Melinnacheres levinseni M'Intosh 1885

Melinnacheres levinseni (M’Intosh, 1885)

(Fig. 11 B, C)

OEstrella levinseni M’Intosh, 1885 —incorrect original spelling (ICZN Art. 32.5.2). Melinnacheres levinseni (M’Intosh, 1885) —new combination: Boxshall (1977).

Original description. M’Intosh (1885): 477, Plate XXXIX A, fig. 11.

Host. Ehlersiella atlantica M’Intosh, 1885 (family Terebellidae).

Type locality. Trawled at H.M.S. Challenger station 63 (Mid Atlantic between Bermuda and Azores), 35º29’ N, 50º53’ W; depth 2,750 fathoms (5,029 m); red clay.

Material examined. Holotype ♀ (NHMUK reg. no. 1934.4.24.2); attached to the posterior fragment of a syntype of E. atlantica (NHMUK reg. no. 1934.85.12.1.373; collected 19 June 1873.

Remarks. M’Intosh (1885) described the body as 4-segmented but Boxshall (1977) showed that the grooves separating these segments are only superficial and are probably artefacts caused by shrinkage during preservation and storage. A similar observational error was made by Sars (1870) who stated that the body of M. ergasiloides was 5-segmented. Bresciani & Lützen’s (1961, 1975) revealed in cross-sections the presence of several pairs of dorsoventral striated muscles which attach to integumental apodemes; contraction of these muscles causes more or less expressed transverse constrictions of the body.

Boxshall’s (1977) re-examination of M. levinseni revealed the presence of three pairs of minute appendages which he identified as the antennules, antennae and maxillae. All three limbs, particularly the antennule, are much more reduced than in the other three members of Melinnacheres. Bresciani & Lützen (1961, 1975) described the antennule in the adult female of M. ergasiloides M. Sars, 1870 and M. steenstrupi Bresciani & Lützen, 1961 as a moderately elongate, indistinctly segmented appendage carrying up to seven setae. O’Reilly (2016) recently confirmed the same condition in M. terebellides (Levinsen, 1870). In M. levinseni the antennule is represented by a small, unarmed, lobate outgrowth and the antenna and maxilla do not show the typical striated pads found in the other congeners (Fig. 11 B, C). Whether this can be interpreted as evidence in support of resurrecting Oestrella as a valid genus will depend on the morphology of the as yet unknown male of M. levinseni. Bresciani & Lützen (1961, 1975) identified the third and fourth pair of appendages in the males of M. ergasiloides and M. steenstrupi as the mandibles and maxillae. The mandibular sexual dimorphism (the mandibles being absent in the female) observed in these species has not been recorded in other copepods and is considered unlikely. The third pair of limbs in the male is here interpreted as the maxillae and the fourth pair as the maxillipeds. This interpretation is in accordance with the condition in highly transformed female poecilostome copepods where maxillipeds are usually expressed in males but often lacking in females.

M’Intosh (1885) observed two rounded lateral prominences at the posterior end of the body where the paired egg sacs attach. These prominences carry the paired genital apertures as in other species of the genus. Bresciani & Lützen (1975) described a small anal prominence dorsally near the genital swellings in M. ergasiloides and confirmed its absence in M. steenstrupi which lacks a functional anus. According to Boxshall (1977) a much larger conspicuous anal prominence is present in M. levinseni and this structure was compared by Stock (1986) with the medio-terminal swelling found between the genital apertures in some Herpyllobius species. However, Boxshall (1977) observed the female in ventral aspect only and made no mention of the genital apertures. Re-examination of the posterior region in dorsal aspect (Fig. 11 B) revealed the presence of paired genital apertures, the right one of which corresponds to the alleged anal prominence.

Melinnacheres levinseni was collected at 5,029 m depth, representing the deepest record of an annelidicolous copepod so far and the only one recorded below the 5,000 m bathymetric mark. The second deepest record is that of Ophelicola kurambia Conradi, Bandera, Marin & Martin, 2015 which was recently collected from an opheliid host at 4987–4991 m in the Kurile Kamchatka trench and abyssal plain (Conradi et al. 2015).