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

LARVAE235Why have a larva?Early hatching can be an economy if it saves the adult from providing largequantities of yolk. Larger numbers of young can be produced and the reproductiveturnover will be faster, but most larvae are fragile and very many will beeaten or die. Larvae often exploit a food source different from the adult’s, andmay colonise a different habitat, thus avoiding competition between adult andyoung.Theymayboth dispersethe species and find a suitablehabitatfor the adult.Which invertebrates have larvae?All phyla have examples of life histories including larvae (see figures in therelevantchapters).In general:Marine invertebrates.The marine plankton is rich in food, readily dispersesanimals, and small floating forms do not require special adaptations.In annelids and crustaceans, for example, marine animals nearly all havelarvae, while freshwater and terrestrial animals do not.Sessile animals. Slow-moving bottom-living animals almost always uselarvae for dispersal. Examples include sponges, hydroids, anemonesand corals, marine molluscs other than cephalopods, barnacles andechinoderms.Parasites. Larvae are important agents of transfer between parasite hosts.Often there is an elaborate life cycle with a succession of hosts, commonlywith one preying upon another, and the volume of reproduction maybe increased by asexual reproduction of some larval stages. Figure 6.4gives examples of two platyhelminth life cycles.Arthropods need to moult as they grow and therefore readily incorporatemetamorphosisinthe life cycle.Larvae can be very unlike the adults.What can larvae tell us about phylogeny?The answer has to be ‘very little’. Larvae are adapted to their environment.However, larvae are the products of early hatching, and therefore may showin their structure the consequences of earlier embryonic development, suchas the method of cleavage and of mesoderm formation, which do have phylogeneticsignificance. Many protostomes, for example, hatch as larvae ratherlike trochophores: this does not necessarily relate them closely to annelids orto molluscs, but it does distinguish them from echinoderms and chordates.Occasionally aberrant forms (such as Sacculina: see Chapter 13) can be placedby larval resemblances, but more often such resemblances must be attributedto convergence. Whether larvae are primitive, to invertebrates as a wholeorin particular phyla, is uncertain; thisis discussed forechinodermsin Chapter17.This brief survey of larvae is very far from complete, even concerning invertebratesmentioned in this book, omitting for example the sessile scyphistomalarva of many jellyfish (Chapter 4) and the great range of aquatic and terrestriallarvae of insects (Chapter15), but it provides anintroductory overview.