BULLETIN OF MARINE SCIENCE, 74(1): 31–51, 2004 THE LATERAL LINE SYSTEM OF TWO SYMPATRIC EASTERN PACIFIC GOBIID FISHES OF THE GENUS LYTHRYPNUS (TELEOSTEI: GOBIIDAE) Harald Ahnelt and Veronika Bohacek ABSTRACT Lythrypnus dalli (Gilbert, 1890) and Lythrypnus zebra (Gilbert, 1890) occur sympatri- cally in rocky subtidal habitats of the eastern Pacific, but occupy distinct microhabitats. The two species belong to different species complexes. We were interested if the differ- ences in microhabitat use of these two gobiids correspond to differences in the pattern of the lateral line system, which is represented by relatively few and large free neuromasts. The neuromast pattern is similar in both gobies with few, but characteristic differences. These differences also occur among other species of the two species complexes (i.e., Lythrypnus cobalus and Lythrypnus gilberti of the Lythrypnus dalli complex and Lythrypnus pulchellus and Lythrypnus rhizophora of the Lythrypnus rhizophora com- plex) and support the recognition of east Pacific species in two species complexes. The topography of the free neuromasts and their innervation is described for Lythrypnus dalli and Lythrypnus zebra. Lythrypnus dalli (Gilbert, 1890) and Lythrypnus zebra (Gilbert, 1890) occur sympatri- cally along the coast of southern California, Baja California (Mexico) and adjacent off- shore islands, where they are most abundant in rocky subtidal habitats (Eschmeyer and Herald, 1983). Both species inhabit shallow, rocky reefs where they display microspatial separation. Lythrypnus dalli occupies a more exposed habitat such as projections and outcrops; whereas, L. zebra is more cryptic. The latter species remains hidden under boulders in crevices, cavities, and caves (Behrents-Hartney, 1989). The free neuromasts (sensory papillae) of the lateral line system of Gobioidei are ar- ranged in series of characteristic rows, lines, or aggregations on the head, trunk, and caudal fin and have been widely used in the classification of gobioid fishes (e.g., Iljin, 1930; Akihito et al., 1984; Miller, 1986; Takagi, 1988; Larson, 2001). Homologies for neuromast rows have been proposed based on superficial topography (e.g., Miller and Wongrat, 1979; Hoese, 1983; Miller, 1986; Gill et al., 1992), and on their innervation (Takagi, 1988; Wongrat and Miller, 1991). We examined the correspondence of micro- habitat use of these two gobies with the cephalic neuromast pattern of the lateral line system and the distribution of this pattern in the L. dalli and Lythrypnus rhizophora spe- cies complexes. In addition, small body size (L. dalli and L. zebra) and distinct cryptobenthic life (L. zebra) are known to be reflected in such morphological adaptations as reductions of the lateral line system, and are considered to be specializations (Miller, 1979, 1996). MATERIALS AND METHODS The following preserved specimens housed in the California Academy of Sciences (CAS) and the Scripps Institution of Oceanography (SIO) were examined (collection number, number of speci- mens, sex, standard length in mm, locality). Determination of the sex by the shape of the urogenital papilla follows St. Mary (1993). Bulletin of Marine Science 31 © 2004 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 32 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 MATERIAL Lythrypnus dalli.—Fifty-nine specimens. CAS 118455; 14 females, 16.1–26.5, seven males, 21.9–28.4, four sex?, 16.1–27.2, two juveniles, 14.3–15.2; Mexico, Baja California Sur, Gulf of California, Isla Partida. CAS 118452; two juveniles, 12.9–15.9; Mexico, Gulf of California, Punta Penasco. CAS 136559; one female, 20.8, one sex?, 24.3 + 6.0, U.S., California, Santa Catalina Island. CAS 118456; one sex?, 25.9; Mexico, Baja California Sur, Bahia San Francisquito. CAS 54911; one juvenile, 14.3; U.S., California, Monterey. CAS uncatalogued, W53-77; one sex?, 24.3; Mexico, Gulf of California. SIO 63-174-59C; two females, 21.4–23.9, seven males, 21.3–30.7, seven sex?, 18.0–28.4, nine juveniles, 15.0–19.8; Mexico, Baja California Norte, Isla Guadalupe. Lythrypnus zebra.—Fifty-six specimens. CAS 25388; nine males, 18.2–34.3, 15 sex?, 18.5– 30.5, one juvenile, 18.5, U.S., California, Santa Catalina Island. CAS 13563; one female, 28.6, one male, 33.8, one sex?, 31.2; U.S., California, Santa Catalina Island. CAS 25391; one sex?, 26.8; U.S., California, Santa Catalina Island. CAS 51318; one sex?, 26.8; U.S., California, Santa Barabara Island. SIO H50-40-59A; one female, 29.4, six males, 20.3–31.5, 12 sex?, 20.4–30.7, six juveniles, 14.6–17.3; Mexico, Baja California Norte, Isla Guadalupe. SIO 54-219-59; one male, 33.0; Mexico, Baja California Norte, Isla Guadalupe. SIO 65-71-95B; one juvenile, 11.3; Mexico, Baja Califor- nia Norte, Isla Guadalupe. COMPARATIVE MATERIAL (NEUROMAST PATTERN) Lythrypnus cobalus.