THE LATERAL LINE SYSTEM OF TWO SYMPATRIC EASTERN PACIFIC GOBIID FISHES OF THE GENUS LYTHRYPNUS (TELEOSTEI: GOBIIDAE)

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1 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 sympatrically 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 differences 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 complex) 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 sympatrically along the coast of southern California, Baja California (Mexico) and adjacent offshore 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 arranged 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 microhabitat 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 species 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 specimens, 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 2004 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 31

2 32 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 MATERIAL Lythrypnus dalli. Fifty-nine specimens. CAS ; 14 females, , seven males, , four sex?, , two juveniles, ; Mexico, Baja California Sur, Gulf of California, Isla Partida. CAS ; two juveniles, ; Mexico, Gulf of California, Punta Penasco. CAS ; one female, 20.8, one sex?, , U.S., California, Santa Catalina Island. CAS ; 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 C; two females, , seven males, , seven sex?, , nine juveniles, ; Mexico, Baja California Norte, Isla Guadalupe. Lythrypnus zebra. Fifty-six specimens. CAS 25388; nine males, , 15 sex?, , 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 H A; one female, 29.4, six males, , 12 sex?, , six juveniles, ; Mexico, Baja California Norte, Isla Guadalupe. SIO ; one male, 33.0; Mexico, Baja California Norte, Isla Guadalupe. SIO B; one juvenile, 11.3; Mexico, Baja California Norte, Isla Guadalupe. COMPARATIVE MATERIAL (NEUROMAST PATTERN) Lythrypnus cobalus. Five specimens. CAS ; two females, , 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, , one female, 17.5, five sex?, , two juveniles, ; Ecuador, Galápagos Islands, Isla San Salvador. SIO ; one male, 22.0; Ecuador, Galápagos Islands, Isla Fernandina. Lythrypnus pulchellus. Fifteen specimens. CAS 18097; one male, 19.1; Mexico, Baja California, Agua Verde Bay. CAS 18458; one male, 20.1; Mexico, Sonora, outer San Carlos Bay. CAS 66901; two males, ; Mexico, Baja California, Isla Partida. SIO F; three males, ; Mexico, Baja California Sur, north of Punta Pulpito. SIO B; three males, ; Mexico, Baja California Sur, Isla del Espiritu Santo. SIO B; three males, , two sex?, ; Mexico, Baja California Norte, Bahia de Los Angeles. Lythrypnus rhizophora. Eleven specimens. CAS 50078; two males, , one female, 16.9, three sex?, , four juveniles, ; Ecuador, Galápagos Islands, Isla de Genovesa. SIO ; 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 applicable 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 homologies should be supported by innervation. The innervation of the neuromasts was investigated in specimens cleared and stained for osteological 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-

3 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, s 1 and s Truncus infrorbitalis (V): innervates neuromasts of the snout (part) and the cheek via ramus buccalis: rows a, c, c 1, c 2, c 1, c 2, cp, r and s 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, i 1 and i 2 ; innervation of f was not discernible), ramus opercularis superficialis (i 3 and z) and ramus hyoideus (i 3, in part); r. opercularis superficialis also innervates the opercle (rows ot, os, oi). 4. Ramus oticus (VII): innervates neuromasts rear of eye (u 1 ). 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, u 2 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. The two anteriormost branches innervate trunk neuromasts, which are often included in the description of the cephalic lateral line system in gobiid fishes Ramus dorsalis: innervates neuromasts of the predorsal area: row h and, in L. zebra, row y First Ramus lateralis subcutaneus: innervates neuromasts of the shoulder: axillary rows as 1 3 and la 2 3. The innervation of the other series of trunk neuromasts (ld, lm, lv) and the neuromasts on the caudal fin (lc) were not discernible. The innervation of the cephalic neuromasts of both Lythrypnus species differs from the description of Wongrat and Miller (1991) for eleotrids (latter condition given in parentheses) in (1) suborbital row b is innervated from the anterior arm of the truncus hyomandibularis (by the ramus buccalis acessorius), (2) preopercular neuromasts i 3 by the ramus opercularis superficialis or the lower neuromast by the ramus hyoideus (all i neuromasts by the ramus mandibularis externus), and (3) the anterior section of the oculoscapular row u by the ramus oticus (entire row u by the ramus supratemporalis). The differences in the innervation are discussed below. To date, the innervation of the lateral line systems of only a few eleotrids and gobiids is known in detail. Thus, it is not

