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BIOLOGY OF

REEF FISHES

The region’s tremendous diversity of inshore fishes is reflected in a wide variety of reproductive habits and life history strategies. The following discussion is intended to give an overview of the most common patterns. More detailed information is available in the scientific literature or semi-popular works such as Thresher’s (1984) Reproduction in Reef Fishes. The majority of reef fishes are egg layers that employ external fertilisation. Relatively few species bear live young that are prepared to fend for themselves at birth. Included in the latter category are sharks, rays, and cusk eels. Basically two patterns of oviparous or egg-laying reproduction is 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)