Amanda C. Hay

Ontogeny of behaviour relevant to dispersal and connectivity in the larvae of two non-reef demersal, tropical fish species

In demersal marine fishes, the dispersal of larvae determines the geographical scale of population connectivity, and larval behaviour may influence dispersal. Yet, little is known of the ontogeny of behaviours that can influence dispersal. The present study examined the development of these behaviours in pelagic larvae of tropical marine fishes (4–21 mm) that occupy non-reef habitats as adults: Eleutheronema tetradactylum (Polynemidae) and Leiognathus equulus (Leiognathidae). In the laboratory, critical speed (Ucrit) increased from 3 to 34 cm s–1 at 1.3–1.7 cm s–1 per mm of size, with the fastest larvae up to 50% faster. In situ speed increased from 4 to 25 cm s–1 at 0.7–2.2 cm s–1 per mm, and was 10–14 body length s–1 (60–90% of Ucrit). Endurance increased from 0 to >40 km at 2.4–4.7 km per mm. In the sea, orientation precision did not change ontogenetically, both species tended to swim in loops, and neither significant overall directionality nor ontogenetic change in orientation was present. Larval orientation of these non-reef species was less precise than that of reef fishes. The two species differed in depth distribution, and one ascended ontogenetically. These behaviours can potentially influence dispersal outcomes over the full size range of these larvae.

A Coral-reef Fish with Large, Fast, Conspicuous Larvae and Small, Cryptic Adults (Teleostei: Apogonidae)

Settlement-stage larvae of the widespread apogonid Gymnapogon urospilotus are described and illustrated from specimens captured with crest nets at Moorea, Society Islands. Identification was confirmed by COI mitochondrial DNA analysis. Otoliths of the larvae had no settlement mark, indicating they had not previously settled, and 33–40 presumed daily rings. The larvae are transparent with large orange/red blotches on the tail, and limited melanophores on head and pelvic fins. The larvae have very long pelvic-fin rays (40–50% standard length [SL]) with black tips and contrasting white bands, and limited inter-ray membranes. They are apparently unique among teleost fishes in having a strongly serrate urohyal. These serrations disappear after settlement. Critical speed of the larvae was measured in a swimming chamber. Larvae of G. urospilotus are the largest (19–24 mm SL), oldest (33–40 days), and fastest (>36 cm s−1) known apogonids at settlement. Photos of larvae of G. urospilotus from Guam, Ryukyu Islands, and Indonesia reveal they swim 2–3 m above coral reefs at night with the pelvic rays spread wide, presumably looking for settlement habitat. They are very conspicuous and may be mimicking small lionfish. In contrast, adults have never been seen alive on the reef. The largest reported adult G. urospilotus is 37 mm SL, which implies that this species undergoes a major portion of its growth as a pelagic larva, a life history pattern that also occurs in some reef gobiids.

Larval development of Achoerodus viridis (Pisces: Labridae), the Australian eastern blue groper

Larvae of the southeastern Australian endemic hypsigenyin labrid Achoerodus viridis are described and illustrated from 30 specimens (3.0–9.6 mm) captured in plankton nets and larval fish beach seines. Development is typical for labrids. Larvae of A. viridis can be distinguished from those of other labrids because they possess distinctive pigment, 28 myomeres, and fin ray counts of D XI,11, A III,11, and P1 16–18. Larvae of closely related hypsigenyin labrid genera are poorly known but are similar in morphology and pigmentation. Adult A. viridis live on coastal rocky reefs. Larvae of 3.0–8.2 mm were captured over the continental shelf off central New South Wales in the Western Tasman Sea. Larvae of 6.6–7.7 mm were captured in a tidal channel leading to an estuarine lagoon, and the smallest larvae captured in seagrass beds in the lagoon were 7.2 and 8.5 mm.