ROLE OF LARVAL STAGES IN SYSTEMATIC INVESTIGATIONS OF MARINE TELEOSTS: THE MYCTOPHIDAE, A CASE STUDYl H. GEOFFREY MOSER AND ELBERT H. AHLSTROM2 ABSTRACT The lanternfish family Myctophidae is the most speciose and widespread family ofmid-water fishes in the world ocean. As presently recognized it contains about 30 genera and 300 nominal species. Their larvae are highly prominent in the plankton and make up about 50% ofall larvae taken in open-ocean plankton tows. Our studies of myctophid larvae, on a worldwide basis, have demonstrated that characters of the larval stages oflanternfishes are ofgreat utility in systematic analysis. The genera and species can be recognized on the basis ofeye and body shape, the shape and length ofthe gut, and pigmentpattern and by the sequence of photophore development. In this paper the larvae of 55 species representing 24 genera are illustrated and used to demonstrate the usefulness oflarvae in understanding the relation­ ships of species within genera. Charactersofthe larvae provide insight into generic affinities oflanternfish, allowing us to construct an evolutionary scheme of tribes and subfamilies that differs in some aspects from those proposed on the basis of adult characters. The concept of using larval characters in combination with adult characters to delineate phylogenetic lines in myctophids is discussed, as is our view of evolutionary strategy in the family. A major facet ofcomprehensive systematic inves­ played a large role in the taxonomy of anguil­ tigations is the search for functionally unrelated liform fishes (Castle, 1969) partly because of the characters. Whether the independence of these conspicuousness ofeel leptocephali and partly be­ characters is actual or merely apparent, they con­ cause ofthe unavailability ofadults ofmany ofthe stitute useful elements in the analysis ofsystema­ families. Bertelsen's (1951) treatment of the tic relationships. Ample evidence of this is the ceratioid fishes is a superb example ofthe value of higher classification ofteleosts (Greenwood et al., utilizing larval stages in a systematic revision ofa 1966) generated by the synthesis of a diverse large group of teleosts. Apart from these two array of classical taxonomic characters. The re­ groups, it is the larvae of myctophiform fishes cent surge ofserological and biochemical studies which have received the most attention from tax­ on fish has placed a fresh group ofcharacters in the onomists. Ege (1953, 1957) relied heavily on lar­ hands of systematic ichthyologists (De Ligny, val stages in his extensive works on the 1969). Likewise, recent advances in fish cytogene­ Paralepididae. Johnson (1971) employed larval tics (e.g., Ohno, 1970; Benirschke and Hsu, 1971; characters in defining species and genera of Ebeling, Atkin, and Setzer, 1971) are providing Scopelarchidae. Bertelsen, Marshall, and Krefn another group oftaxonomic characters. It is likely (pers. commun.) have used larval stages exten­ that behavioral science will be still another source sively in their revision of the Scopelosauridae. of taxonomic characters, as exemplified by the Our studies on the family Myctophidae itself growing body of information on the acoustic be­ (Moser and Ahlstrom, 1970, 1972) indicated that havior of fishes (Fish and Mowbray, 1970). larval characters can aid significantly in differen­ One group ofwell known taxonomic characters, tiating taxa and defining evolutionary lineages those ofthe embryonic and larval stages, has re­ within this family. ceived scant attention from all but a few systema­ The lanternfish family Myctophidae is the most tic ichthyologists. Characters of the larvae have speciose and widespread family of mid-water fishes in the world ocean. As presently recognized it contains about 30 genera and 300 nominal 'This paper was presented atthe International Symposium on the Early Life History ofFish (sponsored by IABO, FAO, ICES, species. Their larvae are highly prominent in the ICNAF, and SCOR) held at Oban, Scotland, 17-23 May 1973. plankton and make up about 50% of all larvae 'Southwest Fisheries Center, National Marine Fisheries Ser­ vice, NOAA, La Jolla, CA 92037. taken in open-ocean plankton tows. Manuscript accepted Agust 197J. FISHERY BULl.ETIN: VOL. 72, NO.2, 1974. 391 FISHERY BULLETIN: VOL. 72. NO.2 Our studies of the larvae of this family have longations of choroid tissue and some have the included material from all oceans. We have been eyes on stalks. Paxton recognized 11 genera in able to identify larvae from all recognized genera the subfamily Myctophinae and distributed them except Hintonia and Dorsadena. Larval evidence into two tribes, the Myctophini and the Gonich­ supports giving generic status to Metelectrona and thyini. Larvae of the species in each of these Parvilux. Including these, we have developmental genera generally conform to a particular morph series for 29 myctophid genera and for many gen­ based on form, pigment, and developmental era we have series for all known species. This has pattern and, although these morphs are remark­ afforded a more