Description of a new species of hake : Merluccius patagonicus sp . nov . ( Gadiformes : Merlucciidae ) from the waters of Argentina *

The hake, genus Merluccius, which inhabits the continental shelf and slope is widely distributed and represents an important resource in marine fisheries. Species of the genus Merluccius are very closely related and many taxonomy studies of this genus have been published, but the most recent reviews of all the species of the genus Merluccius were made by Inada (1981), and Inada in Cohen et al. (1990). Inada (1981) reviewed previous studies on the taxonomy of Merluccius and examined and compared the external and internal meristic and morphometric characters of all the hake species in the world, concluding that the genus Merluccius contains twelve distinct species, with two subspecies each in M. merluccius and M. gayi, and two populations in M. australis. This author presented a diagnostic key using meristic and morphometric characters but admitted that some species could not be separated by these characters because minor overlapping ranges were present. SCI. MAR., 67 (3): 323-326 SCIENTIA MARINA 2003


INTRODUCTION
The hake, genus Merluccius, which inhabits the continental shelf and slope is widely distributed and represents an important resource in marine fisheries.Species of the genus Merluccius are very closely related and many taxonomy studies of this genus have been published, but the most recent reviews of all the species of the genus Merluccius were made by Inada (1981), and Inada in Cohen et al. (1990).Inada (1981) reviewed previous studies on the taxonomy of Merluccius and examined and compared the external and internal meristic and morphometric characters of all the hake species in the world, concluding that the genus Merluccius contains twelve distinct species, with two subspecies each in M. merluccius and M. gayi, and two populations in M. australis.This author presented a diagnostic key using meristic and morphometric characters but admitted that some species could not be separated by these characters because minor overlapping ranges were present.This observation had been made earlier by Ginsburg (1954), who stated that the greatest divergence in proportional measurements in Merluccius seems to be shown by groups of the smallest size; with growth the extent of that divergence appears to decrease and may even disappear in some cases.This stimulated the authors to undertake a revision of this genus to find key characters for differentiating species.
In a recent revision of all the species of Merlucciidae on the world, Lloris et al. (in press) approach this problem with other criteria.They use new characters, and recognise thirteen species of Merluccius, with two subspecies each in M. merluccius, M. albidus, M. polli, M. australis and M. gayi.A diagnostic key for these thirteen species is provided, mainly based in species characters that are accurately determined.
Merluccius is one of the four genera of the family Merlucciidae and the only one of the subfamily Merlucciinae (Lloris et al., in press).Several specimens of hake studied during this review belong to a species of Merluccius that has not yet been described.
The objective of the present paper consists in describing this new species as well as providing a key to differentiate it from other congeneric species occurring in the same biogeographical region.
(Fig. 1) Diagnosis: Body elongated, slender.Superior profile of the head with a depression above the eyes.Upper border of the opercular membrane inclined, descending from its origin, moving away from lateral line.Ocular diameter relatively large.Second dorsal fin with 37 to 38 rays.Anal fin with 37 to 39 rays.Lateral line with 123 to 126 oblique rows of scales.First gill arch with 12 to 17 gill-rakers: 3-4 + 9-13.
Etymology: named after the marine geographic region from which the species comes (Patagonia, Argentina).

Relationships
M. patagonicus belongs to the subgroup of hakes of the southern tip of South America and New Zealand (M.hubbsi and M. australis) characterised by the presence of scales on the lower part of the cheek and preopercle, and their absence on the lower part of the interopercle.Nevertheless, the three species differ by characters given in the key of classification (see below), and also in the shape of some internal structures: the hyomandibular, the sagitta and the urohyal (Fig. 1 top to bottom) are clearly different in the three species of this subgroup.
M. patagonicus shares with M. australis the form of the opercular membrane, their dorsal border being inclined in both species.It differs from M. australis in body shape, which is slender in M. patagonicus and robust in M. australis; upper profile of the head, with a depression in M. patagonicus and straight in M. australis; the lateral line descends gradually from its origin to nearly the middle of the body in M. patagonicus while in M. australis it is gently bowed over the pectoral; and M. australis has a lesser number of rows of scales over the lateral line, about 123-126 versus 144-186 in M. australis.
M. patagonicus shares with M. hubbsi a similar body shape and a similar number of oblique rows of lateral line scales.It differs from M. hubbsi in the upper profile of the head, which has a depression, whereas it is straight in M. hubbsi; in the dorsal border of the opercular membrane, inclined in M. patagonicus and horizontal in M. hubbsi; and the lower jaw of M. patagonicus projects farther than that of M. hubbsi.
The hyomandibular (Fig. 1a, b, c) is different in the three species.In M. patagonicus the intermuscular process of the hyomandibular is longer than the preopercular process, while in M. australis and in M. hubbsi the two processes are of similar length; in M. hubbsi the intermuscular process is bowed and its tip converges into the preopercular process, whereas in M. australis both processes diverge; the inferior process of hyomandibular is straight in M. patagonicus and in M. australis, whereas it is curved forward in M. hubbsi forming an angle with the anterior condyle.
The sagittae of the three species (Fig. 1d, e, f) can be regarded as having apomorphic characters, with a protruding, acute tip, which is characteristic also of other American species of Merluccius studied by Lombarte and Castellón (1991).The sagitta of M. patagonicus presents a well developed excisura ostii, also found in M. gayi gayi from North Chilean waters (Lombarte and Castellón, 1991).The presence of this feature in both species could suggest a close relationship between M. patagonicus and M. gayi gayi.
The shape of the urohyal (Fig. 1g, h, i) and its degree of ossification is different in the three species.The urohyal of M. hubbsi has a short base and its anterodorsal process forms an obtuse angle to the vertical medial plate; it is thickly ossified and triangularshaped in transverse section in examined specimens larger than 50 cm TL; in juveniles and examined specimens of around 25 cm TL it is thin and inverted T-shaped (transverse section).The anterodorsal plate of the urohyal of M. patagonicus forms an acute angle to the vertical medial plate; its ossification increases with the size, and it is thickly ossified in observed specimens longer than 40 cm TL.The urohyal of M. australis is thin regardless the size of the fish and inverted T-shaped (transverse section).
Synonyms: It is very likely that specimens of this species have been mistakenly assigned to M. hubbsi Marini, 1933 and perhaps also to M. australis (Hutton, 1872).

Key of identification
FIG. 1. -On top, with white background, comparison of anterior half of M. hubbsi, M. patagonicus and M. australis, where the existing differences can be observed in the cephalic profile, the ocular diameter, and the distance between the lateral line and the posterior margin of opercle, in all the studied species.At bottom, with dark background, comparative details of hyomandibular (a, b, c), sagitta (d, e, f with acute tip broken) and urohyal (g, h, i) from M. hubbsi (540 mm TL), M. patagonicus (610 mm TL) and M. australis (415 mm TL) respectively.