Lophogaster eurylepis, Bamber & Clark, 2004

Bamber, Roger N. & Clark, Paul F., 2004, A new species of Lophogaster (Crustacea, Mysidacea, Lophogastrida) from the equatorial eastern Atlantic, Zoosystema 26 (3), pp. 419-423 : 420-423

Lophogaster eurylepis n. sp.

( Figs 1-3 )

TYPE MATERIAL. — Off Equatorial Guinea. 3°47.6’N, 8°43.0’E, 46.1 m, III.2002, 1 brooding holotype 13 mm ( NHM 2002.2067 ) ; 1 brooding paratype 14.5 mm ( NHM 2002.2068 ). — West of Point Europa , Bioko, 3°50’N, 8°20’E, 98.9 m, II.2003, 1 brooding paratype 14.9 mm ( MNHN My490). — 3°46’N, 8°36’E, 85.3 m, 1 juvenile paratype 6.75 mm ( NHM 2003.616 ) .

TYPE LOCALITY. — Off Equatorial Guinea, 3°47.6’N, 8°43.0’E.

ETYMOLOGY. — From the Greek eury: broad, and lepis: scale, alluding to the width of the antennal scale.

DISTRIBUTION AND HABITAT. — The specimens were collected on sandy mud; 3°47.6’N, 8°43.0’E (45% sand, 55% silt-clay; mean particle diameter 41.8 µm), 3°50’N, 8°20’E (50.32% sand, 49.68% silt-clay; mean particle diameter 48.38 µm), 3°46’N, 8°36’E (49.32% sand, 50.68% silt-clay; mean particle diameter 41.85 µm) with a depth distribution of 46.1-98.9 m. To date the species is only known from the type specimens.

DESCRIPTION

Carapace ( Fig. 1 ) dorsally 2.3 times as long as wide, posterior dorsum deeply emarginate exposing seventh and eighth thoracic somites, posterolateral margins finely setose, posterior spine short. Carapace surface smooth, without tubercles or postocular spines, but with rugose anterolateral postocular ridges. Rostral plate covering eyestalks and much of eyes, anteriorly tridentate, median process reaching distal margin of antennal peduncle, lateral processes half as long as median processes.

Antennular peduncle ( Fig. 2A ) robust, typical; antennular lamina simple, continuously rounded, margin denticulate, distal pair of setae not in emargination. Antennal scale ( Fig. 2B ) broad, as wide as total length without terminal spine, outer margin slightly convex with five spines including terminal spine. Spine on distal peduncle article of antenna robust, one third length of proximal flagellum segment.

Mouthparts, thoracic appendages, pleopods typical for the genus (where known).

All but last abdominal somites sternally with anteriorly directed hyposphenia (thoracic somites covered by brood); sixth abdominal somite with paired stout posterior spines 0.12 times length of somite.

A

Uropods ( Fig. 3 ) shorter than telson; exopod outer margin naked with distal tooth; endopod slightly longer than exopod.

Telson ( Fig. 2C, D ) 2.15 times as long as last abdominal somite, 2.7 times as long as basal width, dorsal surface concave centrally; margin with two midlateral spines, subapical spine more than twice length of midlateral spines and 0.8 times length of apical spine which is 0.2 times as long as telson. Posterior margin ( Fig. 2D ) with four central spines, paired plumose setae lateral to these, and one outer spine between seta and apical spine (left outer spine absent in paratype, Fig. 2C, D ).

REMARKS

Lophogaster eurylepis n. sp. is the only species of the genus with a combination of a continuously rounded antennular lamina, no postocular spine or tuberculation on the carapace, medial rostral spine twice as long as lateral rostral processes (although it may be expected to be proportionately longer or shorter in the male, as are known to be the cases in, for example, L. muranoi and L. longirostris Faxon, 1896 , respectively), only two midlateral marginal spines on the telson and a broad (“ rotundatus - type ”) antennal scale. Indeed, with a width equal to the length, the antennal scale is broader than any described species (e.g., 0.75 times in L. rotundatus Illig, 1930 ).

From Băcescu (1991) the present species would key out to L. musorstomi Băcescu, 1991 , from which it differs in most features of the rostral plate, the antennular lamina, the antennal scale, the distal telson spination and the carapace posterior margin. Of the other Atlantic species, L. typicus M. Sars, 1857 and L. muranoi have postocular carapace spines; L. spinosus Ortmann, 1906 has a very long medial rostral process and long posterior carapace spines; L. challengeri Fage, 1940 has the three anterior processes of the rostral plate subequal in length (e.g., Tattersall 1955: fig. 2) and a tuberculate carapace.

Nevertheless, with its simple form of the antennular lamina and gross morphology of the antennal scale and telson, L. eurylepis n. sp. appears to be closest to the Atlantic species group including L. typicus and L. challengeri .

Tattersall (1955) remarked on the apparent coldwater preference of species of Lophogaster , referring to the fact that “[…] Dana in 1941 made collections at 18 stations while crossing the Atlantic from Cape Verde to Guiana without taking a single Lophogaster ”; from this she inferred that the genus would not be found in Atlantic Equatorial waters. Fage (1942) referred to material collected from off southern Angola (c. 16°S) in 1912, and examined by K. Stephensen, which “corresponded with the description of” (and was thus attributed to) L. challengeri , although the specimens had only two pairs of lateral telson spines (in brooding females). With limited information on these specimens, such as the presence or absence of carapace tuberculation, it may be the case that they too were of the present species.

As members of this genus are normally mesopelagic, the occurrence of L. eurylepis n. sp. in box-core samples is not regarded as indicative of benthic habit, rather of serendipity.

Figures

FIG. 1. — Lophogaster eurylepis n. sp., carapace; A, lateral view; B, dorsal view. Scale bars: 2 mm.

FIG. 2. — Lophogaster eurylepis n. sp., paratype; A, antennular peduncle; B, antennal scale; C, telson; D, posterior of telson. Scale bars: A, B, 0.5 mm; C, 1.0 mm; D, 0.1 mm.

FIG. 3. — Lophogaster eurylepis n. sp., uropods.Scale bar:0.5 mm