SIZE OF FLATFISH LARVAE AT TRANSFORMATION, FUNCTIONAL DEMANDS AND HISTORICAL CONSTRAINTS

At hatching the larvae of flatfish closely resemble the bilateral symm etric larvae of other teleosts, especially perciforms. Literature data show that transformation to asymmetric benthic juveniles normally occ urs at body lengths between 10 and 25 mm. Unexpectedly, minimal size a t its completion (including eye migration) can be 4.1 mm SL and maxima l size is over 72 mm. In this paper we consider the functional require ments for a successful switch from a symmetric pelagic larva to a typi cal asymmetric juvenile benthic flatfish pal-try based on evidence fro m other teleosts. The unfavourable period of eye migration and transit ion to a benthic habitat requires some food reserves and rewiring and/ or recalibration of vision and gravity-associated structures utilised previously by the still symmetric larvae for e.g. food detection. Bino cular fixation of the prey probably occurs in that stage. Critical or sensitive periods occurring during development of fish larvae suggest that a completely functional symmetric stage of development must prece de transformation. The normal size range in flatfish larvae at transfo rmation seems to confirm our considerations. Recent data on temperatur e effects during development provide an explanation for metamorphosis at the minimal size. Some evidence for paedomorphic heterochrony in fl atfish larvae is presented.