Melanonus zugmayeri Norman , 1930 , captured off Portugal . A review of the current knowledge on this species

The capture of rare deep-sea fishes is of special interest whenever the main biological and morphological characteristics of the species are not yet well defined. At such a low level of knowledge, any new information may lead to new phylogenetic and biogeographic insight (Haedrich, 1996). This is the case of the meso-bathypelagic fishes of the family Melanonidae (Gadiformes, Actinopterygii). This family comprises only the genus Melanonus Günther, 1878, and Melanonus gracilis Günther, 1878 and Melanonus zugmayeri Norman, 1930 are the commonly accepted species (Cohen et al., 1990; Nelson, 1994). M. gracilis has a circumpolar distribution in subantarctic and temperate seas and has been occasionally caught in tropical areas of the southern hemisphere (Cohen, 1990). M. zugmayeri is a circumtropical or subtropical species (Cohen, 1986a) that is rare in the north-east Atlantic (Cohen, 1973) but reaches 40oN or even further north (Cohen, 1986b, 1990). Gordon et al. (1996) captured two specimens at Porcupine Seabight (50oN) and one further north (56oN 9oW, Gordon pers. com.). Torres et al. (1979) and Melendez and Sielfeld (1991) found some specimens in the Pacific Ocean, 33oN and 21oS respectively, and Kotthaus (1970) in the Indian Ocean (5oS 40oE). The morphological data of the few known specimens of M. zugmayeri were not consistent. Also, the characteristics used in the diagnosis of the species of the genus Melanonus were not well defined. The SCI. MAR., 65 (1): 43-46 SCIENTIA MARINA 2001


INTRODUCTION
The capture of rare deep-sea fishes is of special interest whenever the main biological and morphological characteristics of the species are not yet well defined.At such a low level of knowledge, any new information may lead to new phylogenetic and biogeographic insight (Haedrich, 1996).This is the case of the meso-bathypelagic fishes of the family Melanonidae (Gadiformes, Actinopterygii).This family comprises only the genus Melanonus Günther, 1878, and Melanonus gracilis Günther, 1878 and Melanonus zugmayeri Norman, 1930 are the commonly accepted species (Cohen et al., 1990;Nelson, 1994).M. gracilis has a circumpolar distri-bution in subantarctic and temperate seas and has been occasionally caught in tropical areas of the southern hemisphere (Cohen, 1990).M. zugmayeri is a circumtropical or subtropical species (Cohen, 1986a) that is rare in the north-east Atlantic (Cohen, 1973) but reaches 40ºN or even further north (Cohen, 1986b(Cohen, , 1990)).Gordon et al. (1996) captured two specimens at Porcupine Seabight (50ºN) and one further north (56ºN 9ºW, Gordon pers.com.).Torres et al. (1979) and Melendez and Sielfeld (1991) found some specimens in the Pacific Ocean, 33ºN and 21ºS respectively, and Kotthaus (1970) in the Indian Ocean (5ºS 40ºE).
The morphological data of the few known specimens of M. zugmayeri were not consistent.Also, the characteristics used in the diagnosis of the species of the genus Melanonus were not well defined.The

MATERIAL AND METHODS
Two individuals of M. zugmayeri were caught off the south-west coast of Portugal in April 1994 and August 1995 respectively.The first specimen was captured by bottom trawl at 350 m depth (37º30'N, 9º25'W), whereas the second one was caught by long-line at 1500 m depth (38º00'N, 9º20'W).Both specimens were kept in the Oceanographic Museum of the Arrábida Natural Park (Setúbal, Portugal) with catalogue numbers 543MOPNA and 536MOPNA.
In order to compare the data published in previous studies with those of the present work, similar meristic and morphometric measurements were taken, namely number of fin rays, total length, standard length, body height, head length, eye diameter, pectoral fin length and ventral fin length.

