FAU Institutional Repository http://purl.fcla.edu/fau/fauir This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute. Notice: ©1999 Academic Press. This manuscript is an author version with the final publication available and may be cited as: Young, C. M. (1999). Marine invertebrate larvae. In E. Knobil & J. D. Neill (eds.), Encyclopedia of Reproduction, 3. (pp. 89-97). London, England, and San Diego, CA: Academic Press. --------1111------- Marine Invertebrate Larvae Craig M. Young Harbor Branch Oceanographic Institution 1. What Is a Larva? metamorphOSiS Morphological and physiological changes II. The Production of Larvae that occur during the transition from the larval phase to iII. Larval forms and Diversity the juvenile phase: often coincides with settlement in ben­ IV. Larval Feeding and Nutrition thic species. V. Larval Orientation, Locomotion, Dispersal, and mixed development A developmental mode that includes a Mortality brooded or encapsulated embryonic stage as well as a free­ VI. Larval Settlement and Metamorphosis swimming larval stage. VlI. Ecological and Evolutionary Significance of Larvae planktotrophic larva A feeding larva that obtains at least part VlIl. Economic and Medical Importance of Larvae of its nutritional needs from either particulate or dissolved exogenous sources. Planktotrophic larvae generally hatch from small, transparent eggs. GLOSSARY settlement The permanent transition of a larva from the plankton to the benthos. In sessile organisms, settlement atrochal larva A uniformly ciliated larva (cilia not arranged is marked by adhesion to the substratum. It is often closely in distinct bands). associated with metamorphosis and may involve habitat se­ competent larva A larva that is physiologically and morpho­ lection. logically capable of undergoing metamorphosis. trochal larva A larva with cilia arranged in distinct bands. direct development A life cycle that includes neither a distinct larval form nor a dramatic metamorphosis. In direct devel­ opment, the embryo develops into a juvenile by a series of gradual changcs. he larva of a marine invertebrate is a postem­ dispersal The process of moving away from the parents and T bryonic stage of the life cycle which differs from of spreading siblings, either by advection and diffusion of water currents or by active locomotion. the adult morphologically and which is capable of facultative planktotrophy Opportunistic feeding by a lecitho­ independent locomotion. Often the most delicate and trophic larva that is capable of completing metamorphosis vulnerable life history stage, it carries out the vital without external food. functions of dispersal and habitat selection. The over­ indirect development A life cycle which includes a larval stage whelming majority of marine benthic invertebrates, and metamorphosis. many pelagic invertebrates, and most marine fishes juvenile In indirect development, a stage of the life cycle have complex life cycles that include one or more following settlement and resembling the adult, yet not re­ larval stages. In this respect, marine life cycles super­ productively mature. In direct development, any prerepro­ ficially resemble those of many terrestrial and aquatic ductive stage resembling the adult. insects. However, the differences are many. In in­ larval ecology The study of factors influencing the distribu­ sects, volant adults are generally responsible for dis­ tion and abundance of marine larvae and of processes oc­ persal, whereas in marine animals the dispersal stage curring during the larval stage that influence the distribu­ tion and abundance of juveniles and adults. is most often the larva. Metamorphosis of most in­ lecithotrophic larva Nonfeeding larva that receives its nutri­ sects occurs slowly during the sedentary pupa stage, tional needs entirely from yolk supplies stored in the egg but in marine animals the pupa is lacking, and the during oogenesis. Lecithotrophic larvae often develop from major changes of metamorphosis are very rapid. large, opaque, yolky eggs. Metamorphosis marks a transition between habitats Etl(yclopcaia of Rep'oauCLiot! Copyright © 1999 hy Academic Press VOLUME 3 89 All rights of reproduction in any form reserved. 