Development of eggs and larvae of Emmelichthys nitidus (Percoidei: Emmelichthyidae) in south-eastern Australia, including a temperature-dependent egg incubation model

Reared eggs and field-collected material were employed to describe the development of the pelagic eggs and larvae of Emmelichthys nitidus (Emmelichthyidae), a small (36 cm TL) mid-water schooling species common in shelf waters of temperate Australia. Hydrated oocytes from adults trawled from spawning grounds off eastern Tasmania were fertilized and reared to the yolk-sac larval stage, and the data employed to build a temperature-dependent egg incubation model. Embryogenesis lasted 96, 84 and 54 h at mean temperatures of 13.1, 14.4 and 16.5 C respectively, and was divided into seven stages based on extent of epiboly until blastopore closure (stages I–III) and embryo growth (stages IV–VII). Eggs (1.00– 1.05 mm diameter) are spherical with a smooth chorion, small perivitelline space and prominent, unsegmented yolk with a single, posteriorly-located oil globule (0.18–0.20 mm diameter) that becomes pigmented from stage III. Embryos have two distinct snout melanophores, and a paired melanophore row laterally along the trunk and tail. Morphological identification of eggs collected during surveys in October 2005 and 2006 was validated using quantitative PCR amplification of the mtDNA d-loop gene region unique to E. nitidus, producing an 80–100% agreement across all seven stages. Newly-emerged larvae (1.9–3.3 mm) possess a prominent yolk sac with the posteriorly-located, pigmented oil globule, mouth not yet functional and unpigmented eyes. Notochord flexion occurs between 5 and 8 mm while fins are formed by 12 mm. Larvae examined (3.3–17.4 mm) are lightly pigmented and possess percoid features such as an elongate to moderate body, coiled, triangular-shaped gut, preopercular spines and 24–25 myomeres; two prominent pigment patches opposite each other dorsally and ventrally along the tail are diagnostic. Variability of mean egg ages (y) by temperature (t) and stage (i) was best described by the deterministic stage-to-age model y ¼ 35.911exp[ (0.155t þ 0.262i)]i2.436. Developmental changes and model outputs paralleled those reported for laboratory-reared eggs of known clupeoids and scombrids, whereby hatching time and transition periods between stages decrease with increasing temperatures. The suitability of the incubation model to assign ages to staged field-caught eggs of E. nitidus is discussed in terms of its application to estimate spawning biomass using the daily egg production method.