Marine Fishes of South-East Asia. Gerry Allen
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evident in most reef species. Females of many fishes, including the highly visible wrasses and parrotfishes, scatter relatively large numbers of small, positively buoyant eggs into open water where they are summarily fertilised by the male. The spawning event is typically preceded by nuptial chasing, temporary colour changes, and courtship display in which fins are erected. This behaviour is generally concentrated into a short period, often at sundown or shortly afterwards. This pattern is seen in diverse groups such as lizardfishes, angelfishes, wrasses, parrotfishes, and boxfishes. Typically either pair or group spawning occurs in which the participants make a rapid dash towards the surface, releasing their gonadal products at the apex of the ascent.
The fertilised eggs float near the surface and are dispersed by waves, winds, and currents. Hatching occurs within a few days and the young larvae are similarly at the mercy of the elements. Recent studies of the daily growth rings found on the ear bones (otoliths) of reef fishes indicate that the larval stage generally varies from about 1-8 weeks depending on the species involved. The extended larval period no doubt accounts for the wide dispersal of many reef species. For example, many fishes that occur in our region have geographic ranges that extend from East Africa to Polynesia.
A second reproductive pattern involves species that lay their eggs on the bottom, frequently in rocky crevices, empty shells, sandy depressions, or on the surface of invertebrates such as sponges, corals, or gorgonians. Among the best known fishes in this category are the damselfishes, gobies, and triggerfishes. These fishes often prepare the surface prior to egg deposition by cleaning away detritus and algal growth. Bottom spawners also exhibit elaborate courtship rituals which involve much aggressive chasing and displaying. This behaviour has probably been best studied amongst the damselfishes. In addition, one or both parents may exhibit a certain degree of nest-guarding behaviour in which the eggs are kept free of debris and guarded from potential egg feeders such as wrasses and butterflyfishes. A very specialised mode of parental care is seen in cardinalfishes, in which the male broods the egg mass in its mouth. Similarly, male pipefishes and seahorses brood their eggs on a highly vascularised region of the belly or underside of the tail. As a rule the eggs of benthic nesting fishes are more numerous, larger, have a longer incubation period, and are at a more advanced developmental stage when hatched, compared to the eggs and larvae of pelagic spawning fishes. Hatching may require up to one week (in anemonefishes for example) and the larvae then lead a pelagic existence for up to several weeks before settling on the bottom in a suitable reef habitat.
There is very little information on the longevity of most reef fishes. Perhaps one of the longest life spans is that of the Lemon Shark which may reach 50 years or more. Most of the larger reef sharks probably live at least to an age of 20-30 years. In general the larger reef fishes such as gropers, snappers, and emperors tend to live longer than smaller species. Otolith aging techniques indicate that large gropers may live at least 25 years and some snappers approximately 20 years. Most of our knowledge of smaller reef fishes has resulted from aquarium studies. The values obtained from captive fishes may exceed the natural longevity due to lack of predation and the protective nature of the artificial environment. Batfishes (Platax) are known to survive for 20 years and even small species such as damselfishes and angelfishes may reach an age of 10 years or more.
Many-spotted Sweetlips and small reef fishes (mainly Anthias) are typical reef inhabitants throughout the region. (R. Steene)
ECOLOGY OF
REEF FISHES
The majority of fishes included in this book are generally considered to be inhabitants of coral reefs. However, reefs are highly complex systems, consisting of numerous microhabitats. In general, coral reef fishes are finely synchronised to their environment.
Each species exhibits very precise habitat preferences that are dictated by a combination of factors including the availability of food and shelter, and various physical parameters such as depth, water clarity, currents, and wave action. The huge number of species found on coral reefs is a direct reflection of the high number of habitat opportunities afforded by this environment.
Coral reef fishes generally exhibit a higher degree of habitat partitioning than do fishes from cooler seas. A good example of the fine scale on which this principle operates is the Urchin Clingfish (Pl. 12-6). It is usually found amongst the spines of Diadema sea urchins or nearby branching corals and feeds primarily on the tube feet of its host urchin or on coral- burrowing molluscs. The coral reef offers numerous examples of fishes that have similar narrow habitat and feeding requirements. Water depth is also an important partitioning factor, and again there are numerous examples of coral reef fishes that have well defined depth ranges. In the very broadest sense there are three main depth categories for reef fishes: shallow (0-4 m), intermediate (5-19 m), and deep (20 m+). The depth limits of these zones may locally vary depending largely on the degree of shelter and sea conditions. The shallow environment is typified by wave action which in highly protected areas such as coastal bays or lagoons may exert its effect down to only a few cm. On the contrary in exposed outer reef structures the effect of surface waves may sometimes be felt below 10 m. The intermediate zone harbours the greatest abundance of fishes and live corals. Here wave action is minimal, although currents are often strong, and sunlight is optimal for reef-building corals. The deep outer reef slope is characterised by reduced light levels, hence fewer corals and fishes. Although species numbers are reduced the species that occur in this habitat are among the most interesting of coral reef fishes. A high percentage of the new fishes that have been discovered on coral reefs in the past three decades were collected on deep reefs by SCUBA diving scientists.
The region’s reef environments can be broadly classified into two major categories: sheltered inshore reefs or lagoons and outer reefs. Under optimum conditions both of these environments can support extensive beds of nearly 100 per cent coral cover. Inshore or coastal reefs may be strongly influenced by freshwater runoff and resultant siltation. Underwater visibility on these reefs is often greatly reduced, particularly during the wet season when rivers are flowing at their maximum. Coastal reefs and lagoons are further characterised by extensive sand or silt bottom areas that may support broad seagrass beds. In most coastal reef or lagoon situations the maximum depth seldom exceeds 25 m and due to heavy siltation coral growth is usually sparse below 15 m depth.
Outer reefs often have a classical reef structure consisting of a broad shallow reef flat, a raised algal ridge, reef front zone of surge channels, and a steep outer slope. But on some islands the bottom plunges into the depths directly from the rocky shore. The clearest waters are found on outer reef slopes and underwater visibility may sometimes exceed 30 m. Coral growth is most abundant between about 5 and 15 m depth, although in some areas appreciable growth may extend well below this limit. In shallower water corals are inhibited by the pounding surge, and in deeper water by the much reduced penetration of light. Although most reef-building corals do not thrive below 30-40 m, certain reef fishes may penetrate well below these depths. Observations made in research submarines at Hawaii and Enewetak Atoll indicate that reef species, including some damselfishes, butterflyfishes, and squirrelfishes, may occur to depths approaching 200 m.
A perfect day on Australia’s Great Barrier Reef. (R. Steene)
CLASSIFICATION OF
FISHES
Although the fundamentals of biological nomenclature and classification are common knowledge to many, it is my experience that the average non-biologist frequently has little idea of the basis of scientific names or how fishes are classified. It therefore seems worthwhile to include a brief section on the rudiments of this subject.
Every described organism, be it a single-celled amoeba, crab, bird, fish or mammal has a scientific or Latin name. It is composed of two parts and is generally italicised. The first part is the genus or generic name and the second is the species or specific name. For example the Five-lined Seaperch is Lutjanus quinquelineatus. The generic name Lutjanus pertains to a group of closely related species which share a number of common features related to general