring reduced
lamellae (Rudman), that can be retracted when disturbed. The foot extends from
below the mantle, and continues posterior to the gills at a length greater than
the mantle, giving the body en elongated shape. When handled the body is firm
and has the common slimy feel noted with many soft bodied marine species.
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Figure 2 |
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Figure 3 |
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Size | |
Most likely due to the confusion
caused by the varied colour morphs, size estimates for this animal vary. General consensus and personal and community field observations indicate that it is quite large compared to most other nudibranch species. Individuals used for Rudman's observations were approximately 80-115mm, and Marshal & Willan noted a maximum length of 130mm.
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Colouration | |
As previously stated, there is a vast variety of colour
morphs within this species, so diverse that they were thought to be many
separate species instead of one. As the nudibranch eye is rather basic, these
bright and varied colour schemes are likely to be a form of aposomatic warning
colouration to alert predators that they are unpalateable (Behrens).
In the current specimen, body colouration is a mottled
mixture of oranges, brown and cream, overlaid with orange spots. The gill
structures are orange and cream, and with bright orange rhinophores tipped in
purple. The mantle is bordered completely by a thick, unbroken purple line. The
foot is bordered by a non-uniform shaded purple to blue line.
One of the best examples of the diversity of colour morphs
is the difference between Western Australian C. trilobatum, compared to east coast specimens, which occur at
similar latitudes on opposing ends of the continent. Additionally, a specimen
found in the Northern Territory and initially identified in print as C. corallinum (Marshall & Willan), a
name no longer accepted, featured a bright magenta colouration with a white
mantle ridge.
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Figure 4 |
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Similarity to other species | |
It is worth noting that
this species, particularily the colouration found in south-east Queensland, is frequently confused with C.
tenue, due to their similar shape and appearance. However they can be
distinguished by the lack of a third lateral lobe, whoch is present in C. tenue, occurring posterior to the head
and anterior to the gills. Additionally, in C. tenue, the purple border
around the mantle has a ‘dashed’ line appearance, whereas in C. trilobatum it is
solid.
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Figure 5 |
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Figure 6 |
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Ecology |
Habitat | |
The species is found in a variety of habitats and depths. Around North Stradbroke Island, the population appears most abundant at Amity Point, a shallow(<8m), sandy bottomed coastal gradient with patches of rocks and artificial substrates such as jetty pilings and debris. Individuals have also been noted at the island's north eastern offshore rocky reefs, at approximately 16m in depth. At Heron Island, a few individuals were found at a subtidal region with patches of course sand, and plentiful living coral, between 14-22m in depth (Marshall & Willan). Habitat choice is likely driven by location of prey species.
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Feeding Ecology | |
C. trilobatum, like many cryptobranch dorid nudibranchs, are carnivorous and eat sponges, most likely the siliceous variety (Marshall & Willan). They are likely to prey on a limited number of species and unless
dislodged by an outside force, individuals likely remain in locations where
preferred prey is found (Willan & Coleman). The photographic evidence
provided for C. trilobatum in eastern
Australia, as most are found on or nearby sponges, supports this.
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Predation | |
Not many animals appear to
target nudibranch species as a prey item (Marshall & Willan), however they
have been observed being consumed by fish. It is possible that there is a heavy
predation pressure on Ceratosoma nudibranchs
is some regions, as many specimens, including some C. trilobatum, have been found missing the ‘lure’ like acid gland
protuberance (Rudman).
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Defenses | |
The only apparent defence in
this species is it’s toxic ‘lure’ and accompanying aposomatic colouration. The
term ‘lure’ refers to the high, curved dorsal protuberance present behind the
gills, that acts as a physical shield and terminates in a concentrated acid
gland (Behrens). These acids are produced using chemicals found in the animals
prey sponges, which are extracted and modified and moved to the storage area
without damaging the nudibranch. By advertising this high appendage, would-be
fish predators will attack it first, resulting in minimal losses for the
individual. Additionally, the acids likely have an unpleasant taste and may
directly damage the predator’s mouth, resulting in avoidance of the species and
similarly coloured individuals. The success of this defence can be noted
in healthy individuals collected missing this protuberance (Rudman).
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Figure 7 |
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Life History and Behaviour |
Reproduction | |
Unfortunately, no reproduction was observed during this study.
Nudibranchs are hermaphroditic, and have fully functioning organs of both sexes, however they do not self fertilise. Mating behavior consists of two individuals lining up right side to right side, or 'top to toe', and both sets of organs are utilised. Both individuals will then lay spawn, which in C. trilobatum is orange (Marshall & Willan).