4 34 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Figure 1. Lythrypnus dalli, CAS , male, 24.5 mm. Innervation of the neuromasts of the head. RB, ramus buccalis; RBA, ramus buccalis acessorius; RD, ramus dorsalis; RH, ramus hyoideus; RLP, ramus lateralis posterius; RME, ramus mandibularis externus; ROS, ramus opercularis superficialis; RS, ramus supratemporalis; TH, truncus hyomandibularis; TS, truncus supraorbitalis. Arrows = neuromasts of i 3, innervation of dorsal neuromast not discernible in this specimen. an, pn = anterior and posterior nostrils. Neuromast of s 3 on opposite side of anterior nostril. Scale bar = 5 mm. clear if these differences in the innervation of these two lineages of Gobioidei are of phylogenetic significance. TOPOGRAPHY OF THE LATERAL LINE SYSTEM OF LYTHRYPNUS DALLI (FIGS. 1 6, TABLES 1 AND 2) All nine series of neuromast rows listed by Sanzo (1911) are developed in both species (Tables 1,2). The neuromast pattern does not vary individually, and the numbers of neuromasts per row between individuals are constant. Cephalic canals. Absent. Free neuromasts (= sensory papillae) (Figs. 1 6, Tables 1, 2). On the head, trunk, and caudal fin; neuromasts comparatively large; rows represented by few neuromasts or by a single neuromast only. Head. Preorbital. Median series in four rows represented by a single neuromast each, r and s 1 3 ; r behind posterior nostril, and median to the s-series; s 1 median and anterior to posterior nostril, s 3 at tip of snout close to upper lip. Lateral series in four rows, c 2, c 1, c 2 and c 1 ; c 2 as a single longitudinal row approximately between anterior and poste-

5 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 35 Figure 2. Lythrypnus dalli, SIO C, male, 18.9 mm. Innervation of the preopercularmandibular row i 3 (indicated by arrows), the oculoscapular row z and the opercular rows oi, os and ot by the ramus opercularis superficialis (ROS). RH, ramus hyoideus; TH, truncus hyomandibularis. Scale bar = 1 mm. rior nostrils, c 1, c 2 close to upper lip, and c 1 as a short transverse row between margin of eye and origin of suborbital neuromast row d 1. Suborbital. Longitudinal, suborbital, neuromast type. Five series on cheek, four longitudinal rows, a, b, c, cp and d; rows a and c along lower edge of orbit, overlapping where they meet; row b of two small neuromasts below posterior part of eye; row cp a single neuromast in ventral elongation of anteriormost a and posteriormost c neuromasts, thus the ventral end of a short transverse row; longitudinal row d divided into anterior (d 1 ) and posterior (d 2 ) sections; d 1 short from below c 1 parallel to upper lip, not exceeding corner of mouth posteriorly, last neuromast below first of c; d 2 shorter, below center of eye, posteriorly not extending through a vertical elongation of cp. Lythrypnus zebra. More neuromasts in longitudinal row d. Gobioid fishes can be grouped either in transverse, or in longitudinal suborbital neuromast types (Aurich, 1938; Miller, 1973; Hoese, 1983), which seemingly have arisen one from the other several times, and independently (Miller, 1973). The arrangement of these rows in species of the genus Lythrypnus Jordan and Evermann, 1896 is similar to the cheek pattern of small gobiids of the tropical Pacific, e.g., Eviota Jenkins, 1903, Trimma Jordan and Seal, 1906, Pandaka Herre, 1927, Trimmatom Winterbottom and Emery, 1981 and in part, Priolepis Valenciennes (ex Ehrenberg) in Cuvier and Valenciennes, 1837 (Akihito et al., 1984: Figs , 72 73, 171 Akihito et al., 2000: figs ). In

