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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 Larvae

TABLE 1 Taxon Larval forms Known Larval Forms in Major Taxa of Marine Invertebrates Echinoidea Echinoplutcus Holothuroidea Vitellaria, doliolaria, auricularia, Taxon Larval forms pentactula Phylum Porifera Amphiblastula, parenchymula Phylum hcmichordata Tornaria Phylum Cnidaria Phylum Chordata Hydrozoa Planula, actinula Urochordata Tadpole Anthozoa Planula, Edwardsian larva, hal­ Cephalochordata Amphioxides campoides larva, Semper's larva (zoanthina. zoanthclla) Phylum Ctenophora Cydippid, planuloid larva Phlum Platyhelminthes phyla and are typical of sponges, cnidarians, and Turbellaria Muller's larva, Gone's larva, Lu- plathyelminth worms, Although some atrochal lar­ ther's larva vae, including sponge parenchymulae and cnidarian Monogenea Onchomiricidia planulae, are simple ciliated spheres or spheroids, Trematoda Miricidia, redia, cercaria others, including the Muller's larvae of flatworms, Cestoda Coracidium have more complicated shapes and may have orga­ Phylum Rhombozoa lnfusoriform nized ciliary fields with cilia of different lengths, Phylum Nernertea Pilidium. Desor's larva Trochal larvae, which have cilia organized into Phylum Loncilera Higgin's larva one or more discrete bands, are also found through­ Phylum Priapulida Priapulid larva out much of the animal kingdom and it bas been Phylum Annelida Trochophore, nectochacta. cndo­ larva, cxolarva, mitraria. aulo­ argued by Nielsen that an organism very like modern phora trochophore larvae may have been the ancestor of Phylum Mollusca the metazoans. Virtually all trochal larvae may be Polyplacophora Trochophore assigned to one of two larval types, each of which is Aplacophora Trochophore regarded as characteristic of a major subdivision of Bivalvia Trochophore, bivalved vcliger, the coelomate metazoans. The spiralian eucoelo­ pediveliger, pericalymma mates or protostornes, including the annelids and Gastropoda Trochophore, veliger. echino­ molluscs, characteristically have trochophores or spira trochophore-like larvae. In their basic form, trocho­ Scaphopoda Trochophore, veligcr phores and veligers have two parallel bands of com­ Phylum Arthropoda pound cilia termed the prototroch and mesotroch. Mcrosiomata Trilobite larva Pycnogonida Protonymphon Additional ciliary bands (the metatroch, neurotroch, Crustacea Nauplius, zoea, megalopa, phyllo­ and telotroch) may also be present, particularly in soma, pucrulus, cyprid, gau­ later larval stages. The deuierostome phyla, including cot hoe the echinoderms and the hernichordates, characteris­ Phylum Echiura Trochophore tically have larvae with a single ciliary band, though Phylum Sipuncula Trochophore, pelagosphera the shape of this band may be complex and the Phylum Bryozoa Coronate larva, cyphonautes convolutions vary substantially even within larval Phylum Entoprocta Trochophore forms (Table 1). Larvae in two phyla, Arthropoda Phylum Phoronida Actinotroch and Chordata (subphylum Urochordata) lack exter­ Phylum Brachiopod Tripartite larva, gloltidia (inartic­ nal cilia and swim by means of muscular contrac­ ulate) larva tions. Phylum Echinodermata Crinoidea Doliolaria Some life cycles include several different larval Ophiuroidea Vitellaria, ophioplutcus forms. This is particularly true among parasites, in Asteroidea Bipinnaria, brachiolaria, "barrel- which there may be definitive and intermediate hosts, shaped larva" each infected initially by a particular larval form. In M (II inc III V C It c1n ate La r v (I C 93 a typical marine digenean fluke, for example, the some of their nutrition. However, the lines between ciliated miricidium larva infects a molluscan interme­ planktotrophy and lecithotrophy have been blurred diate host by burrowing into it. Within the intermedi­ in recent years by the discoveries that some lecitho­ ate host, the miricidiurn transforms itself and propa­ trophic larvae take up dissolved organic matter and gates asexually to form a second larval form, the that some phyla have intermediate larval forms that cercaria, which leaves the host and propels itself by depend to different degrees on maternal provi­ means of a muscular tail to burrow into the definitive sioning. Facultative planktotrophs, an example of host or to locate a settlement site where it is likely the latter, arc fully capable of collecting food particles to be eaten. but have