Brianella corniger Wilson 1915

Brianella corniger Wilson, 1915

(Figs. 48–56)

Hosts. Spotback skate Atlantoraja castelnaui, smallnose fanskate Sympterygia bonapartii, bignose fanskate Sympterygia acuta, Magellan skate Bathyraja magellanica, Rio skate Rioraja agassizii, zipper sand skate Psammobatis extenta and smallthorn sand skate Psammobatis rudis.

Attachment site. Olfactory sacs (in A. castelnaui, S. bonapartii, S. acuta, P. extenta and R. agassizii) and fins (in B. magellanica and P. rudis).

Locality. Waters off Buenos Aires province, Argentina (34°–41°S, 53°–62°W). Additionally, specimens from B. magellanica were collected in waters off Patagonian region, Argentina (49°– 54.5°S, 63°–69°W).

Material examined. Twenty females from A. castelnaui, one female from S. bonapartii, two females from S. acuta, two females from B. magellanica, seven females from R. agassizii, one female from P. extenta and three females from P. rudis, collected between November 2011 and November 2014. Voucher specimens MLP-Cr 26.985 (five females from A. castelnaui, two females from R. agassizii, one female from S. acuta and one female from P. r u di s) are deposited in the Carcinological Collection of the Museo de La Plata (CCMLP), La Plata, Argentina.

Prevalence. A. castelnaui = 42.9% (CI = 23.3–64.6), S. bonapartii = 2% (CI = 0.6–6), S. acuta = 18.2% (CL = 3.3–50), P. r u di s = 3.3% (CL = 0.2–17.7), R. agassizii = 19% (CL = 6.8–40.3), B. magellanica = 11.1% (CL = 2– 33).

Mean intensity (range). A. castelnaui = 2.2 ± 1.4 (1–5), S. bonapartii = 1 ± 0 (1–1), S. acuta = 1 ± 0 (1–1), P. rudis = 1 (1–1), R. agassizii = 1.8 ± 0.5 (1–2), B. magellanica = 1 ± 0 (1–1).

Remarks. Brianella corniger was originally described from the gills of an unknown ray host from Lota, Chile (Wilson 1915) and then reported from an unidentified species of Psammobatis Günther, 1870 collected off Antofagasta, Chile (Atria 1967). More recently, Romero & Kuroki (1987), after identifying the host species studied by Atria (1967) as a member of the genus Sympterygia Müller & Henle, 1841, reported this species parasitizing the gills (Castro Romero pers. commn) of Sympterygia brevicaudata (Cope, 1877) from Antofagasta, and provide a complete redescription of the appendages. Although specimens of B. corniger were found in different microhabitats in the present study than those reported from Chile (olfactory sacs in A. castelnaui, S. bonapartii, S. acuta, R. agassizii and P. extenta and fins, with the branched processes at the tips of the maxillae embedded under the skin of the host, in B. magellanica and P. rudis), their general habitus, as well as the morphology of most appendages was the same as that described by Romero & Kuroki (1987) and confirmed by SEM (Figs. 48–56). Therefore, all specimens from the Atlantic and the Pacific were considered as conspecific. The only differences from the diagnosis by Wilson (1915) and Romero & Kuroki (1987) were (1) the presence of a vestigial, very small and transparent bulla; (2) second endopodal segment of antenna has a well-developed hook, a spinulate prominence and three naked setae (instead of only one), two at the base of the hook and the third in the centre of the prominence; and (3) the fused tips of the maxillae have a different number of dichotomous branches in different specimens. Observations of several specimens allowed us to determine that the number of branches varied from two to six among the specimens studied, and that some specimens showed asymmetry in the branching between the two processes, as well as between some bifurcations in each maxillae. This variability could depend on the stage of development of the copepods or on any small scale conditions at the particular site of attachment.

The confirmation of the presence of a vestigial bulla (Fig. 54) and the variability in the symmetry and branching of the holdfast (Figs. 55, 56) indicates that the differential diagnoses should be reconsidered for three related genera of lernaeopodids parasitic on chondrichthyans, namely Brianella, Schistobrachia Kabata, 1964 and Dendrapta Kabata, 1964. The presence of a vestigial bulla (in some instances virtually nonexistent) has been used as a diagnostic character to discriminate Schistobrachia from Brianella and Dendrapta, while the latter two genera were distinguished by the degree of branching of the holdfast, which is profusely branched in Dendrapta (Dippenaar et al. 2004). In addition, Dendrapta differs from Brianella in having a cephalothorax about as broad as long, a maxilliped claw that is not reduced, and the maxillae positioned close to the rest of mouthparts (Kabata 1964; Kabata & Gusev 1966).

In light of the new evidence, the presence of a vestigial bulla is no longer a discriminating character, at least not between Brianella and Schistobrachia. In addition, Schistobrachia jordaanae Dippenaar, Olivier & Benz, 2004 is particularly different from its four congeners by having a long cephalothorax with the origins of the maxillae separated from the rest of mouthparts by a relatively long neck-like region (Dippenaar et al. 2004), which do not conform with the Type-A body scheme of the genus according to Kabata (1979). These features, together with the length of the posterior processes and the branched nature of the holdfast, closely resemble those in Brianella, from which S. jordaanae can be distinguished by the position of the maxillipeds (near the mouth parts in S. jordaanae and halfway between the maxillae and the mouth tube in B. corniger). Based on this character, the classification of S. jordaanae in Schistobrachi a seems doubtful; however, there is an additional character that differentiates both genera. Indeed, in all species of Schistobrachia four processes arise from the fused tips of maxillae, whereas in Brianella there are only two such processes. Consequently, despite the overall similarity between S. jordaanae and B. corniger, the former is provisionally kept in Schistobrachia until further evidence, probably from molecular genetics, can resolve its systematic position.

It is noteworthy that the character used by Boxshall & Halsey (2004), i.e. the location of the posterior processes being ventral to oviduct openings, to discriminate Brianella from Schistobrachia and Dendrapta in their key to genera of Lernaeopodidae, is incorrect. In fact, we observed that these processes are dorsal to the oviduct openings as described by Wilson (1915). This position is therefore shared by all three genera.