6 36 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Figure 3. Lythrypnus zebra, CAS , sex?, 34.4 mm. Innervation of the neuromasts of the head. Abbreviations as in Figure 1. Arrows = neuromasts of i 3. Scale bar = 5 mm. addition, Eviota shows a similar abbreviated type (Miller, 1987) of neuromast pattern on the head, and has a similar suborbital papillae pattern (Akihito et al., 1984: figs ; Akihito et al., 2000: figs ). This resemblance may be viewed as convergent simplification due to small size and similar life styles. Difficulties in assigning species with a very reduced number of free neuromasts to either the transverse, or longitudinal pattern have been mentioned by Hoese (1983). This is also the case in Lythrypnus. Winterbottom and Burridge (1992) for Priolepis (in part) and Hoese and Gill (1993) for Thalasseleotris Hoese and Larson, 1987 introduced the term reduced transverse pattern for a similar type of suborbital neuromasts, a longitudinal pattern, which may have secondarily originated from a transverse state. Secondary loss of transverse rows (McKay and Miller, 1997), or most of them may be due to large eyes and narrow cheek depth. This may also be the case for Lythrypnus, but see oculoscapular rows below. A large orbit may also result in the shift of row b to behind the lower border of the eye. Row b reaching anteriorly below the eye is seemingly the plesiomorphic state (McKay, 1993). This backward shift of row b in the two Lythrypnus species is especially evident from the innervation. The small, middle neuromast of the short transverse suborbital series is innervated from the ramus buccalis, thus not part of row b (for innervation of this row see below). But the transverse arrangement of these neuromasts is also not the remnant of a former long transverse row because they are not innervated by a single twig of the r. buccalis. Each neuromast is directly innervated by a separate branch of this ramus (Figs. 1, 3). Seemingly, due to the large eye and narrow cheek depth, three rows have been shifted

7 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 37 Table 1. Numbers of neuromasts in the head lateral line system of L ythrypnus dalli. Isla Partida, Gulf of California SL mm * ** n = 27, 26, 25 Isla Guadalupe SL mm * n = 25, 24 Isla Partida Isla Guadalupe PO PO OS OS r 1 1 r 1 1 x 3 3 x s s u1 2 2 u1 2 2 s s u u s s z z c c y y c c as as c c as 2 3 * 2-3 * as c c as * as IO IO la 2 1 * 1 * la p 4 4 p la 3 1 * 1 * la SO SO OP OP a a 3 3 ot ot b b os os * c c oi oi * cp 1 1 cp 1 1 AD AD d d n 1 1 n 1 1 d d o 1 1 * o 1 1 PM PM g g e e m 1 1 * m 1* 1 e e h 3 * 3 * h i i i i i i f 2 2 f 2 2 close together below the orbit to form this short transverse row: from dorsal to ventral rows a, c and cp. Reduced suborbital rows have been described also from other gobiids with large eyes (Miller, 1987). In the nomenclature of the suborbital neuromast pattern we follow Miller (1987: figs. 6,8) and Larson (1999a: fig. 15; 1999b: fig. 2), as e.g., for Mugilogobius Smitt, 1900, Brachygobius Bleeker, 1874, Pandaka Herre, 1927, Hemigobius Bleeker, 1874, or Calamiana Herre, However, the terms longitudinal and transverse only refer to types of neuromast patterns and do not necessarily implicate close relationships. After passing through the foramen hyomandibularis in the hyomandibula, the truncus hyomandibularis splits into two rami. The posterior branch gives rise to the ramus opercularis superficialis and the ramus hyoideus. The anterior arm gives rise to the ramus buccalis accessorius, the ramus mandibularis externus, and before splitting into these two branches, to a twig, which innervates the longitudinal row b (Figs.1,3). This is different to eleotridins, in which this cheek row is innervated from the r. buccalis accessorius (Wongrat and Miller, 1991). Gobiinae (sensu Pezold 1993) from the northeastern Atlantic and the Mediterranean (see Appendix) exhibit an innervation of row b similar to that of

8 38 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Table 2. Numbers of neuromasts in the head lateral line system of L ythrypnus zebra. Santa Catalina Island SL = mm * ** n = 25, 24, 23 Isla Guadalupe SL = mm * ** n = 25, 24, 23 Santa Catalina Island Isla Guadalupe PO PO OS OS r 1 1 r 1 1 x x s s u1 2 2 u1 2 2 s s u u s s z z 4-6 * 4-6 c c y 1 1 y 1 * 1 c c as as c c as as * 1-3 c c as as * IO IO la la * 1-3 p 4 4 p la la * SO SO OP OP a 3 3 a ot ot b 2 2 b 2 2 os os c c oi 3-4 * 3 oi cp 1 1 cp 1 1 AD AD d d n 1 1 n 1 1 d d o 1 * 1 o 1 1 * PM PM g 2 2 g e e m 1 1 m 1 1 e e h h i i i i i i f f the two Lythrypnus species, i.e., from a nerve rising from the anterior arm of the truncus hyomandibularis before it splits in the r. buccalis accessorius and the r. mandibularis externus (Ahnelt, unpubl.). Preopercular-mandibular. Three longitudinal rows, external e on lateral edge of lower jaw and preopercle, internal i medial, and mental f; papillae of i larger than of e; external series shorter than internal; rows e and i divided in mandibular (e 1, i 1 ) and preopercular (e 2, i 2, i 3 ) sections by a distinct gap at the lower jaw articulation; both preopercular sections are positioned more medial, thus usually e 2 lies in the extension of i 1 and i 2 internal to i 1 ; papillae of i 1 proceed in a groove, the rest of a mandibular canal; in dorsal extension of i 2 two neuromasts in course of the lost preopercular canal, the dorso-preopercular section, which we name in numerical order i 3 (i 1 of Miller (1963), por of Herler et al. (1999); figs. 1,3,4). Mental row f short, in a groove posterior of the mandibular symphysis and distinctly separated from the anterior ends of e 1 and i 1. Row i replaces the mandibular (i 1 ) canal and the horizontal preopercular section (i 2 ) of the preopercular canal. In basal gobioids, the preopercular canal extends anteriorly along the horizontal section of the preopercle (e.g., Rhyacichthys aspro (Valenciennes, in Cuvier and Valenciennes, 1837), Bostrychus sinensis Lacepede, 1801 (Miller, 1973; Akihito,