—Five specimens. CAS 205778; two females, 13.2–26.1, one sex?, 13.8; Costa Rica, Isla del Coco. SIO uncatalogued UAZ 89-1; one female, 18.3; Costa Rica, Isla Manuelita. SIO 72-97; one female, 17.3, one sex? 18.5; Colombia, Isla de Malpelo. Lythrypnus gilberti.—Eleven specimens. CAS 39236; two males, 17.1–17.5, one female, 17.5, five sex?, 13.2–20.0, two juveniles, 9.8–12.5; Ecuador, Galápagos Islands, Isla San Salvador. SIO 78-181; one male, 22.0; Ecuador, Galápagos Islands, Isla Fernandina. Lythrypnus pulchellus.—Fifteen specimens. CAS 18097; one male, 19.1; Mexico, Baja Califor- nia, Agua Verde Bay. CAS 18458; one male, 20.1; Mexico, Sonora, outer San Carlos Bay. CAS 66901; two males, 15.6–18.8; Mexico, Baja California, Isla Partida. SIO 65-319-59F; three males, 19.5–21.8; Mexico, Baja California Sur, north of Punta Pulpito. SIO 61-277-59B; three males, 19.8–21.9; Mexico, Baja California Sur, Isla del Espiritu Santo. SIO 62-227-59B; three males, 18.7–26.4, two sex?, 16.6–19.9; Mexico, Baja California Norte, Bahia de Los Angeles. Lythrypnus rhizophora.—Eleven specimens. CAS 50078; two males, 20.5–22.3, one female, 16.9, three sex?, 17.8–24.3, four juveniles, 8.7–13.0 ; Ecuador, Galápagos Islands, Isla de Genovesa. SIO 78-181; one juvenile, 9.7; Ecuador, Galápagos Islands, Isla Fernandina. NEUROMAST ROW TERMINOLOGY.—Lateral line canals and free neuromasts show a common basic distribution pattern in teleosts and are innervated by the same trunks and branches of the anterior and posterior lateral line nerves (Coombs et al., 1988; Wongrat and Miller, 1991). Sanzo (1911) developed a terminology for the neuromast patterns of gobies. This terminology is seemingly ap- plicable to all gobioids (Wongrat and Miller, 1991; Gill et al., 1992). We favor a description of a neuromast pattern based on its topography on defined regions of the head and the trunk (Sanzo, 1911; Iljin, 1930). It allows a comparison of the neuromast pattern of large numbers of specimens in a relatively short time. For establishing phylogenetic relationships, the recognition of homolo- gies should be supported by innervation. The innervation of the neuromasts was investigated in specimens cleared and stained for osteo- logical studies according to Dingerkus and Uhler (1977). Illumination of the head from the anterodorsal aspect gives the nerves a whitish appearance. The neuromasts of L. dalli and L. zebra are large and also visible in cleared and stained specimens, thus it was possible to determine which nerve innervated which neuromast row. The nomenclature of innervation follows Wongrat and Miller (1991) and Jakubowski (1966). Abbreviations of lateral line nerves are given in the text of figures. Series of free neuromasts (sen- AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 33 sory papillae) on the head are identified as: AD, anterior dorsal; IO, interorbital; OP, opercular; OS, oculoscapular; PM, preopercular-mandibular; PO, preorbital; SO, suborbital. Unless otherwise stated, the arrangement and the innervation of the free neuromasts of the lateral line system is the same in both, L. dalli and L. zebra. RESULTS INNERVATION OF THE CEPHALIC NEUROMAST ROWS (FIGS. 1–3) The anterior and posterior lateral line nerves leave the central nervous system with the cranial nerves (V) Nervus trigeminus, (VII) Nervus facialis, (IX) Nervus glossopharyngeus and (X) Nervus vagus (Harder, 1964). Roman numbers in parentheses are those of the cranial nerves. Nervus lateralis anterius (V, VII).—The anterior lateral line nerve is divided into three trunks that give rise to several branches, all innervating neuromasts on the head. 1. Truncus supraorbitalis (V): innervates neuromasts of the snout (part), along upper margin of orbit and anterior nape: rows n, o, p, s1 and s2. 2. Truncus infrorbitalis (V): innervates neuromasts of the snout (part) and the cheek 1 2 3 via ramus buccalis: rows a, c, c , c , c1, c2, cp, r and s . 3. Truncus hyomandibularis (VII): innervates neuromasts of the lower jaw and rear cheek via a short nerve (row b), ramus buccalis accessorius (rows d and e), ramus mandibularis externus (rows f, i1 and i2; innervation of f was not discernible), ramus opercularis superficialis (i3 and z) and ramus hyoideus (i3, in part); r. opercularis superficialis also innervates the opercle (rows ot, os, oi). 4. Ramus oticus (VII): innervates neuromasts rear of eye (u1). Nervus lateralis posterius (IX, X).—The posterior lateral line nerve gives rise to two branches, the first innervates neuromasts on the head and the second on the body and caudal fin. 1. Ramus supratemporalis (IX): innervates neuromasts of the mid-nape to upper opercular groove: rows g, m, u2 and x; rows q, trp absent. 2. Ramus lateralis posterius (X): this ramus gives rise to a series of branches and innervates neuromasts on the predorsal area, body and caudal fin.