enough yolk to complete metamorphosis Nonparasitic invertebrates may also have multiple without feeding. larval forms within a single life cycle. For example, Some predatory larvae are found among the Crus­ crustaceans typically pass through instar stages tacea and the Polychaeta, but most planktotrophic which are separated by molting events and desig­ larvae feed on phytoplankton. Ciliated larvae use at nated with separate numbers or names. The early least three different mechanisms for collecting food instal'S of brachyuran crabs are known as zoeal stages particles. Planula larvae of some cnidarians and a few and the terminal instar, or megalops, is the settlement polychaete trochophores secrete a strand of mucus stage. Likewise, barnacles pass through numerous which is pulled behind the larva like a fishing line. feeding naupliar stages then undergo metamorphosis The strand itself is ingested along with any adherent to the nonfeeding cyprid stage, which selects a habi­ particles. Trochophores, veligers, and related larval tat and undergoes a second metamorphosis to be­ forms of protostomous eucoelomates use a dual-band come the juvenile barnacle. In some holothurians, food collection mechanism in which particles arc embryos give rise to a yolky, uniformly ciliated vitel­ trapped in a food groove that lies between two ciliary laria larva which then organizes its cilia into bands bands. The anterior prototroch is composed of large. to form a barrel-like doliolaria. The doliolaria be­ compound cilia that function simultaneously in loco­ comes a pentactula, the larval stage which settles and motion and particle collection. Particles are deflected grows into a juvenile. Some species of sipunculans into the food groove, apparently with the aid of a pass through both trochophore and pelagosphera lar­ secondary band, the mesotroch, whose function re­ val stages. Similarly, polychaetes begin larval life as mains incompletely understood. Echinoderm and he­ trochophores and then become nectochaeta larvae michordate larvae collect food with convoluted with the onset of segmentation. Molluscan trocho­ bands of simple cilia. Upon encountering suitable phores become veligers as the larval shell develops. particles, cilia reverse the direction of their beat, Bipinnaria larvae of starfish become brachiolaria lar­ deflecting the particles toward the mouth and caus­ vae with the addition of the attachment organs used ing them to concentrate in the circumoral field. They at settlement. also capture some particles without ciliary reversal, apparently by directing the flow of particles into the oral region. Cyphonautes larvae of anascan bryozo­ IV. LARVAL FEEDING AND ans use stiff cilia of the locomotory band as a sieve NUTRITION to capture particles. Herbivorous crustacean larvae capture particles by Lecithotrophic larvae rely entirely on maternally means of fine spines and hairs on their locomotory provided yolk for their sustenance, whereas plankto­ appendages. Raptorial forms, such as megalopae, feed trophic larvae acquire nutrients and energy from ex­ on individual prey items in much the same way as ternal sources, either by concentrating and collecting adult crabs, using the mouthparts to sort food and food particles or by absorbing organic molecules chelipeds and periopods to hold the prey and disas­ from seawater. A few larval forms, including some semble it. The larva of the polychaete Mage/ana papil­ cnidarian planulae and ascidian tadpoles, carry sym­ licornis is a specialist predator that preys on bivalve biotic algae from which they may perhaps derive veligers, which are captured on very long tentacles. 94 Mu ri n r Invertebrate larvae

Recent work on nutrition of larval invertebrates in unnatural, unidirectional light. which is very dif­ indicates that many species have very specialized ferent from the scattered light found in the sea. In food requirements. In this respect, polyunsaturated the most common pattern of diel larval migration, fatty acids have been shown to be especially impor­ larvae approach the surface at night and then move tant. Larvae are most easily cultured on mixed algal into deeper water by day. diets that ensure a proper balance of essential amino High fecundities of marine animals attest to the acids and lipids. Many species of both plankiotrophic fact that mortality is severe during the embryonic, and lecithotrophic larvae are capable of taking up larval, and