9 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 39 Figure 4. Neuromast patterns of the head in dorso-lateral view. A) Lythrypnus dalli, CAS , female, 23.8 mm. B) Lythrypnus zebra, CAS 25388, male, 18.2 mm. an, pn: anterior and posterior nostrils. Arrows = neuromasts of i 3. Scale bar = 1 mm.

10 40 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Figure 5. Neuromast patterns of trunk and caudal fin in lateral view. A) Lythrypnus dalli, CAS , male, 27.7 mm. B) Lythrypnus zebra, CAS 25388, male, 22.0 mm. Anteriormost scale row and scales at origin of caudal fin shown. AN, anus; UG, urogenital papilla. Scale bar = 2 mm. 1986; Wongrat and Miller, 1991). This canal is reduced to its vertical section in most other gobioid fishes. Many gobiid taxa have a further reduced head canal system by loss of canal sections, or even the complete canal system, which is generally replaced by free neuromasts (e.g., Miller, 1973; Akihito et al., 1984; Takagi, 1988; Akihito et al., 2000). The papillae of i 1 and i 2 are such replacement neuromasts (Wongrat and Miller, 1991), but those of i 3 [= dorso-preopercular division of the preoperculo-mandibular line of Takagi (1988)] could represent a paedomorphic condition with primary neuromasts not enclosed in a canal. This is supported by: (1) their position, which is about the same as it would be within a preopercular canal (Afzelius, 1956; fig. 1; Herler et al., 1999; fig. 6C); (2) they are distant from the dorsal end of i 2 and more widely spaced than the neuromasts of this row; and (3) they are innervated by the ramus opercularis superficialis (both neuromasts) and the ramus hyoideus (only ventral neuromast) and not by the ramus mandibularis externus as it is characteristic for i 2 (Figs. 1 3). According to Lekander (1949), primary neuromasts, which remain superficial, are homologous to canal neuromasts. Thus, the two neuromasts (i 3 ) on the vertical section of the preopercle should have the same innervation as the neuromasts in the preopercular canal of Gobiidae. This is the case in species

11 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 41 Table 3. Numbers of neuromasts in the head lateral line system of of juvenile and adult specimens. Lythrypnus dalli. Comparision Juvenile SL n = 17 Adult SL n = 20 Juvenile Adult PO PO OS OS r 1 1 r 1 1 x 3 3 x s s u1 2 2 u1 2 2 s s u u s s z z c c y y c c as as c c as as c c as as IO IO la la p p 4 4 la la SO SO OP OP a 3 3 a ot ot b b 2 2 os os c c oi oi cp 1 1 cp AD AD d d n 1 1 n 1 1 d d o 1 1 o 1 1 PM PM g g e e m 1 1 m 1 1 e e h 3 3 h 3 3 i i i i i i f 2 2 f 2 2 examined by Afzelius (1956; fig. 1) and by one of us (H.A.): both neuromasts of the preopercular canal are innervated by the r. opercularis superficialis, or by the r. opercularis superficialis (dorsal neuromast) and the r. hyoideus (ventral neuromast), and is in accordance with various euteleosts. In the latter, the neuromasts of the horizontal section of the preopercular canal are innervated by the r. mandibularis externus, but those of the vertical section are supplied by the r. opercularis superficialis, the r. hyoideus or directly from the truncus hyomandibularis (Haller v. Hallerstein, 1934; Lekander, 1949; Jakubowski, 1963, 1966, 1967; Freihofer, 1978). In an eleotrid species (Perccottus glenii Dybowski, 1878) with i 2 extending along the entire preopercle, all neuromasts, even the uppermost, are innervated by the r. mandibularis externus (Wongrat and Miller, 1991). This suggests that the preopercular canal and the canal neuromasts are replaced by superficial neuromasts. Row i along the entire length of the vertical section of the preopercle has been described by Sanzo (1911; plate 9, figs. 1, 6), Wongrat and Miller (1991; fig. 1) and figured by Barlow (1961; fig. 1) and Akihito et al. (1984; figs , 47, 114, , 148), but seems to be rare among Gobiinae (sensu Pezold, 1993; e.g., Stonogobius xanthorinica Hoese and Randall, 1982, Vanderhorstia lanceolata Yanagisawa, 1978). In the majority of gobiids without a preopercular canal and i 2 not extending along its entire length, two

