22 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



Its size and rate of development are useful, as well as whether 

 it is round or oval. Some fish larvae have eyes borne on stalks 

 that reach an extreme in Idiacanthus, while others develop an 

 area of choroid tissue. Migration of the eye in flatfish larvae 

 from a symmetrical position to one side of the head is well 

 known. The sequence of development of ossified structures is 

 proving to be a powerful tool in systematic studies offish larvae. 

 The losses and fusions of bones, which are generally assumed 

 based only on adult material, can and should be tested using 

 developmental studies. The caudal fin skeleton has provided 

 excellent developmental characters to be used for systematic 

 inferences, mainly at the order-generic levels. The development 

 of scales has been little studied but may prove valuable, espe- 

 cially in fishes with precocious scales (e.g., some anthiins, hol- 

 ocentrids). 



The Transformation Stage 



Between the larval and juvenile stages, there is a transitional 

 stage which may be abrupt or prolonged and which, in many 

 fish, is accompanied by a change from planktonic habits to 

 demersal or schooling pelagic habits (Fig. 12). In some fishes 

 migration to a "nursery" ground occurs during or just before 

 this stage. Morphologically the transformation stage is charac- 

 terized by a change from larval body form and characters to 

 juvenile-adult body form and characters. At the end of this stage 

 the fish generally looks similar to the adult, with major differ- 

 ences only in pigmentation patterns. Two ontogenetic processes 

 occur during this stage of transition between the larva and ju- 

 venile: I ) loss of specialized larval characters, and 2) attainment 

 of juvenile-adult characters. Changes that occur during this stage 

 include pigment pattern, body shape, fin migration (e.g., in clu- 

 peids and engraulids), photophore formation, loss of elongate 

 fin rays and head spines (e.g., in epinepheline serranids and 

 holocentrids), eye migration (pleuronectiforms), and scale for- 

 mation. 



In several groups, where the transformation stage is pro- 

 longed, the fish have developed specializations that are distinct 

 from both the larvae and juveniles. This stage has been desig- 

 nated the prejuvenile stage (Hubbs, 1943). The specializations 

 generally involve body shape and pigmentation. In many, the 

 morph resembles a herring-like fish and is apparently adapted 

 for neustonic life. The dorsal aspect of the fish is dark green or 

 blue and the lateral and ventral is silvery or white. The body 

 tends to be herring shaped and the mouth terminal. Fins are 

 generally unpigmented. Such a stage is present m Gadiformes 

 (Urophycis), Beryciformes (Holocentrus), Perciformes (e.g., Po- 

 malomus, MuUidae, Mugilidae) and Scorpaeni formes (e.g., 

 Scorpaenichthys, Hexagrammos). In other fishes, such as some 

 myctophiforms and carapids, the prolonged transformation stage 

 may have distinctive body and fin shapes. 



Implications of Larval Fish 

 Morphology 



When studying the appearance of larval fishes, one is im- 

 mediately struck with their diversity and morphological dissim- 

 ilarity to adults. This dissimilarity led early workers to establish 



names for several of these forms, not realizing that they were 

 the young stages of known adults. After establishing the identity 

 of many fish larvae in a variety of groups, we hypothesize that 

 the larvae of all species are recognizably distinct. The use of 

 diversity of larval form in vertebrate systematics was discussed 

 some time ago by Orton (1953b, 1955c, 1957) and in this vol- 

 ume we examine this use in detail in numerous groups of fishes. 



Why are the larvae so diverse?— Despite the tremendous mor- 

 tality associated with living in the planktonic realm during the 

 larval period, survival must be sufficient to maintain the species 

 and provide a dispersal mechanism for it. To different degrees, 

 various taxa apparently rely on survival and longevity of in- 

 dividual larvae. The amount of reliance is presumably related 

 to fecundity and importance of dispersal and colonization to 

 the taxon. A number of structures have evolved that would be 

 expected to enhance larval survival in the plankton. Practically 

 no experimental work has been done to investigate the function 

 of larval structures, but some structures probably assist flotation 

 and feeding while others decrease predator mortality. Conver- 

 gence on characters that are apparently functionally important 

 to larval survival in the plankton is seen. These specializations 

 develop in conjunction with the basic ontogeny of the taxon. 

 In studying systematics using larval fishes, both the basic pattern 

 of development and the specialized structures must be analyzed. 



Why are these larvae so morphologically unlike the adults?— 

 Most larvae are adapted to survive in an ecological realm (gen- 

 erally the plankton) that is far different from that of the adult. 

 These are small organisms, compared to adults, and they live 

 in the plankton, having to find and capture food there and avoid 

 becoming food. They float and migrate vertically in a milieu 

 that may be moving much faster than they are. During this 

 larval period, these fish undergo extreme changes in morphology 

 yet remain a functioning (eating, avoiding predators) organism 

 and eventually end up in a suitable nursery area for the juvenile 

 stage. 



How then can larval morphology help us understand the evolu- 

 tion of these fishes?— Mler recognizing that each species has a 

 morphologically distinctive larva, generally we see that species 

 of the same genus are phenetically similar, and larvae of mem- 

 bers of a family also share common features. Even larvae of 

 suborders and orders share some larval characters. This would 

 be expected since evolution operates on all stages in the life 

 cycle, not just the adult. Evolutionary pressures on the larval 

 stage seem to be particularly intense in those groups that rely 

 on the larvae for widespread dispersal in the ocean. Here the 

 larvae appear well adapted for life in the planktonic realm, and 

 it can truly be said that the larva and the adult perform in "two 

 quite separate evolutionary theaters" (Moser and Ahlstrom, 

 1974). In this volume we are focusing on what we know to date 

 about larval evolution within various groups of fishes (Table 4). 



Northwest and Alaska Fisheries Center, 2725 Montlake 

 Blvd. E., Seattle, Washington 98112 and Southwest 

 Fisheries Center, P.O. Box 271, La Jolla, California 

 92038. 