comprehensive view of the range ably diverse, we can find no character or set of and variability of larval characters, and we are characters that would divide the genera into increasingly impressed with the functional inde­ tribes. Within each genus of the subfamily, how­ pendence ofthe larval and adult characters. It is ever, the larval characters are indispensible in apparent that the world of the larvae and the delineating groups ofrelated species or subgenera. world of the adults are two quite separate This is best illustrated by examining the impor­ evolutionary theaters. Our studies of larval tant genera of the Myctophinae. lanternfishes have disclosed a full range ofcharac­ Protomyctophum larvae have a slender shape ters, from generalized to specialized and from con­ (Figure 1). For all species exceptP. anderssoni, the servative to labile, equal in scope to those of the gut is short during most of the larval period and adults. These characters fall into several characteristically there is a marked interspace categories. An important group is the shape of between the anus and the origin of the anal fin various structures such as the eye, head, trunk, (Figure lA-D). The gut elongates dramatlically in guts, and fins, especially the pectoral fins. Another late larvae, to fill the interspace. Gut development group is the sequence of appearance and the posi­ is completely dissimilar in P. anderssoni, where tion of fins, photophores, and bony elements. the gut is long at all larval sizes, in fact longer Another is the size of the larvae when fins and than in most other lanternfish larvae (Figure IE). other features appear and the size of the larvae Series of ventral tail melanophores are formed in when they transform intojuveniles. Pigmentation some species of both recognized subgenera provides an important group of characters based (Heirops and Protomyctophum sensu stricto), for on the position, number, and shape of melano­ example in P. Protomyctophum normani (Figure phores. Finally, there are the highly special­ 1A) and P. Heirops thompsoni (Moser and Ahl­ ized larval characters such as voluminous fin strom, 1970). Larvae ofthe subgenera can be sepa­ folds, elongated and modified fin rays, chin bar­ rated, however, on the basis ofeye shape, the eyes bels, preopercular spines, etc. It is our purpose ofHeirops (Figure 1C, D) being characteristically here to point out some of these characters and narrower than those of Protomyctophum sensu demonstrate how they can be of advantage in stricto (Figure lA, B). Choroid tissue is absent defining taxa and establishing phylogenetic from the ventral surface ofthe eye in all species of lineages. the genus exceptP. anderssoni, which has a well­ developed "teardrop" (Figure IE). Larvae of P. THE SUBFAMILY MYCTOPHINAE anderssoni are so markedly different from those of all other species of Protomyctophum, which The most trenchant character of larval myc­ otherwise form a rather cohesive group, that this tophids is eye shape. Our studies show that species should be placed in a separate subgenus or lanternfish larvae fall naturally into two groups perhaps even in a distinct genus. This suggestion on the basis ofeye shape-those with narrow ellip­ is supported by the unique placement of certain tical eyes and those with round or nearly round photophores and by the structure of the sup­ eyes (Moser and Ahlstrom, 1970). The species racaudalluminous tissue in adults ofthis species. composition of these two groups agrees closely Larvae of the genus Electrona are a less with that ofthe two subfamilies, Myctophinae and homogeneous group but are united by a common­ Lampanyctinae, established by Paxton (1972) on ality ofbody shape and especially gut shape (Fig­ the basis of osteological and photophore charac­ ure 2). A marked interspace is present between ters of adults. Larvae of the Myctophinae have the end of the gut and the origin of the anal fin. elliptical eyes; some species have ventral pro- This space is closed only at the termination ofthe 392 MOSER anJ AHLSTROM: ROLE 01' LARVAE IN SYSTEMATICS A -- --------- -- ------- ~ 8 ....---- E FIGURE I.-Larvae of Protomyctophum. A. P. Protomyctophum normani, 15.2 mm; B. P. Protomyctophum teni­ soni, 14.5 mm; C.P. Hierops subparallelum. 15.2 mm; D.P. Hierops chilensis, 11.0 mm; E.P. anderssoni, 15.7 mm. 393 FISHERY BULLETIN: VOL. 72. NO. A B c o FIGURE 2.-Larvae of Electrona and Metelectrona. A. E. antarctica, 12.7 mm; B. E. carlsbergi, 11.1 mm; C. E. subaspera, 10.5 mm; D. M. ahlstromi, 10.3 mm. 394 MOSER and AHLSTROM: ROLE OF LARVAE IN SYSTEMATICS larval period. None of the species forms photo­ are synonyms, however, the uniqueness of the phores during the larval period other than the Br2 larva strongly suggests the resurrection of pair. Metelectrona as a valid genus. The characters that most clearly separate the The genus Benthosema IS the least cohesive of three developmental lines in Electrona are eye any genus inthe subfamily Myctophinae, from the shape and the amount ofchoroid tissue developed viewpoint of larval structure (Figure 3).