12 42 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Figure 6. Arrangement of the neuromasts of ld 3 on the caudal peduncle between second dorsal and caudal fins in A) Lythrypnus dalli and B) Lythrypnus zebra. Neuromasts shown somewhat enlarged. neuromasts (i 3 ) occur at the rear edge of the preopercle (Akihito et al., 1984; Miller, 1986; Herler et al., 1999; Scsepka and Ahnelt, 1999; Akihito et al., 2000). Oculoscapular. Four longitudinal (u, x, la 2 3 ) rows and four transversal (z, as 1 3 ) rows, including the axillary series; u divided, proceeding from posterior edge of orbit (u 1 ) to above opercle (u 2 ); x dorsal and parallel to u 1, with a short anterior section plus a single papilla close to origin of opercle; z short, dorsal and slightly posterior to the dorsal preopercular row i 3 ; transverse axillary rows as 1 (dorsal of the origin of the opercular membrane), as 2 (above the dorsal origin of pectoral fin) and as 3, latter two associated with la rows dorsally; transverse rows q, tr and y lacking. Lythrypnus zebra. y present as a single neuromast immediately anterior to as 1, forming the anterior tip of a triangle with the two neuromasts of this axillary row (Figs. 3,4B). All rows are very short or reduced to one neuromast only. A posterior section (x 2 ) of row x is seemingly lacking. The posteriormost neuromast of row x lies anterior to the opercle and to the branch of the ramus supratemporalis, which innervates the occipital rows g and m. In gobiids a distinct section, x 2, lies generally dorsal to the opercle (e.g., figures of Miller, 1986; Gill et al., 1992) and posterior to the branch of the ramus supratemporalis (Wongrat and Miller, 1991: Figs. 2,4,6). The lack of x 2 seemingly cannot be correlated with the small size of the Lythrypnus species. It is present in several small, but not necessarily related, Indo-Pacific gobionelline and gobiine genera as e.g.,

13 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 43 Figure 7. Neuromast patterns of the head in dorso-lateral view. A) Lythrypnus gilberti, CAS 39236, male, 17.5 mm. B) Lythrypnus rhizophora, CAS 50078, male, 22.3 mm. an, pn: anterior and posterior nostrils. Arrows = neuromasts of i 3. Scale bar = 1 mm.