juvenile stages. Predation, starvation, and dissolved organic matter from seawater. Although transport to unsuitable habitats are probahly the ma­ the actual implications of this remain unknown jor sources of larval mortality, though fertilization pending a better understanding of the composition failure may also be a Significant source of gamete and quantity of naturally occurring organic mole­ wastage in free-spawning species. Larvae are preyed cules in the sea, it appears that fatty acids arc proba­ upon by a wide assortment of planktonic and benthic bly more important than dissolved amino acids be­ predators and filter feeders, but the relative impor­ cause of their relatively greater energetic content. tance of benthic and planktonic predation remains unknown. The dispersal period varies in marine invertebrates V. LARVAL ORIENTATION, from a lew minutes in lecithotrophie larvae of ovovi­ LOCOMOTION, DISPERSAL, viparous ascidians and bryozoans to more than I AND MORTAUTY year in some planktotrophic species from tropical seas. Larvae in many phyla arc capable of riding Virtually all larvae are capable of locomotion by ocean currents across ocean basins. These far­ means of either cilia or muscles. Muscular locomo­ wandering forms are known as teleplanic larvae. tion is generally faster, at least for short distances, Teleplanic larvae often have adaptations that facili­ than ciliary locomotion. With the possible exception tate locomotion and reduce sin king rates. In a re­ of late-stage puerulus and rnegalops larvae of crus­ markable display of dispersal ability. one lobster larva taceans, larvae are not capable of swimming faster captured off south Florida was traced to a species than the currents they encounter in their environ­ that lives only in the Indian Ocean. In some species, ment. Navigation is accomplished by vertical move­ the length of the dispersal period is variable. Once ments that cause larvae to encounter currents moving larvae become competent to undergo metamorpho­ in different directions and at different speeds. By sis, they can in some cases delay metamorphosis moving vertically, larvae can apparently control the indefinitely until encountering a suitable substratum. dcgree to which they arc either exported from or The ability or larvae to extend their free-swimming retained in estuaries, enhance offshore dispersal or period is dependent on nutritional resources and also return to the coast, and ensure retention in the down­ on the availability of suitable substrata. In feeding stream eddies of islands. The behaviors which con­ larvae, there appears 10 he little disadvantage to de­ trol these vertical movements may change on diel laying metamorphosis apart from the risk of plank­ cycles, or they may shift gradually with ontogeny. tonic mortality. Indeed, larvae that grow to a larger Thorson noted long ago that many larvae of benthic size in the plankton may achieve a refuge in size that invertebrates arc initially photopositive and geonega­ reduces juvenile mortality. In Iecithotrophs, how­ tive to maximize dispersal, and they become photo­ ever, individuals delaying metamorphosis may often negative and geopositive at the end of larval life to sacrifice some viability, presumably because nutri­ increase chances of encountering the benthos. How­ ents needed in the juvenile period arc consumecl ever, numerous variations on this theme exist, and prior to metamorphosis. much of the data that led to this conclusion have At the end of the swimming period, larvae must recently been challenged as having been generated find the bottom and select an appropriate substra- Marine Invertebrate Laf-vae 95 tum. Downward movement may be accomplished in algae, including but not limited to crustose coral­ with a combination of behavioral changes and pas­ line forms. Gregarious settlement in barnacles and sive mechanisms that increase sinking rate. In le­ oysters is mediated by proteins found in the adult cithotrophs, specific gravity often increases toward cuticle or shell, but bacterial metabolites also appear the end of larval life as buoyant lipid yolk is con­ to play a role. The cue for gregarious settlement sumed. Some larvae, including those of many echino­ in sabellariid polychaetes has been more completely derms, grow ever-larger calcite skeletons during the characterized than those for other inducers. It con­ larval stage, causing their sinking rate to increase as sists