Dietary composition of myctophid larvae off Western Australia

Abstract

Mesopelagic fishes of the southeastern Indian Ocean are represented by a high number of lanternfish species (Myctophidae). The horizontal distribution patterns of their larvae have been previously studied; however, research on larval feeding habits is non-existent. The goal of this study was to describe the diet and trophic-based strategies of the most abundant myctophid larvae off Western Australia through stomach content analysis. Samples were collected in autumn 2007 off southwestern Australia when this oligotrophic area was under the influence of the seasonal Leeuwin Current, and in autumn 2010 on the tropical continental shelf of northwestern Australia. The species investigated during these periods were Benthosema fibulatum, B. pterotum, B. suborbitale, Dasyscopelus asper, Diaphus “deep” and “slender” morphotypes, Hygophum hygomii, Lampadena luminosa, Lampanyctus alatus, Lampanyctus spp. and Myctophum sp. Generally, myctophid larvae showed a diurnal feeding pattern. Only postflexion stages in L. alatus, L. luminosa, B. pterotum, Diaphus “slender”, and Myctophum sp. preyed at dawn or during the night. The highest feeding incidence was found in L. luminosa and the Diaphus “deep” morphotype, while the lowest, in H. hygomii. Generally, larvae were zooplankton feeders with nauplii and copepodites being the most abundant dietary groups, except for B. suborbitale and Myctophum sp. where the small non-calanoid genus Oncaea was dominant. Soft organisms such as appendicularians were frequent items in most fish diets throughout their entire larval phase, and highly abundant in Diaphus “deep” morphotype. The wide prey-size spectrum indicates that the majority of the fish species were not size-specific feeders for most of their larval life. Nevertheless, B. suborbitale, B. fibulatum, and L. alatus preferred larger prey, ignoring the smaller items as they grew. Larval feeding patterns of similar genera from other ecosystems are discussed and compared with those in Western Australia.