Haplomesus tropicalis Menzies, 1962

Kavanagh, Fiona A., Wilson, George D. F. & Power, Anne Marie, 2006, Heterochrony in Haplomesus (Crustacea: Isopoda: Ischnomesidae): revision of two species and description of two new species, Zootaxa 1120, pp. 1-33 : 25-28

Haplomesus tropicalis Menzies, 1962 ( Fig. 15 )

Haplomesus tropicalis Menzies, 1962: 120 –121, fig. 20K–M.

Type fixation. Holotype male AMNH 12062; paratype ("allotype") AMNH 12063.

Type locality. Mediterranean Sea, 34º14'N, 24º10'E, 2526 m.

Remarks on type material. Paratype ("allotype") female is decalcified, with antennae and several pereopods broken. Although the container received from the American Museum contained labels for both " Holotype " and "Allotype," the male holotype specimen described by Menzies (1962) was not in the jar. Menzies (1962) also reports another specimen from a different locality that was not examined here: AMNH 12206, LGO Biotrawl no. 95, Vema 15–10, 7 November 1958, 4071 m, 14º05'N, 75º25'W, central part of Columbia Abyssal Plain.

Material examined. Paratype female, AMNH 12063, Lamont Geological Observatory Biotrawl no. 76, Mediterranean Sea, Vema 14–32, 9 July 1958, 2526 m. Holotype male not examined, data taken from illustration.

Diagnosis. Male with small spines on pereonites 1 and 4 only; female with spines on pereonite 1 only, spines short (length distinctly less than pereonite 1 lateral margin) and acutely pointed. Male pereonite 1 with 3 pairs of low spines on posterolateral margin, minute, acute (see Menzies 1962: 120, fig. 20K–M). Pleotelson posterolateral margin anterior to uropods with simple spines, less than half uropoda length. Pereopod I carpus ventral margin with 1 proximal robust seta and with 2 distal robust setae on palm. Pleopod I of male with simple setae on distal margins.

Description of female. Body length 4.1 mm; pereonite 7 undeveloped. Head length 0.8 width; lobe on ventrolateral margin absent in lateral view. Pereonite 1 width 0.15 total body length. Pereonite 5 length 3.5 width, 0.4 total body length. Pleotelson length 1.6 width; dorsal surface axial ridge weakly vaulted, separated from lateral fields only by shallow elongate concavities; posterolateral margin adjacent to uropods convex.

Antennula and antenna. Antennula article 2 length 0.9 head width, with 2 elongate ventromedial setae. Antenna article 3 length 0.1 anterior body length, length 3.2 width.

Mouthparts. Maxilliped palp article 2 wider than 3.

Pereopods. Pereopod I propodus ventral margin with 1 robust seta. Pereopod VII absent in adults.

Pleopods and uropods. Pleopod II operculum with narrow proximal neck, laterally convex, broadening posteriorly to rounded angles, posterior margin weakly convex, without plumose setae. Uropods extending near posterior margin of pleotelson; length 0.2 length of pleotelson.

Description of male. Body length 3.1mm. Head length 0.8 width. Pereonite 1 width 0.14 total body length. Pereonite 4 anterolateral simple spines, minute, length distinctly less than pereonite 1 length. Pereonite 5 length 3.7 width, 0.4 total body length. Pleotelson length 1.5 width;

Antennula and antenna. Antennula article 2 length 1.1 head width. Antenna article 3 length 1.5 width.

Pleopods and uropods. Uropods extending near posterior margin of pleotelson, length 0.2 length of pleotelson.

Distribution. Mediterranean Sea, 2526m;?North Atlantic Ocean, Caribbean Sea, Columbia Abyssal Plain, 4071 m.

Remarks. H. tropicalis is easily distinguished from other species without pereopod VII by having the shortest spines on pereonites 1 and 4 ( Fig. 15 A). It is also distinguished in males by the group of posterolateral minute spines on pereonite 1. The holotype of H. tropicalis Menzies, 1962 is an immature male specimen, based on our inspection of the original description ( Menzies 1962: 120–121). A fully grown male would be expected to have lateral horns on pleopod I, and well developed spines both on the pleotelson and pereonites 1 and 4. Although the holotype was not examined because it has been apparently lost (see above remarks on the type material), the dorsal illustration by Menzies (1962, fig. 20K) shows the lack of development of pereonite 7, which implies that pereopod VII is also absent. The "allotype" is a preparatory female with oostegites. This specimen also lacks pereopod VII and the seventh pereonite is undeveloped, a fact overlooked by Menzies (1962).

Distribution of species lacking the seventh pereopod

Species without the seventh pereopod exist throughout the Atlantic Ocean (see fig. 1), with species collected as far north as the Arctic circle ( H. angustus ), and as far south as the Southern Ocean, just north of the Antarctic circle ( H. corniculatus ), at depths from 1333– 4071 m. All species, however, have limited distributions thus far confined to small regions. The exception is H. tropicalis , with specimens recorded by Menzies from both the type location in the Mediterranean, and from the Columbia abyssal plain. An undescribed species similar to H. tropicalis is known (Wilson, unpublished data) from the Gulf of Mexico [DGoMB stns (see Rowe 2003), sp. 239], that has short spines on pereonites 1 and 4, but lacks the lateral group of short sharp denticles on pereonite 1. The specimen from the Columbia Abyssal Plain reported by Menzies should be examined in relation to this Gulf of Mexico material. At least 1 other unidentified species has also been collected, from the Pacific Ocean ( Clipperton Clarion Fracture Zone, sp. 153; see Thistle & Wilson 1996). Species of this group may have been collected elsewhere but may have been considered as manca specimens or the missing last pereopod has not been noted. Therefore, we suspect that this species group has a cosmopolitan deep­sea distribution and may be an older clade than Heteromesus , which is restricted to the North Atlantic ( Cunha & Wilson, personal communication)

Figures

FIGURE 15. Haplomesus tropicalis (female allotype AMNH 12063). A, dorsal view; B, ventral view; C, maxilliped; D, lateral view; E, pereopod IV; F, pleotelson ventral; G, pereopod I (scale bar 1 mm).