The complex structure of the GBR gives an impression of stability and permanence. The tracts of coastal fringing reefs, mid-shelf platform reefs and ribbon reefs of the outer barrier provide a parallel series of distinctive habitats extending along a north-south axis for 1000 km. The different habitats support distinctive fish communities. At a number of localities along the reef the habitats intergrade, especially in the vicinity of large coastal islands (Fig. 28.1). At these localities, elements of the inshore and mid-shelf reef assemblages may mix.
The geological history of the GBR shows that this structural and biological partitioning of the reef has
Figure 28.7 Mimicry in GBR Fishes. A, noxious tetradontid Canthigaster valentini; B, monocanthid Paraleuteres prionurus that mimics C. valentini; C, poison fang blenny Meiacanthus lineatus; D, E, mimics of M. lineatus, Scolopsis bilineatus (D) and Petroscirtes fallax (E). (Photos: A, B, D, Gerry Allen; C, E, Roger Steene.)
occurred very recently, and that sea level changes associated with glacial cycles have profoundly modified Australian shallow water reef systems. For the last 130 ka sea levels have fallen on a global basis from levels that were similar to those of the present day to a low of 140 m below present levels only 20 ka. The biogeo-graphical consequences of this were profound, with isolation of the north-east Australian coast from the west due to the closure of the Torres Strait from 116 ka to 30 ka, and then by the emergence of the exposed coasts of Papua New Guinea to the north and east. Rapid sea level rises linked to the termination of the last glacial period 19 ka ago flooded continental shelves so that reef habitats increased from 50 000 to 225 000 km2 at present. Thus, over the last 6000 years the north-eastern Australian coast has been transformed from a shelf habitat dominated by fringing reefs to one supporting the extensive and ecologically diverse system of the present day GBR. One consequence of the recent emergence of a new reef system is that the majority of the fish fauna must be the product of a rapid colonisation process from external sources. Where did the present day fish fauna come from and are there any evolutionary and ecological signatures of this colonisation process?
To explore this question we will consider two of the most prominent members of the reef fauna, the bar cheeked coral trout (Plectropomus maculatus) characteristic of inshore reefs and the leopard coral trout (Plec-tropomus leopardus) common on mid-shelf reefs. The geographical source of these populations and their recent evolutionary history was investigated by examining and comparing sequences of molecules amplified from mitochondrial genes. Such sequences are passed on through female parents and as they are inherited they provide a method for disentangling patterns of evolutionary descent in the sampled population. They also provide a species identification code and allow assessment of the degree of relatedness among groups of species.
An analysis of mitochondrial sequences of GBR Plec-tropomus species was carried out to establish the pattern and degree of relatedness of three species. These were P. leopardus, P. maculatus and P. laevis that are very similar in appearance and colour pattern (Fig. 28.8). In addition, the historical biogeography of the most abundant species, P. leopardus, which occurs on both the eastern and western tropical coasts of Australia, was carried out. The distribution of this species is of considerable
BOX 28.3 DISGUISE, DEFENCE AND AGGRESSION
Mimicry is a special kind of resemblance that involves co-evolution of colour and morphology, and even behaviour to enhance the deception. Mimicry among coral reef fishes, once thought to be rare, appears to be a general and widespread phenomenon, with about 100 cases now reported. Many of the known cases of interspecific mimicry in fishes involve one or more species of the family Blenniidae. Mimicry also appears to be particularly important during juvenile stages, with more than 25% of mimic species losing their mimic colouration when they outgrow their models and become less vulnerable to predation.
Most of the cases of interspecific mimicry reported so far can be classified as Batesian, Mullerian or Aggressive mimicry. Batesian mimicry is the resemblance of a harmless or palatable species to a harmful or unpalatable one. An example of Batesian mimicry among GBR fishes is that between the noxious Cathigaster valentini (Fig. 28.7A) and the triggerfish Paraleuteres prionurus (Fig. 28.7B). In Mullerian mimicry both species possess some undesirable qualities. This type of mimicry appears to be rare among fishes although it may contribute to the mimetic complexes involving members of the blenniid tribe Nemophini. Aggressive mimicry is the resemblance of a predatory species to a harmless or non-predatory form. Aggressive mimicry is the most prevalent type of mimicry in coral reef fishes, constituting about half of all known cases. An example among GBR fishes is the aggressive mimic blenny Aspidontus taeniatus, which closely resembles the colour and behaviour of the cleaner wrasse Labroides dimidiatus, and uses this deceit to closely approach and bite pieces from unsuspecting prey fishes.