14 44 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Brachygobius, Pandaka (Miller, 1987: fig. 8), Eviota and Priolepis (Lachner and Karnella, 1978: fig. 2; Akihito et al., 1984: figs , 72 74; Akihito et al., 2000: figs , ), although apparently lacking in Trimma (Akihito et al., 1984: fig. 59; Akihito et al., 2000: figs ; Takagi, 1988: fig. 13c,d). In gobiids lacking postorbital sections of the oculoscapular canal, these are replaced by transverse (q, tr) and/or longitudinal (u) rows of neuromasts. According to Wongrat and Miller (1991), transverse row series (q, tr) are lacking in species of the longitudinal type. These rows are also lacking in the Lythrypnus species and may support the assumption that their neuromast arrangement is abbreviated longitudinal and possibly did not derive from a former transverse pattern. The longitudinal neuromast row (u) is generally developed as a more or less continuously row in gobiids (Sanzo, 1911: figs. 1 2, 4 5; Miller, 1986; Wongrat and Miller, 1991: fig. 1; Larson, 2000: fig. 5), and is entirely innervated by the posterior lateral line nerve (ramus supratemporalis) in an eleotridine species, Perccottus glenii, (Wongrat and Miller, 1991). In both Lythrypnus species the anterior two neuromasts of this row (u 1 ) are innervated by the anterior lateral line nerve (ramus oticus), as in the two neuromasts in the otic canal of gobiids (Afzelius, 1956; Ahnelt, unpubl.; figs. 1,3). Opercular. Three rows, one transversal (ot) and two longitudinal rows (os and oi); ot long, ventrally reaching close to ventral edge of subopercle; os short, oblique; oi short. Anterior dorsal (occipital). Three generally longitudinal (g, m and h) and two generally transversal rows (n and o), but n, o and m each represented by a single neuromast only; n opposite posterior most neuromast of p; o, g, and m as a series of neuromasts extending rearwards and lateral, last papilla (m) relatively close to anteriormost neuromast of h; o median and anteriormost, m lateral and posteriormost; g as a short longitudinal row between o and m; h with two neuromasts immediately anterior to origin of first dorsal fin and a third one distinctly antero-lateral and in elongation of the o-g-m rows. Lythrypnus zebra. Row h only as the two neuromasts close to the origin of the first dorsal fin and distinctly separated from row m. An isolated neuromast of L. dalli, which lies in the vector of the o-g-m rows (Figs.1, 4A) is assigned to row h because, like the two other papillae of h, it is innervated by the ramus dorsalis, a dorsal ramification of the ramus lateralis subcutaneus of the posterior lateral line nerve. The arrangement of the rows n, o, g, and m differs somewhat from descriptions of Sanzo (1911), but is still in accordance with his nomenclature. If one prefers to follow Takagi (1988), n + o correspond to the anterior line (at) and g + m to the medial division of the middle line (mdm). Row h is not considered by Takagi (1988; Fig. 2, Table 4). Interorbital. Longitudinal row p bilateral, along dorsal margins of both eyes; from posterior and lateral of preorbital row r rearwards to opposite of occipital row n. This row replaces the supraorbital canal. Trunk and caudal fin. Trunk. Three series of longitudinal and transversal rows (ld, lm, and lv); if not explicitly mentioned, each row is positioned on a single scale; position and number of rows in each series in a very constant pattern (Fig. 5A), thus each single row is numbered. Dorsal lateral series ld in three rows close to dorsal midline; first (ld 1 ) as short transverse row below midline of first dorsal fin; second (ld 2 ) as a single neuromast approximately below origin of second dorsal fin; third (ld 3 ) as longitudinal row of three neuromasts on caudal peduncle, each on a single scale (Figs. 4A, 5A); it is the only longitudinal row on the trunk and the only one extending over more than one scale. Me-

15 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 45 Table 4. Numbers of neuromasts of juvenile and adult specimens. in the head lateral line system of Lythrypnus zebra. Comparision Juvenile SL n = 13 Adult SL n = 20 Juvenile Adult PO PO OS OS r 1 1 r 1 1 x x s s u1 2 2 u1 2 2 s s u u s s z z c c y 1 1 y 1 1 c c as as c c as as c c as as IO IO la la p 4 4 p 4 4 la la SO SO OP OP a 3 3 a 3 3 ot ot b 2 2 b 2 2 os 3 3 os c c oi oi cp 1 1 cp 1 1 AD AD d d n 1 1 n 1 1 d d o 1 1 o 1 1 PM PM g 2 2 g 2 2 e e m 1 1 m 1 1 e e h h i i i i i i f f dian lateral series lm with (18: 29, 19: 7, 20: 1) transverse rows, a few represented by a single neuromast only; lm 1 always as a single neuromast and usually one scale row below lateral midline; lm 2 below ld 1 and two three scales posterior to lm 1 ; lm 3 five nine scales posterior to lm 2, below origin of second dorsal fin; these two rows (lm 1 2 ) are separated from each other by a distinct gap; lm 3 to last lm a continuous series of short rows from below second dorsal fin to the rear of the caudal peduncle; these rows are separated by one scale from the preceding row except of lm 7 and lm 8 ; usually three scales without neuromasts between these two rows, forming a small posterior gap in the lm series; lm 5 and lm 12 a single neuromast each; last lm displaced one scale row ventrally and posteriorly; no lm-neuromasts on scales on caudal fin. Ventral lateral series lv in three short transverse rows on ventral side of the abdomen; lv 1 below ventral origin of pectoral fin, lv 3 above anus, lv 2 near to lv 3 ; lv 1 and lv 3 close to ventral midline, lv 2 somewhat higher positioned. Lythrypnus zebra. The dorsal lateral row ld 3 on two scales only and the three neuromasts arranged in the shape of a triangle (Figs. 4B, 5B); median lateral series from lm 3 to the last lm each on a successive scale except for lm 7 lm 8, separated by two scales

