Larval feeding habits of Diaphus theta, Protomyctophum thompsoni, and Tarletonbeania taylori (Pisces: Myctophidae) in the transition region of the western North Pacific

Abstract

We examined the larval feeding habits of the 3 dominant myctophids, Diaphus theta, Protomyctophum thompsoni, and Tarletonbeania taylori, in the transition region of the western North Pacific. Feeding incidence for D. theta and T. taylori larvae was higher during the day than at night (mean: 70.7 to 84.4% vs. 2.2 to 4.1%), indicating that they are daytime visual feeders. Larvae of P. thompsoni were also visual feeders, with a daytime feeding incidence of 83.0%. The proportion of nighttime feeding in P. thompsoni gradually increased with development, which suggests faster lar- val adaptation of this non-migrant species to the poor light conditions in the mesopelagic zone. D. theta larvae ≤7.9 mm in body length (BL) fed mainly on copepod nauplii, which were replaced by calanoid copepodites and Oithona spp. in larvae ≥8.0 mm BL. The most important prey for smaller P. thompsoni and T. taylori larvae ≤7.9 mm BL were nauplii and Oithona spp., which were replaced by calanoid copepodites and ostracods in P. thompsoni and copepod eggs in T. taylori with increased development. Competition for prey among the larvae of these 3 myctophids was mostly avoided because of diet and habitat-depth segregation. Interspecific comparisons of feeding habits among the 3 species, in addition to larvae of 3 other myctophids found in the study area, suggested that their feeding strategies were associated with mouth size, i.e. larvae with larger mouths tended to take larger prey rather than a large number of smaller prey. This interspecific difference in feeding strategy is reflected in the diverse morphological characters of myctophid fish larvae.