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Larval Stage | |
Specific larval information for this species is not readily available. As in all nudibranchs, there is likely a planktotrophic larval stage, before settlement occurs when appropriate prey is located.
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Locomotion | |
Locomotion is achieved as in other dorid species, by muscular waves through the foot. As a larger individual, it is unlikely that cilia are used much, if at all, in movement (Behrens). This species has been observed in captivity to leave a slime trail behind on the tank glass (community observation), so mucus secreted by the foot likely assists in forward movement. A form of 'rearing' was also observed in the field.
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Anatomy and Physiology |
Internal Anatomy | |
As is common amongst most
gastropods, there is distinct cephalisation resulting in a clear anterior head.
A basic eye, likely only capable of sensing changes in light, direct
observations to the brain via an unrefined optic nerve (Behrens). Rhinophores
with a lamellate (plated) structure are used for detecting chemicals dissolved
in the environmental medium, comparable to terrestrial smell, and for sensing
vibrations. These can be retracted when stressed or harassed. The mantle in
this species is reduced, with the complete loss of a mantle cavity and the
‘firmness’ experienced to the touch is attributed to a thickening of the body
wall (Rudman). All mantle glands are concentrated to the dorsal protuberance
(Rudman; Behrens). Reproductive organs are located on the right side, due to
partial detorsion common in many nudibranchs.
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Evolution and Systematics | |
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Biogeographic Distribution | |
The species global distribution
is wide, and likely incomplete. Populations seem to occur throughout the
northern sections of Australia, with sightings from Byron Bay in New South
Wales in the east, to Ningaloo Reef in the west. It is possible that they
prefer coastal habitats, as the species is noted as 'rare' on Heron Island
(Marshall & Willan). Individuals have been noted at the following foreign
locations:
· East
China Sea
· New
Caladonia
· Phillipines
· Red Sea
· Seychelles
· Tanzania
· United
Arab Emerites
· Zanzibar
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Conservation and Threats | |
No direct studies looking at
threats to this species have been done, and it has not been examined under the
criteria for the IUCN Red List of Threatened Species. On a broader scale,
threats to their prey species, sponges, could filter down into the population
of C. trilobatum. This would be
exacerbated by the specialised nature of nudibranch diets, where most restrict
themselves to a particular species(Willan & Coleman). Many sponge species rely on symbiotic
relationships with microbes, which may suffer detrimental effects under
increased global sea temperatures. Additionally, as extremely active water
filtration organisms, sponges may be affected by the increasing mass of
microplastic being found in our oceans. Reduction in fitness and biomass of
sponges fed upon by C. trilobatum,
particularly due to the large resources needed for reproduction1, could
result in a mirrored effect in the population.
1It is noted in Wallin & Coleman that nudibranchs kept in captivity will, over time, reduce in size due to the large amount of resources devoted to reproduction.
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References |
Bibliography | |
"A sponge - nudibranch
interaction", 1985, Toxicon, vol. 23, no. 1, pp. 33-33.
Behrens, D.W., Petrinos, C. &
Schrurs, C. 2005, Nudibranch behavior, New World Publications, Jacksonville,
Fla.
Karuso, P. & Scheuer, P.J.
2002, "Natural Products from Three Nudibranchs: Nembrotha kubaryana,
Hypselodoris infucata and Chromodoris petechialis", Molecules, vol. 7, no.
1, pp. 1-6.
Marshall, J.G. & Willan, R.C.
1999, Nudibranchs of Heron Island, Great Barrier Reef: a survey of the
Opisthobranchia (sea slugs) of Heron and Wistari Reefs, Backhuys, Leiden.
Rudman W.B. (1984) The
Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review
of the genera. Zoological Journal of the Linnean Society 81 (2/3): 115-27
Willan, R.C. & Coleman, N.
1984, Nudibranchs of Australasia, Australasian Marine Photographic Index,
Sydney.
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Acknowledgements | |
This compilation benefited greatly from the assistance of the greater Brisbane and Gold Coast diving community, including "Scuba Buddies Brisbane" members. Many thanks must go to Peter Grigg, Anna Toma and Loren Mariani for their generous photo contributions. Special thanks must go to Ryan Goldfinch for his help, time and patience in answering my questions.
Additional thanks go to Manta Lodge and Scuba Center at Point Lookout,
for their help on the initial dive where I first found my study species.
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