Megalopae of several crab species exhibit active habitat selection when settling. These megalopae usually select structurally complex habitats which can provide refuge and food. The portunid mud crab, Scylla serrata, is commonly found within the muddy estuaries of the Indo-West Pacific after attaining a carapace width > 40 mm. Despite substantial efforts, the recruitment mechanism of juvenile mud crabs to estuaries is not understood because their megalopae and early stage crablets (carapace width < 30 mm) are rarely found. We used laboratory experiments to determine whether megalopae and early stage crablets are selective among three estuarine habitats which commonly occur in Queensland, Australia. These animals were placed in arenas where they had a choice of habitats: seagrass, mud or sand, and arenas where they had no choice. Contrary to the associations exhibited by other portunid crab megalopae, S. serrata megalopae were not selective among these estuarine habitats, suggesting that they tend not to encounter these habitats, or, gain no advantage by selecting one over the others. The crablets, however, strongly selected seagrass, suggesting that residing within seagrass is beneficial to the crablets and likely increases survival. This supports the model that for S. serrata, crablets and not megalopae tend to colonise estuaries, since a selective behaviour has evolved within crablets but not megalopae. Organisms are constantly faced with choices, for example choices about feeding, habitat, and breeding. It is a tenet of evolutionary theory that, collectively, these choices or selections serve to enhance the fitness of future generations (Gould and Lewontin 1979; Krebs and Davies 1997). The postlarvae of several decapod species actively choose to settle to habitats that provide refuge or abundant food, or move into these beneficial habitats shortly after settling (
Artificial-reef units (rolls of plastic garden mesh) attached to subsurface
floats were used to study settlement behaviour of larval reef fishes. These
units were located 3, 5, 7 and 9 m above the bottom in water 15–19 m
deep in the Great Barrier Reef Lagoon, 1 km from natural reefs. Larvae of 50
species (15 families) settled on these units. The nine most abundant reef-fish
taxa were in the families Apogonidae, Blenniidae, Gobiidae, Monacanthidae,
Pomacentridae and Tetraodontidae. The less abundant of these taxa
(n = 4) settled uniformly. The more abundant taxa
(n = 5) had clumped settlement. Four taxa
preferred structurally complex reef units, whereas five showed no preference.
Apogonids, gobiids, tetraodontids and a pomacentrid preferred deep units, one
pomacentrid had no depth preference, and a blenniid and a monacanthid
preferred shallow units. Experiments evaluated visual, olfactory and auditory
cues that reef-fish larvae may use to locate and settle onto reefs. Visual
cues (large white panels) did not enhance settlement. Experiments on olfactory
cues (corals in vented containers) and auditory cues (‘the nocturnal
chorus’ of tropical reefs) were compromised by low and highly variable
settlement, but show the potential of the method. The advantages of subsurface
moorings for study of settlement behaviour are discussed.
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