Siphamia Weber

Gon, Ofer & Allen, Gerald R., 2012, 3294, Zootaxa 3294, pp. 1-84 : 7-10

Genus Siphamia Weber

Neoscopelus Castelnau 1875: 46 (type species, Scopelus cephalotes Castelnau , by monotypy; preoccupied by Neoscopelus Johnson

1863). Beanea Steindachner 1902: 337 (type species, Beanea trivittata Steindachner , by monotypy; suppressed name, ICZN, Opinion 1481). Siphamia Weber 1909: 168 (type species, Siphamia tubifer Weber , by monotypy). Adenapogon McCulloch 1921: 132 (type species, Apogon roseigaster Ramsay and Ogilby , by original designation). Scopelapogon Whitley 1933: 74 (type species, Adenapogon woodi McCulloch , by original designation; replacement name for

Neoscopelus Castelnau 1875 ). Fodifoa Whitley 1936: 26 (type species, Foa fistulosa Weber , by original designation and monotypy).

Diagnosis: Dorsal rays VI–VII + I,7–11; anal rays II,7–11; pectoral rays 11–16; lateral line scales 0–24, rarely with 1–2 scales beyond hypural plate; median predorsal scales 0–6; gill rakers 2–6 + 7–16; developed gill rakers 0–3 + 6–16 = 6–18; gill rakers on ceratobranchial 6–10. Supraneurals 1 or 2; uroneurals and basisphenoid absent; two epurals; and hypurals 1–2 and 3–4 fused into two plates. Body short and compressed to relatively slender, its depth 2.2–4.8 in SL and width 1.6–2.8 in the depth; eye large to moderate its diameter 2.5–4.2, and snout moderate to short its length 3.7–9.85, both in head length; pectoral and pelvic fins moderately long, 3.7–6.7 and 3.8–6.4 in SL, respectively; caudal peduncle stout to somewhat slender, its depth 1.15–2.8 in its length. Preopercular edge smooth to fully serrate with up to 38 serrations; preopercular ridge smooth. Scales usually spinoid, sometimes cycloid or ctenoid. Lateral-line scales usually with a vertical row of free neuromasts. Bacterial bioluminescent system present.

Colour in life: unknown for most species, but available underwater photos show variable patterns. Body translucent, usually revealing silvery peritoneum, often with purplish tinge, and with variable amount of dark brown to black or orange to reddish brown chromatophores, or both, which appear as dots and are referred to as such in both the live and preserved colour descriptions; dark dots may expand and coalesce to form 1–3 dark lateral stripes, or irregular markings and blotches on body; orange or reddish brown dots may join into dashes or concentrate along scales margins; fins pale or with variable amount of dark dots along spines of first dorsal and pelvic fins; orange-brown dots frequently present on all fins; bioluminescent system consisting of ventral silvery area from below pectoral-fin base gradually tapering to caudal peduncle and edged in black, at least dorsally.

Colour in alcohol: similar to colour in life, but without orange-brown pigment; bacterial bioluminescent system manifested externally by silvery or pale area along ventral edge of body with dark vertical to slanted striations, or with or without dark dots.

Remarks: The osteology of this genus was described by Fraser (1972) and the structure of the cephalic lateralis system by Rodman-Bergman (2004). In species with an incomplete lateral line one or more scales with vestigial tubes may be present posterior to the last tubed scale. Specimens of several species had cysts of myxosporidians on the caudal fin, usually along the rays’ edges ( Fig. 1a). These parasites are host-specific (Horst Taraschewski, University of Karlsruhe, Germany, pers. comm.) and therefore potentially useful for host species identification.

Weber (1913: 244–246) described the light organ of Siphamia as “a tubular extension of the peritoneum” and suggested that it could be a buoyancy adaptation characteristic of deep-water apogonids. Comparing Adenapogon with Siphamia, McCulloch (1921: 132) disagreed with Weber (1913) and apparently was the first to associate this structure with light emission which he referred to as phosphorescence. In the years that followed several detailed studies were done on the structure and development of the bioluminescent system of Siphamia ( Iwai 1958, 1960, 1971; Matsubara and Iwai 1958; Haneda 1965; Leis and Bullock 1986; Dunlap et al. 2009). These studies described a single luminous organ found in the body cavity above the pelvic girdle and connected to the gut. Recently, Fishelson et al. (2005) found that luminous bacteria are also present in the anterior end of the paired sacs extending from the gular area into the mouth cavity, i.e. on each side of the tip of the tongue ( Fig. 2a). They also observed oral luminescence in feeding specimens at night and, consequently, suggested that there are two luminous organs in Siphamia . However, they did not investigate how the bacteria get to the tip of these sacs and how the bacterial population is maintained.