of free fatty acids found in the matrix that binds a function of age. Many late-stage larvae are less together the adult sand tubes. responsive 10 light and also less active than earlier Most chemical cues that stimulate metamorphosis larvae, so they naturally tend to move toward the are bound to surfaces and must be detected by tactile sea floor. chemorcceptors, so settlement choices tend to occur on tiny spatial scales. However, settlement behavior can also control distribution on larger scales if larvae VI. LARVAL SETTLEMENT drifting in the current test the bottom periodically AND METAMORPHOSIS and either accept or reject the site. Larvae of Phestilla sibbogae, an opisthobranch that feeds obligatcly on Settlement and metamorphosis have been studied two species of coral, can detect corals from a distance for many larval groups, though specific inducers of using waterborne cues. Oyster larvae have also been metamorphosis have been isolated and identified for shown to change their behavior in the presence of only a few species. Some larvae settle gregariously waterborne cues associated with adult oysters. in response to the presence of other conspecifics, Metamorphosis involves multiple physiological presumably because established individuals indicate processes and morphogenetic changes, including cell that the habitat has recently been suitable or that death, resorption of tissues, reorganization of tis­ potential mates are available. Associative settlement, sues and organs, and activation of organ systems. settlement in response to other species, is especially Some of these processes take place at dramatic speed, common among predators and herbivores that re­ whereas others, particularly those that require quire specific food items and also among parasites growth and reorganization of tissues, may take longer that require a particular host. Many kinds of larvae to complete. In echinoids (Fig. 2), the juvenile body can be stimulated to settle in the presence of a bacte­ ("echinus rudiment") forms in an invagination on the rial biolilm. Some require sediments of a particular left side of the body while the larva is still swimming. grain size. A number of species are known to select Metamorphosis is completed very rapidly by casting habitats in which overgrowth competition or preda­ off the larval tissues after the juvenile rudiment at­ tion is unlikely. Negative phototaxis is common taches to the substratum. Ascidian tadoples provide among settling larvae, and this response aids in se­ a contrasting example of rapid metamorphosis. In lecting habitats protected from algal overgrowth, silt­ the initial phase, anterior papillae evert, attaching ation, and large predators. The charge and texture the animal to the substratum. Immediately after at­ of a surface is also important in determining where tachment, the muscles and notochord of the tail are a larva will settle. rapidly taken into the trunk (Fig. 2). After these Metamorphosis can often be stimulated by neuro­ initial rapid events, slower processes, including rota­ active compounds such as y-aminobutyric acid and tion of the body, formation of blood cells, and exten­ by a wide variety of organic and inorganic chemi­ sion of epidermal ampullae, take place over the next cals, but the specific substances that are involved several days. Equally rapid and spectacular metamor­ in metamorphic induction under natural conditions phoses have been described in pilidium larvae of have been largely elusive. A number of molluscs are nernerteans, coronate and cyphonautes larvae of known to settle in response to compounds found bryozoans, actinotroch larvae of phoronids, cyprid 96 Marin e Inveneb r a te Larvae

A

B

FIGURE 2 Meta morp hosis of an echino id and an ascidi an . (A) Late ec hino pluteus larva of th e echinoid Strongylocentrotus j rancisc(//1l(s showing the large, opaque echinus rudime nt on th e left side of the larval bod y. (B) Ju venil e S. jranciscanus attached by tub e feet to the su bstratum shortly after larval tissues have been cas t off. (C ) Tadpole larva of th e asc idia n Boltenia villosa JU Sl after attac h ment to the subst ratum. (D -F) Progressive stages of tail resorp tion, occ urring within minutes of attac hment. (G) Juvenile asci dian surrou nded by epide rma l amp ullae 24 hr after se ttlement. (H) Two -week-o ld j uve nile asci dian with rotated trunk, completed circulatory system, and epidermal ampullae.