In some cases, where two or more species of fishes are involved in a mimetic complex, elements of all three types of mimicry may be present. The spatial distribution of mimics also appears to be limited by that of their model species, although some mimic different models or different colour morphs of the same model in different habitats or in different parts of their range. For example, the juvenile coral bream Scolopsis bilineatus (Fig. 28.7D) and the blenny Petroscirtes fallax (Fig. 28.7E) both mimic the yellow and black-striped poison-fang blenny Meiacanthus lineatus (Fig. 28.7C) on the GBR, but in Fiji where an all yellow morph of M. lineatus occurs, it is mimicked by an unusual yellow colour form of the coral bream and also by a yellow form of the aggressive sabretooth blenny Plagiotremus laudandus, which preys on the soft tissue of other fishes. These examples suggest a high degree of phenotypic plasticity in mimetic colouration and little genetic differentiation among different mimics of the same species. For further reading see:
Randall, J. E. (2005). A review of mimicry in marine fishes. Zoological Studies 44(3), 299-328.
Moland, E., Eagle, J. V., and Jones, G. P. (2005). Ecology and evolution of mimicry in coral reef fishes. Oceanography and Marine Biology: An Annual Review 43, 455-482.
interest as it occurs on the Australian tectonic plate, the western and southern Pacific (Papua New Guinea, Solomons, New Caledonia) and the Indo-Philippine archipelagos, Taiwan and southern Japan. It achieves high abundances on the Australian plate, including New Caledonia, but abundances decline through the reef at lower latitudes, then increase in higher northern latitudes, which suggests an anti-tropical distribution.
Analysis of the geographical structure of P. leopardus populations includes some surprises. Eastern and western Australian populations are distinct, a reflection of the long period of closure of the Torres Strait and the sparse reef environment in the present day Arafura Sea. In fact, the closest relatives of the west Australian population appear to be from Taiwan. The GBR populations had their strongest affinities with New Caledonian fish, and the analysis of larval migration patterns strongly suggests an east to west gene flow. This provides a key to the question of the rapid colonisation of the GBR reef over the last 6-7 ka. During the period of low sea level stands the Coral Sea was characterised by large shallow areas of actively growing reefs—the Queensland and Marion plateaus. These are now inundated and well below the level of active reef growth and are now represented by only scattered groups of Coral Sea reefs and islands. However, during periods of low sea level, driven by glaciation cycles over the last 500 ka, these reefs and those of New Caledonia further to the east, would have served as recruitment sources when rising sea levels provided the opportunity for recolonisation of the GBR.
One of the most important messages is that the process of colonisation of the newly forming GBR and the partitioning of species into different habitats happened over a very short period. Are there any genetic signatures of these events? Genetic analysis of the relationships among species of Plectropomus shows that P. leopardus, P. maculatus and P. laevis are closely related, something that is reflected in their colour patterns. On the west coast of Australia P. leopardus and P. maculatus form distinct monophyletic groups or clades. However, on the east coast P maculatus has the same mitochondrial genetic signature or haplotype as P. leopardus. Using mi-tochondrial genes, Plectropomus maculatus from the GBR cannot be distinguished from P. leopardus, which is in striking contrast to the pattern observed in Western Australia. The most parsimonious explanation for these distinctive coastal patterns is that on the east coast the species have a history of hybridisation, with male P. maculatus joining spawning groups of P. leopardus (Fig. 28.8; Box 28.4). The opportunity for the mixing and overlap of the species populations on the east coast was greatly enhanced by the turbulent history of the reef, with very rapid episodes of colonisation of new reef structures and sorting among the newly formed habitats. In contrast, the coast of Western Australia has had a far more stable history, with limited effects of sea level change and with populations of each species clearly partitioned between coastal and oceanic offshore reefs.
The distinctive genetic structures of P. leopardus and P. maculatus populations on the GBR, with evidence of past episodes of hybridisation, are in effect a signature of a distinctive geological and evolutionary history of this reef system. It is highly probable that other species will show similar evidence of a reef system subject to rapid structural and biological change.
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