90 Marine Invertebrate Larvae in both aquatic insects and benthic marine inverte­ redia stage and coronate larvae of some cyclostome brates; the former emerge from water to land, bryozoans (order Tubuliporata), which are produced whereas the latter settle from plankton to benthos. by polyembryony. Although larvae are, by definition, immature stages incapable of sexual reproduction, some oceanic starfish larvae do propagate asexually. I. WHAT IS A LARVA? Sexually produced larvae may originate by ovipa­ rous, ovoviviparous, or viviparous development. In The term "larva" has been used in a variety of ways oviparous development, which is common among by marine biologists, and there is no firm consensus marine animals, gametes are shed into seawater, on the definition. Some definitions, including the where embryogenesis and larval development occur one presented in this article, incorporate ecological without any parental protection. The larvae eventu­ and behavioral attributes, whereas others use mor­ ally seek suitable adult habitats where they undergo phological criteria alone. Morphologically, the begin­ metamorphosis to the juvenile stage. This form of ning and ending points of the larval stage may be development is found in at least some species of most difficult to define within the developmental contin­ phyla, with the most notable exception being the uum and different criteria may be appropriate for phylum Arthropoda. Ovoviviparous development, in different taxa. Some definitions specify that the larval which embryos are brooded by the parent without stage ends at metamorphosis, but even this is prob­ any postovarian nutritive contribution and the larva lematic for some groups (e.g., polychaetes) which is the first free-living stage, is common in crusta­ have a gradual metamorphosis. Some workers extend ceans, sponges, and sessile clonal animals such as the definition of larva to include not only free-swim­ bryozoans, hydrozoans, and colonial ascidians. Vi­ ming stages but also brooded embryos that pass viparous development, wherein parental nutrients through their entire developmental period within, are transferred to the embryo through a direct tissue on, or under the adult yet have some structural char­ connection, is relatively uncommon in the marine acteristics of related larval forms. Even the asexually environment and often is associated with direct de­ propagating polyps of benthic hydrozoans and the velopment. Nevertheless, some larval forms are pro­ large physonects of siphonophores are regarded as duced viviparously among the ascidians and the echi­ larval forms by some. noderms. Many molluscs, flatworms, phoronids, and By the definition used in this article, an embryo polychaetes deposit their eggs in capsules or gelati­ becomes a larva when it begins to swim or crawl nous egg masses, in which the embryos either de­ (regardless of whether this occurs at the blastula or velop directly to the juvenile stage or hatch as plank­ gastrula stage) and the larva becomes a juvenile after tonic larvae. The latter case is known as mixed all exclusively larval structures have been resorbed, development. transformed, or cast off. Unciliated cleavage stages are not considered larvae by this definition, even though they may drift passively in the plankton. III. LARVAL FORMS AND DIVERSITY Likewise, brooded or encapsulated developmental stages do not qualify as larvae even though cilia may The shapes and forms of invertebrate larvae are cause them to rotate rapidly within the confines of spectacularly diverse (Fig. 1; Table 1). Larvae have their capsules. been classified by form and ciliary arrangement, nu­ tritional mode, locomotory method, and dispersal potential. Of these, ciliary arrangement appears to II. THE PRODUCTION OF LARVAE be the most useful for elucidating phylogenetic rela­ tionships, whereas the other methods of classification Most, but not all, larvae are products of sexual find greater application in studies of larval ecology. reproduction. Exceptions include miricidia larvae of Yolky atrochallarvae, which have uniform ciliation trematode flatworms that arise asexually from the over all or part of the body, are present in most Marine In vertebrate Lar va e 91 F Q FIG URE 1 Repr esentative larval forms of marine invertebrates. A, Planula larva of a ceriaru hid anthozoan (sea anem one); B, coral planula; C, Muller's larva of a polyclad llatworrn; D, cercaria larva of a marin e digenean trematode (nuke ); E, pilidium larva of a nem ertean ; F, late trochophore larva of a serpulid pol ychaete worm; G, troch ophore of a sabe lJarid polychaete, bearing long protective seta e; H, veliger larva of a gastropod mollusc; I, nauplius larva of a cirriped crustacean (barnacle); J ,. cyp rid larva of a cirripede; K, cyphonautes larva of an anasca n bryozoan; L, actino troc h larva of a ph oronid; M, echinopluteus larva of an echinoid echinoderm (sea urchin); N, glottidia larva of an inarticulat e brachiopod ; 0 , bipinnaria larva of an asteroid echinoderm (starfish); P, auri cularia larva of a holothuroid echinode rm (sea cuc umber) ; Q, pentactul a larva of a holothuroid echinoderm; R, tadp ole larva of a colonial ascidian (sea squirt )[pho tos provided by W. B. Jae ckle (C, D, L, M, N, P, Q) and C. M. Young (remainder)] . 92 Marine invertebrate