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Symphysanodon rhax ZBK , new species

Maldives Slopefish

(Figures 9-15; Tables 4, 5, 8, 9)

Holotype: BMNH 2003.1.5.1, 144 mm SL; Maldive Islands, northern Indian Ocean; 04°11’ N, 72°55’ E; 218 m; R/V DR FRIDTJOF NANSEN cruise: Maldives station no. 13; 21 August 1983.

Paratype. BMNH 2003.1.5.2, 134 mm SL, from the same station as the holotype.

Other specimens, not part of the type series. BMNH 1997.9.17:2, one specimen, 64 mm SL; R/V DR FRIDTJOF NANSEN survey of the Maldives, 04°05’ N, 73°23’ E - BMNH 1997.9.17:6-8, three specimens, 128-136 mm SL; R/V DR FRIDTJOF NANSEN survey of the Maldives, 04° 11’ N, 72°55’ E.

Diagnosis. A species of Symphysanodon ZBK (Figure 10) separable from all other species of the genus, except S. berryi ZBK , by the following combination of characters: segmented rays in anal fin 7, tubed lateral-line scales 50, total gillrakers on first gill arch 35-38 (10 or 11 + 25-27), sum of lateral-line scales and gillrakers on individual specimens 85-88, depth of body 20.6-24.8% SL (4.0-4.9 times in SL), length of depressed anal fin 21.8-23.9 % SL, length of anal-fin base 12.6-14.8% SL, length of first anal-fin spine 3.0-4.8% SL, hypurals 1 & 2 autogenous, hypurals 3 & 4 represented by a single plate, and first caudal vertebra without parapophyses. It can be separated from the Atlantic species S. berryi ZBK by a number of morphometric characters (see Table 4 and Figures 11-15). It also differs slightly from S. berryi ZBK in counts of pectoral-fin rays (17 or18, mean = 17.33, for S. rhax ZBK vs. 16-18, mean = 16.99, for S. berryi ZBK ).

Description. The characters included in the combined description of S. mona ZBK , S. parini ZBK , and S. rhax ZBK and those presented in the species diagnosis form part of the species description. Counts for the holotype are indicated by asterisks. Dorsal-fin rays IX, 10.* Pectoral-fin rays 17 or 18*. Procurrent caudal-fin rays 13 or 14 dorsally, 12 or 13 ventrally. Trisegmental pterygiophores 2 or 3* associated with dorsal fin, 2 or 3 with anal fin. Epineurals associated with first 8* or 9 vertebrae (first 9 vertebrae in 5 of 6 specimens). Uroneurals 2 pairs*. Caudal-fin lobes damaged on all available specimens. Internarial distance contained 4-6 times in snout length; other morphometric data are presented in percentages of SL in Table 5.

Comparisons of S. rhax ZBK with other Indian Ocean species of Symphysanodon ZBK . Symphysanodon rhax ZBK can be distinguished from S. andersoni ZBK in having fewer gillrakers (total on first gill arch 35-38 vs. 41or 42) and lateral-line scales (50 vs. 60 or 61), from an undescribed species from the Comoros in having more gillrakers (total on first gill arch 35-38 vs. 28), and from S. maunaloae ZBK (assuming that species occurs in the Indian Ocean) in having more lateral-line scales (50 vs. 42-47).

Sexuality and sexual dimorphism. Histological examination of the gonads of two specimens of S. rhax ZBK (134 & 136 mm SL) showed both to be females with no evidence of hermaphroditism. Four specimens have short pelvic fins (20 ->26% SL) and two have produced pelvic fins (>64 &>67% SL). Because both specimens known to be female by histological examination have short pelvic fins and because females of the apparent closest relative of this species, S. berryi ZBK , have short pelvic fins (in contrast with males which frequently have very well-produced pelvic fins), it is reasonable to assume that the individuals of S. rhax ZBK with produced pelvic fins are males. Symphysanodon berryi ZBK is also sexually dimorphic with regard to lengths of caudal-fin lobes. Unfortunately, both caudal-fin lobes are damaged on all six specimens of S. rhax ZBK , leaving the question of sexual dimorphism in this character unanswered.

Distribution. Known only from the northern Indian Ocean off the Maldive Islands (Figure 9). Depth of capture, 218 m, is available for only one collection.

Remarks. Two of the six specimens of S. rhax ZBK (i.e., the type specimens) examined were reported under an account of S. berryi ZBK by Anderson (2003:1306), who wrote “2 specimens(134 and 144 mm standard length), indistinguishable from this species [ S. berryi ZBK ], have been obtained off the Maldives, southwest of Sri Lanka in the Indian Ocean.” Three other specimens (BMNH 1997.9.17:6-8, 128-136 mm SL) reported herein, along with another two (MRS 0455/97, 118 & 146 mm total length) from the Maldives, are listed under Symphysanodon ZBK sp. in Adam et al. (1998). Meristic and much of the morphometric data for S. rhax ZBK agree closely with those for specimens of S. berryi ZBK , but ranges for some measurements do not overlap or overlap only slightly with those of S. berryi ZBK (see Table 4).

It can be argued that the differences noted in Table 4 are artifacts of the small sample size from the Maldives and not sufficient to recognize those specimens as representing a species distinct from Symphysanodon berryi ZBK , but when the data for the measurements shown in Table 4 are plotted (see Figures 11-15), it is obvious that the Atlantic and Indian Ocean populations belong to different species. Also, closure of the Tethys Sea by Late Miocene (Smith et al., 1994) would have provided the long-term geographic isolation necessary for allopatric speciation to occur. Gill and Kemp (2002) addressed the problem of the taxonomy of widely distributed Indo-Pacific shore fishes, maintaining “that well-diagnosed geographic forms and subspecies should be awarded full-species status” (p. 165). We concur with their thinking and believe that it applies also to fishes, such as species of Symphysanodon ZBK , that occur over the continental shelf, upper continental slope, and submarine ridges and in insular waters of similar depths. Consequently, we have chosen to describe the Maldive form as a new species.

Etymology. The name rhax (berry or grape) is from the Greek and is an allusion to the similarity of this new species to S. berryi ZBK . The new name is a feminine noun in apposition to Symphysanodon ZBK .


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