16 46 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 Figure 8. Neuromast patterns of trunk and caudal fin in lateral view. A) Lythrypnus gilberti, CAS 39236; one female, 17.5 mm. B) Lythrypnus rhizophora, CAS 50078; one sex?, 24.3 mm. Scale bar = 2 mm. without neuromasts. As in L. dalli, the vast majority of specimens with 18 lm rows (18:31, 19:2, 20:1). Caudal fin. Three longitudinal (lcd, lcm, lcv) and one transverse (lct) rows (Fig. 5). Longitudinal rows on interradial membranes extending close to posterior edge of caudal fin; anteriormost neuromasts in each row largest, gradually decreasing in size from about mid distance rearwards; typically lcd between third and fourth (3/4) branched caudal fin rays, lcm between 6/7 and lcv between 8/9; former two close to the dorsal, latter close to the ventral fin rays; in a few specimens an additional branched fin ray may occur dorsally, thus each row is placed in these individuals between the fin rays 4/5, 7/8 and 10/11. Transverse row lct on interradial membranes from the sixth to the tenth branched caudal fin rays, immediately posterior to last scale row and anterior to the three longitudinal neuromast rows; typically lct starts dorsally with two longitudinal-arranged neuromasts anterior to lcd and one neuromast each anterior to lcm and lcv; it ends ventrally with one neuromast on the interradial membrane between ninth and tenth branched caudal fin rays. Transverse rows on the caudal fin of gobioid fishes have been rarely described (Sanzo, 1911; Mortara, 1918; Miller and Wongrat, 1979; Ahnelt et al., 2000; Ahnelt and Duchkowitsch, 2001), but seem to occur in many gobioid taxa (Ahnelt, unpubl.).

17 AHNELT AND BOHACEK: LATERAL LINE SYSTEM OF GOBIID FISHES 47 DISCUSSION Macdonald (1972), who figured a stylized, combined, and partly incomplete head neuromast pattern for L. dalli and L. zebra, correctly mentioned that both species are very similar in the arrangement of the neuromasts. This is very likely based on a close relationship of both species, and due to small size, an abbreviated neuromast pattern. Contrary to Macdonald (1972), several neuromast features may be used to separate the species. Besides having a slightly lower number of neuromasts in some rows (Tables 1,2), L. dalli differs from L. zebra (values of latter in parentheses) in four features: (1) two neuromasts of as 1 dorsal of posterior margin of opercle, row y lacking [three neuromasts arranged in triangular pattern (y + as 1 )]; (2) row h with three neuromasts, two immediately anterior to the origin of the first dorsal fin, and one neuromast antero-lateral, in elongation of the o- g-m rows and relatively close to m (two neuromasts immediately anterior to the origin of the first dorsal fin, distinctly separated from row m); (3) the three neuromasts of ld 3 in a longitudinal arrangement each on a successive scale (arranged as triangular patch on two scales); and (4) rows of lm series from lm 3 to last lm separated by a single scale without neuromast (lm 3 to last lm on a successive scale; Figs. 1 6). Reduced numbers of neuromasts have been described and/or figured for many gobiids (e.g., Akihito et al., 1984; Miller, 1986, 1987; Takagi, 1988; Herler et al., 1999; Larson, 1999a). A decrease in the number of neuromasts per row is a modification often associated with size reduction (Brownell, 1978). Small body size and a cryptic life style are known as reflected in morphological adaptations and considered specializations (Miller, 1979, 1996). Lythrypnus dalli and L. zebra inhabit shallow, rocky reefs where they display microspatial separation. Lythrypnus dalli occupies a more exposed habitat, such as projections and outcrops; whereas, L. zebra remains hidden under boulders, in crevices, cavities, and caves (Behrents-Hartney, 1989). Despite this microspatial separation, we could not find obvious differences in the topography of the neuromasts, which could be attributed to the more cryptic life style of L. zebra compared with the more exposed of L. dalli. However, the zebra goby has (slightly) larger numbers of neuromasts in some suborbital rows (Tables 1, 2), which are not size-related. Juvenile specimens (<19 mm SL) have the same number per row as adult specimens (>20 mm SL; Tables 3, 4). Yet, both species have very low numbers of neuromasts in nearly all rows. Several rows are represented by a single papilla only. A further reduction as result of a cryptic life style would include the entire loss of neuromast rows and consequently the loss of the function of the lateral line system in entire regions of the head or body. Based on color, meristics, and lack or presence of a predorsal crest, Bussing (1990) grouped the ten species of the genus Lythrypnus of the eastern Pacific into two species complexes, the L. dalli complex (three species) and the L. rhizophora complex (seven species). We compared the neuromast pattern of L. dalli and L. zebra with those of four other Pacific Lythrypnus species: L. cobalus Bussing, 1990, L. gilberti (Heller and Snodgrass, 1903; Figs. 7A, 8A), L. pulchellus Ginsburg, 1938 and L. rhizophora (Heller and Snodgrass, 1903; Figs. 7B, 8B). All six species have a similar arrangement of the free neuromasts on the head, body, and caudal fin. They represent the pit-organ type (neuromast type) of Takagi (1988) (cephalic canals of the lateral line system are lacking) with a reduced number of sensory papillae per row [ abbreviated neuromast type; Miller (1987)]. Lythrypnus cobalus and L. gilberti belong to the L. dalli complex, the two other

