Akko Birdsong and Robins

James L. Van Tassell & Carole C. Baldwin, 2004, A review of the gobiid genus Akko (Teleostei: Gobiidae) with description of a new species., Zootaxa 462, pp. 1-15 : 10-14

[[ Akko Birdsong and Robins ]]

Discussion

All species of Akko are easily recognized by their large teeth, elongate body, small eyes, anterior nostril located within a fleshy tube, and by having the dorsal, caudal, and anal fins connected by membrane. The species can be differentiated by numbers of lateral scales, vertebrae, and fin rays, and by color. In A. dionaea the lateral scales do not overlap on the caudal peduncle, and there are approximately 76 in the series; A. brevis and A. rossi both have overlapping scales on the caudal peduncle, with 53-60 and 155 scales, respectively, in the lateral series. Color patterns are distinctive, even in preserved specimens. Akko dionaea has no trunk or head melanophores, and only the distal two-thirds of the caudal fin and posterior portion of the anal fin have some melanophores; A. brevis possesses melanophores on the upper portion of the trunk, on the interradial membranes of the dorsal fin, and on the caudal fin; A. rossi has a heavily pigmented dorsal trunk, and all fins are dark brown. Vertebral counts in A. dionaea and A. brevis are 11 precaudal and 16 caudal, 11+17 in A. rossi ; additional specimens of A. rossi are needed to determine if this vertebral count is characteristic for the species. Akko dionaea can be separated from the Pacific species by having fewer pectoral-fin rays: 17 in A. dionaea , 19-21 in A. brevis , and 19 in A. rossi . The second dorsal and anal fins both have 15 elements in A. dionaea and A. brevis but 16 in A. rossi . The pelvic-fin rays are branched in the three species, with A. dionaea and A. brevis having all rays dichotomously branched. In A. rossi , rays 1,2,3 are serially branched, and rays 4,5 are dichotomously branched. Differences that readily separate the three species are given in Table 3.

The head pores in all species of Akko are very small and difficult to observe. All species of Akko have a short cephalic head canal with two pores, B’ and G’. Pore G’ was reported as absent by Birdsong and Robins (1995) in A. dionaea , and, although it is absent in the holotype, we have observed it in several of the paratypes. The three species also possess small fleshy flaps on the posterior dorsal edges of the upper and lower lips, a character not mentioned in the original description of A. dionaea . Birdsong and Robins (1995) stated that row ot in A. dionaea has a disjunct extension on the branchiostegals. Our examination of the types of A. dionaea with 0.5mm fiber optic lighting shows this row to be complete, not disjunct, and similar to our Figure 2 of A. brevis . A large ossified lacrimal is present in all species of Akko . In all other Gobiosomatini and most members of the Gobiinae the lacrimal is small. Akko brevis has the ability to open its mouth in a complete circle equal to the body diameter. The large lacrimal, which is attached to the maxilla by a broad tendinous sheath, may play a role in this movement.

Akko belongs within the tribe Gobiosomatini based on the presence of seven spines in the first dorsal fin, a dorsal-pterygiophore formula of 3-221110, and a vertebral count of 11 precaudal and 16 caudal vertebrae (17 in rossi ). Although Birdsong and Robins (1995) placed Akko within the tribe, they did not comment further on its relationships within the group because of the large number of autapomorphies possessed by A. dionaea . Additional information now available from mtDNA places Akko within the “ Microgobius ” group of the Gobiosomatini (Birdsong et al. 1988) and provides support for a sister-group relationship with Microgobius (Ruber et al. 2003).

Further supporting the placement of Akko within the “ Microgobius ” group are patterns of sensory papillae. There are two general categories of sensory papillae patterns within the Gobiosomatini: In the “ Gobiosoma ” group, row n on the dorsal surface of the head is short, the right and left elements never joining at the dorsal midline, and rows x1 and x2 are not connected to form a continuous row (Fig. 5A); in the “ Microgobius ” group row n is generally elongate, the left and right elements frequently joining at the dorsal midline, and rows x1 and x2 are united to form a single row (Fig. 5B). This arrangement of sensory papillae has been observed in most species of Microgobius , Bollmannia , and Parrella of the “ Microgobius ” group by one of us (JVT). All species of Akko possess a papillae pattern similar to that of the “ Microgobius ” group.

Myological features, particularly origin and insertion patterns within the adductor mandibulae complex, also support the proposed relationship between Akko and other species of the “ Microgobius ” group. In the “ Gobiosoma ” group (Fig. 6A) the adductor mandibulae1 complex (A1) originates along the dorsal half of the preopercle and along the lateral edges of the pterotic and sphenotic. The A1 complex separates into A1 beta and A1 alpha sections. An additional separation of the A1, the A1 gamma, occurs in all genera of the “ Gobiosoma ” group except Aruma , Barbulifer , Gymneleotris , Chriolepis , Eleotrica , Pycnomma , Gobulus , Nes (Van Tassell, 1998). The A1 gamma inserts on the maxilla and on the A2 gamma, the A1 beta inserts on the maxilla, and the A1 alpha inserts on the coronoid process of the dentary. The adductor mandibulae 2 (A2) originates on the ventral half of the preopercle and on the quadrate. The A2 separates into two sections, one inserting along with the A1 alpha on the coronoid process and the other inserting on the maxilla and A1 gamma, near or at the insertion of the primordial ligament.

In the “ Microgobius ” group the A1 complex does not originate along the sphenotic, and the A1 gamma is absent. In Bollmannia (Fig. 6B) the A1 originates along the dorsal half of the preopercle and on the pterotic; it inserts via two heads, one on the maxilla, in conjunction with the A1 beta, and the second via a tendon on the coronoid process of the dentary. An A1 beta is present medial to the large A1 alpha, originating on the hyomandibular and inserting with the A1 alpha on the maxilla. In Microgobius there is a partial separation of an A1 beta from A1 alpha anteriorly, just prior to the insertion on the maxilla; otherwise, the A1 remains a single muscle mass. The adductor mandibulae 2 is a single mass originating along the ventral half of the preopercle and on the quadrate. It inserts, along with the A1 alpha, via a tendon onto the dentary. The cheek myology pattern of Akko is most similar to that of the “ Microgobius ” group (Fig. 4). Cheek myology of Parrella and Palatogobius in the “ Microgobius ” group has not been examined.