An Introduction to the Invertebrates, Second Edition - tiera.ru

WHAT IS THE ROLE OF CRUSTACEAN LARVAE?179three pairs of appendages, the smallest number that can filter-feed.Even within a directly developing egg, a pause at a recognisablenauplius stage can often be detected.Copepods hatch as a nauplius, followed by a metanauplius withfour pairs of appendages, and then a series of stagesprogressively more like the adult. Metamorphosis is verygradual, with adults remaining in the larval habitat. Theyretain some larval characters, for example there is no carapaceand only a simple ‘nauplius’ eye.Cirripedes have a far-reaching metamorphosis. After thenauplius, distinctive with ‘horns’ (Figure 13.7a), and metanaupliusthere is a ‘cypris’ larva (Figure 13.7b) that does not feedbut selects the adult habitat, seeking a rock where a protein‘message’ has been left by an earlier barnacle occupant.In a sessile animal, larvae are particularly important for dispersaland habitat selection.Malacostraca nearly all hatch beyond the nauplius stage.Freshwater malacostracans have direct development, marineones often hatch as a zoea (Figure 13.7c) followed by a larvamore like the adult. For example, the crab zoea is followedby a megalopa (Figure 13.7d), a small crab with the abdomenextended instead of being tucked underneath.Fig.13.7 Some crustacean larvae: (a) cirripede (barnacle) nauplius; (b) cirripede cypris,which does not feed but searches for a site and settles; (c) crab zoea, the first larval stage;(d) crab megalopa, a later larval stage, with abdomen still extended.