Links between the timing of life-history transitions and dietary and morphological variation during early life history in the sand goby, Pomatoschistus minutus

Abstract

Bipartite life histories involve a suite of morphological changes that support the pelagic to demersal transition and an expanded range of prey options and microhabitats. Pelagic individuals are thought to shift (settle) to their preferred benthic habitat at the earliest opportunity once they have attained a minimum level of morphological competency to access their new environment. In theory, early changes in larval morphology (collectively termed ‘metamorphosis’), habitat and diet—a measure of habitat-use—ought to be synchronous. Yet relationships may be decoupled by factors linked to behaviour, prey availability or morphological complexity, and few descriptions exist to allow such synchrony to be assessed. The sand goby, Pomatoschistus minutus, is a common coastal fish across north-western Europe, with a size at larval metamorphosis and settlement of around 10 and 16–18 mm standard length (SL), respectively. We sampled shoreline larval and juvenile populations to examine relationships between morphology, diet and life stage. Prey diversity increased with body length; however, dietary change was clearest at 16–18 mm SL, with a reduction in calanoid copepods and shift to larger prey such as Nereis polychaetes and mysid and amphipod crustacea. Early growth in five prey capture and processing morphologies was rapid. Four of these showed a subsequent marked shift to slower growth, but none of these changes were aligned with size at metamorphosis and only that of mouth width coincided with body size at settlement. Early life history in P. minutus appears geared towards a protracted morphological reorganization prior to demersal life and an alternative suite of prey resources. Larval metamorphosis seems to be of limited consequence in this regard. Comparable studies of other Baltic Sea fishes would confirm whether these dynamics relate to shared environmental pressures or to factors intrinsic to P. minutus biology.