RESULTS AND DISCUSSION
The capture of the two individuals of M. zugmayeri confirmed the presence of this species off the mainland coast of Portugal.Apart from the records from Madeira (Maul, 1952) and Azores (Roule and Angel, 1933;Santos et al., 1997), the only other reference to this species near the Portuguese coast is based on unpublished data (Cohen pers. com.).
The specimen scales were cycloid and coloured uniformly black, and several striae were found in the opercular apparatus.Both specimens had nematode parasites (Anisakidae) in the body cavity.Only one of the specimens had stomach contents, which included remains of unidentified shrimps (Decapoda, Natantia).The only data on the food habits of this species were reported by Koefoed (1953), who found remains of Euphausia krohnii, Nematoscelis sp. and Phronima sp. in the stomach of one individual.The second specimen reported in the present paper (Table 1), represented in Figure 1, is the longest individual of this species ever recorded (294 mm total length).Both specimens had the characteristics considered to be distinctive of the species M. zugmayeri according to Blache et al. (1970) and Cohen (1986b), namely the lower jaw teeth of uneven size (see Fig. 1) and the number of pectoral fin rays (see Table 1).Two dorsal fins were observed in both individuals, with a clear separation between the last ray of dorsal I and the first ray of dorsal II, both linked to the back body's skin by independent membranes; the rays of the first dorsal were longer than the rays of the second.
The analysis of the available data on morphometric and meristic characteristics for the specimens of the genus Melanonus caught in several geographical areas revealed a considerable discrepancy (Table 1) in the morphological characters reported for M. zugmayeri.This was also reflected in the published illustrations of this species.In their figures, Blache et al. (1970) and Cohen (1986b) representing Norman's holotype specimen (Norman, 1930), show two dorsal fins well separated by a gap.Maul (1952) and Cohen (1986a) represent their specimens with only one dorsal fin with the first rays slightly higher than the remaining posterior rays.Melendez and Sielfeld (1991) describe it as being a single one with its first 6 rays higher.Cohen (1986b) and Cohen et al. (1990) admit that in some specimens but not in  Günther, 1887;2, 4: Brauer, 1906;3: Norman, 1930;5: Norman, 1930;6: Zugmayer, 1911;7: Beebe, 1932;8: Maul, 1952;9: Koefoed, 1953;10: Kotthaus, 1970;11: Melendez and Sielfeld, 1991;12 others the first dorsal rays may appear separated.Finally, Nelson (1994) assumes a single long-based dorsal fin for the Melanonidae family.Besides the dorsal fin, other aspects of external morphology and relative measurements of the body are also very variable in the published work (e.g.Fowler, 1936;Maul, 1952).Thus, new biometrical data needed for a better description of the external morphology of this rare species, and its geographical variation.
The diagnosis characters traditionally used, such as the number of pectoral fin rays and the existence (or not) of two separated dorsal fins (Cohen, 1986b), do not seem to be reliable for species discrimination purposes.In fact, the range of values presented for several meristic and morphometric characters overlap in most cases.Regarding the dorsal fin characteristics, even in the original description of M. gracilis (Günther, 1887) a slight separation between dorsal fins is reported (in both text and figure).This separation has been considered as one of the discriminating characteristics between M. gracilis and M. zugmayeri (Cohen, 1986b).The shape of the teeth of the lower jaw seems to be the only morphological characteristic that is coherently used for species discrimination.However, the differences reported may be due to individual variation rather than interspecific variation.Given the small number of specimens studied so far, there is a lack of data to support the hypothesis of the existence of more than one species of the genus Melanonus.
Further work is needed to better characterise this genus.Though Melanonus zugmayeri has recently been captured for a variety of studies in deep-sea fish physiology (e.g.Phleger and Laub, 1989;Douglas and Thorp, 1992;Patridge et al., 1992;Marshall, 1996), these specimens were not used to improve the knowledge on the systematics of this species.Any capture of individuals of this genus should be carefully analysed from the taxonomic point of view.