The light organ of Siphamia has two different colour patterns. The better known pattern consists of dark striations on the ventral silvery area of the fish ( Fig. 2b). In the second pattern the dark striation is replaced with a variable amount of irregular dark dots ( Fig. 2c). Some preserved specimens have no dark pigmentation on the abdominal and caudal parts of the light organ, but in such cases the original pattern usually can be seen in the gular area under the gill cover. The dotted light organ apparently was overlooked by most past authors even though it is clearly shown on the illustration of Apogon tubulatus in Weber (1913). Lachner (1953) and Tominaga (1964) described and illustrated this pattern for S. elongata and the striated pattern for the other species they examined, but they evidently did not realize the taxonomic importance of this character. The light organ proved to be a useful character for separating species mostly in the S. tubulata species group. Its growth rate and the extent of its reach posteriorly on the caudal peduncle differed between certain species of this group ( Fig. 3a–c). Moreover, we found sexual dimorphism in this character in at least one species ( Fig. 3c). Lachner (1953: 414–416) reported sexual dimorphism in size and several morphometric characters in some species, but his samples were small, usually less than 10 specimens of each sex per species. The striated and dotted colour patterns of the light organ are also found in other fishes with light organs. For example, species of the perciform genus Acropoma have a dotted light organ (O. Gon, personal observations) and species of the beryciform genus Paratrachichthys have a striated light organ ( Gon 1983). Siphamia is apparently exceptional among all bioluminescent percomorph fishes in having both colour patterns in one genus, an intriguing situation. But then the family Apogonidae is unique in Percomorpha in having at least four structurally different and two functionally different kinds of luminous systems, all in one family.

While examining material for this study the first author came across specimens of many species in which the tongue was covered by a whitish tissue of spongy texture ( Fig. 1b). Fishelson et al. (2005) described this tissue as “epithelium rich in mucus-producing goblet cells” that they had found in males and females as well as in mature and immature specimens of S. permutata and S. cephalotes . They postulated that this tissue plays a role in feeding by trapping planktonic organisms attracted to light in the mouth cavity in the mucus on the tongue. We observed this tissue in many specimens of most Siphamia species , but not in the Red Sea specimens of S. tubifer (= S. permutata of Fishelson et al. 2005). We found this tissue in about 30% of the specimens examined and, overwhelmingly, in sexually mature or maturing males (determined visually); we observed it only in one sexually mature female which had large gonads and eggs ready for spawning. If this tissue were to be involved in feeding associated with the oral luminescence one would expect it to be present in all fish of both sexes from the moment they start using this light organ. Since this was not the case an alternative explanation could be a disease, or infection, to which males are more susceptible than females.

Weber (1909) described four species with a silvery stripe along the ventral margin of each side of the body in three different genera, including Apogon , Foa and Siphamia . The first of these was Apogon argyrogaster that Weber named with reference for this characteristic silvery stripe. This was probably the primary reason that several recent authors (Paxton et al. 198; Allen 1999; Hutchins 2001) placed this species in Siphamia . Other characters that A. argyrogaster shares with several species of Siphamia that support this allocation include 15 pectoral-fin rays and 1+8 developed gill rakers. Although A. argyrogaster shares similarities with A. fuscomaculatus and A. photogaster described by Allen and Morrison (1996) and Gon and Allen (1998), respectively, these authors overlooked a possible relationship with Weber’s species due to it’s placement in Siphamia . Instead, these authors indicated a close relationship between their new species and members of Jaydia (sensu Gon 1996) . Eschmeyer (2009) considered A. photogaster and A. fuscomaculatus junior synonyms of A. argyrogaster . The second author [Allen] observed A. argyrogaster and A. photogaster underwater and noticed differences in colour pattern (presence or absence of large dark spots, respectively) as well as habitat differences. Apogon argyrogaster was collected from an open rubble bottom in 25 m depth in turbid water in an area of strong current, in contrast to the lagoon coral reef habitat of A. photogaster .