larvae of barnacles , veliger larvae of molluscs, and mechan ism for the larvae themselves and would not, others. therefore, reflect any particular stag e in the phylog­ en y of the spec ies. Th e study of selective pressures on early life histo ry stages of marine invert ebrates VII. ECOLOGICAL AND remains an imp ortant resea rch focus. A large bod y EVOLUTIONARY SIGNIFICANCE of theoretical literature deals with the trad e-offs and OF LARVAE cons traints associated with the evolutionary switch from planktotrophy to lecith otrophy, the control of The imp ortance of the larval stag e has long been egg size, the optimal strategy for parental investm ent recognized by biolo gists interes ted in evolution and and protection, fert ilization tactics, and constraints ecology. When the major larval forms were identifi ed on the evolution of larval body forms. and described during the latter half of the nineteenth Because the larval stage is generally resp onsible cen tury , many were held forth as examples of the for dispersal, larval processes must be taken into "bioge netic law" (ontogeny recapitul ates ph ylogeny). account in studies of gene flow , recruitment, and Wa lter Garstang refuted the recapitulation argument ecology. In recent years, larval supply has received in the first half of the twentieth century by arguing mu ch attent ion as a potential factor controlling pop ­ that larvae are susce ptible to mortality and should ulations of invertebrates and the structure of benthic thus be a focal point for the forces of natural selec­ communities. Marin e eco logists now recogniz e that tion . He reasoned, for example , that gastropod tor­ pos tsettlement processes, such as predation, compe­ sion, a major developmental event that takes place tition, and mortalit y by ph ysical stress, must act on in the larval stage, may have evolved as a protective the patterns established initially by larval recru it- Marine Invertebrate Lanae 97 ment. Thus, large research efforts now seek to under­ See Also the Following Article stand how oceanographic processes influence larval distribution and abundance. Despite a long and rich MARINL tNVFRTFlJRA I LS. MODES OF RfTRlll)UCTlON IN biological tradition focused on larval evolution and ecology, our understanding remains rudimentary in many respects. Larvae are microscopic. temporary Bibliography drifters in the plankton and as such are not easily studied in their natural habitats. Chia, F. 5.. and Rice, M. E. (Eds.) (1978). Seulemf/lt (//](/ Metamorphosis of Marine Tnvertefmlle Larvae. Elsevier­ North Holland, New York. VIII. ECONOMIC AND MEDICAL Chia, F. 5., Buckland Nicks, j., and Young, C M. (1984). IMPORTANCE OF LARVAE Locomotion or marine invertebrate larvae: A review. Can. I ZooI. 62, 1205-1222. From a practical standpoint, the study of larvae is Crisp, D. ]. (1974). Factors influencing the settlement of marine invertebrate larvae. In Chemoreception in Marinc particularly important in the control of fouling and Organisms (P. T. Grant andA. M. Mackie, Eds.), pp. 177­ in the management of invertebrate fisheries. Fouling 265. Academic Press, New York. studies have been a mainstay of naval research for Giese, A. C, Pearse, j. 5., and Pearse, V. B. (Eds.) (1987). centuries. Modern work on the fouling of ships and Reproduction of Marine Invertebrates, Vol. 9, Sec/ling Unity in other manmade objects focuses on the study of meta­ Diversitv. Blackwell/Boxwood Press, Palo Alto, CA/Paci{ic morphic cues and their disruption. Management of Grove, CA. fisheries for oysters, clams, prawns, lobsters, abalone, McEdward, L R. (Ed.) (1995). Ecology of Marine Invertebrate etc. has depended on studies of larval dispersal and Larvae. CRC Press, Boca Raton, FL abundance since the early twentieth century. In addi­ Nielsen, C (1995). Animal Evolution. Intenclationships ,ij the tion, larval invertebrates are often seasonally impor­ Living Phyla. Oxford Univ. Press, Oxford, UK. tant foods for the young stages of commercially im­ Pawlik,]. R. (l992). Chemical ecology or the sculemenr or portant finfish. benthic marine invertebrates. Oceanogr. Mar. BioI. Ann«. Rev. 30, 273-335. Cercaria larvae of trematodes and planula larvae Schcltcma, R. S. (1986). On dispersal and planktonic larvae of cnidarians may cause minor skin irritations in or benthic invertebrates: An eclectic overview and summary swimmers. One such larval irritant, known colloqui­ of problems. Bull. Mar. Sci. 39, 290-322. ally as "sea lice" or "sea bather's eruption," is the Thorson, G. (1950). Reproductive and larval ecology of ma­ nematocyst-bearing planula of the scyphozoan Li­ rine bottom invertebrates. BioI. Rev. 25, 1-45. nuche unguiculata. This larva has caused significant Young, C M. (990). Larval ecology or marine invertebrates: economic losses to coastal communities. A sesquicentennial history. Ophelia 32, j -48.