Feeding pattern and the visual light environment in myctophid fish larvae

Abstract

The trophic spectrum and feeding pattern of two myctophid larvae, Benthosema glaciale and Myctophum punctatum, were analysed in relation to changes in daily light intensity. The larvae of both species live relatively deep, occurring in the first 100 m, though distribution of M. punctatum extends to 150 m depth. The present study, carried out in the western Mediterranean (Alboran Sea), indicated that the larvae of the two species exhibit different foraging strategies. Both started to feed at dawn, but while feeding of M. punctatum was high at dawn and dusk, the feeding of B. glaciale remained high throughout the day. The light intensity profiles taken during the day indicated that at the depths where the species dwelt, light intensity was enough to provoke a feeding response. The larvae of these species, in contrast to the majority of fish larvae, had an enhanced sensitivity due to their pure rod‐like retina, an adaptation for foraging at low light intensities. Both species showed an ontogenetic change in their diet: B. glaciale preflexion larvae fed mainly on copepod eggs and nauplii, while postflexion larvae consumed calanoid copepodites; M. punctatum larvae showed a more diversified diet, composed of larger prey items. The stalked and elongated eye of M. punctatum larvae would enable the detection of a greater range of prey in terms of shape and size. In addition, the retina of this species was characterized by a higher summation ratio and longer photoreceptors, indicating a preference for dimmer environments. This could explain the decreasing feeding activity of M. punctatum during the high light intensity of the middle daylight hours. As a clear relationship existed between feeding pattern and light intensity in these myctophid larvae, the visual characteristics of each species could help to explain the different strategies of foraging behaviour, therefore avoiding a possible overlap in their trophic niche.