18 48 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 species and L. zebra to the L. rhizophora complex. Interestingly the few differences in the arrangement of neuromasts between L. dalli and L. zebra do not only reflect a difference between these two species, but between the two species complexes. All three species of the L. dalli complex share the features: (1) two papillae dorsal of the posterior margin of the opercle; (2) row h with three papillae; (3) row ld 3 on the caudal peduncle as a more or less distinct longitudinal row of three papillae, but always each on a separate scale; and (4) lm series from lm 3 caudad each row separated by a scale without papillae. The three investigated species of the L. rhizophora complex, L. pulchellus, L. rhizophora and L. zebra, display: (1) three papillae dorsal of the posterior margin of the opercle; (2) row h with two papillae and a distinct gap to row m; (3) the three papillae of row ld 3 on the caudal peduncle as a triangular patch on two scales; and (4) lm series from lm 3 caudad each row on a successive scale. The arrangement of the free neuromasts of the lateral line system in the six investigated species of Lythrypnus plus the (incomplete) descriptions of neuromast patterns of additional three species (Bussing, 1990) supports the recognition of two species complexes in the Pacific as proposed by Bussing (1990). Finally, it may be noted that L. gilberti and L. rhizophora occur syntopically in shallow, rocky reefs of the Galápagos Islands where they have a microspatial segregation similar to L. dalli and L. zebra. Lythrypnus gilberti, like L. dalli, is conspicuous and lives a more exposed lifestyle, and L. rhizophora, like L. zebra, is cryptic, living in crevices (R. Rosenblatt, SIO, pers. comm.). Seemingly, the members of each species complex exclude each other from their preferred habitats e.g., L. rhizophora and L. pulchellus (Bussing, 1990), but species of different complexes may occur syntopically, as in L. dalli and L. zebra or L. gilberti and L. rhizophora. ACKNOWLEDGMENTS We thank H. Larson (NTM), F. Pezold (NLU) and R. Winterbottom (ROM) for commenting on an earlier version of the manuscript. We are indebted to B. Eschmeyer, T. Iwamoto, D. Catania (CAS) and to P. Hastings, R. Rosenblatt, C. Klepadlo, H.G. Walker (SIO) for their support and hospitality during our visits and for the loan of material, and to R. Rosenblatt for information on Lythrypnus species from the Galápagos Islands. This study was supported in part by a grant of the International Relations Office, Vienna to V.B. to visit the collections of CAS and SIO. LITERATURE CITED Afzelius, B. A Seitenorgane und Schleimkanalknochen bei Periophthalmus koelreuteri und Gobius minutus. Z. Anat. Entwicklungsgesch 119: Ahnelt, H., A. Abdoli, M. Naderi and B. W. Coad Anatirostrum profundorum: a rare deepwater gobiid species from the Caspian Sea. Cybium 24: and M. Duchkowitsch The lateral line system of two Ponto-Caspian gobiid species (Gobiidae, Teleostei): a comparision. Folia zool. 50: Akihito, P Some morphological characters considered to be important in gobiid phylogeny. Pages in T. Uyeno, R. Arai, T. Taniuchi and K. Matsuura, eds. Indo-Pacific fish biology: Proceedings of the second international conference on Indo-Pacific fishes. Ichthyolog. Soc. Japan, Tokyo., M. Hayashi and T. Yoshino Suborder gobioidei. Pages [English text], plates in H. Masuda, K. Amaoka, C. Araga and T. Uyeno, eds. The fishes of the Japanese Archipelago. Tokai Univ. Press, Tokyo.

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