Heteromesus bifurcatus Menzies, 1962

Cunha, Marina R. & Wilson, George D. F., 2006, The North Atlantic genus Heteromesus (Crustacea: Isopoda: Asellota: Ischnomesidae), Zootaxa 1192 (1192), pp. 1-76 : 13-15

Heteromesus bifurcatus Menzies, 1962

( Figure 1)

Heteromesus bifurcatus Menzies 1962: 122 , fig. 22C; Wolff 1962: 85, 265. Material examined

Holotype: brooding female (fragment, 4 mm), North Atlantic , Caribbean, Colombia Abyssal Plain, 14º05’N, 75º25’W, 7 Nov 1958, R / V Vema stn V­15–9 (Biotrawl nº94), 4077 m ( AMNH 12061 ).

Remarks on type material: The holotype is a headless fragment of a brooding female (pereonites 4–7 and pleotelson with some incomplete pereopods). The fragment is deformed and decalcified (almost transparent). The suture between pereonite 5 and 6 is evident. The spine on each side of the pleon that was drawn and described by Menzies (1962: 122, fig. 22C) is not evident in dorsal view, but can be seen in ventral view. The spines, however, are damaged and present only as stubs in this specimen.

Diagnosis (Female only)

Pereonite 5 length 2.8 width. Pleotelson pleonite 1 region with no dorsal spines or tubercles; posterolateral margin anterior to uropods with simple spines. Pereopod bases proximal shoulder with no spines; pereopods II– VII bases granulate, with no spines. Uropods length 0.40–0.45 length of pleotelson.

Description (Female characters only)

Body length 4 mm (approximately; fragment); granular. Pereonite 4 length 0.60 width. Pleotelson length 1.15 width; posterolateral margin at uropod insertion produced from margin; posterolateral margin anterior to uropods with low and broad posterolateral spines (damaged, possibly with distal seta).

Distribution North Atlantic, Caribbean, Columbia Abyssal Plain, 4077 m.

Remarks

Menzies (1962: 122) commented that H. bifurcatus is similar to H. dentatus , but differs from it by having an incision in the pleotelson. This is in error because the pleotelson is not incised and only the anal plates are seen in dorsal view. Ischnomesidae have anal openings and the associated plates close to the terminus of the pleotelson. This condition is similar to that of the Munnidae where the anus and anal plates are completely terminal. Ischnomesids, however, have the anus somewhat covered by the dorsal surface of the pleotelson and the anus itself is somewhat angled under the body. Some badly preserved specimens, as in this case, will show the anal plates in dorsal view.

In fact, H. bifurcatus is unlike other species that also have spines on the posterolateral margin of the pleotelson, because it apparently lacks other distinctive features such as dorsal rook spines (as in H. dentatus ), or elongate spines on the pereopods (as in H. spinosus ). The male and the head of the female, however, are unknown, so this species remains poorly characterised.