The Isopods of Abyssal Depths in the Atlantic Ocean

92
two specimens (both larvae, one of them on Liparis
frigidus); Station 113, latitude 69°31'N., longitude
7° 06' W., 2465 meters, temperature — 1.0°, one
specimen (larva, taken on Liparis frigidus).
Gnathia albescenoides, new species
Figure 3 F-H
Synonyms: None.
Diagnosis: Gnathia with produced frons, eyes
lacking. Mandible with slight redan but lacking teeth
on cutting edge. Pylopod triarticulate, last article
minute. Last two large peraeonal somites without a
It is possible to divide the Asellota into three groups
of equivalent rank, as shown in the following key.
The characteristics used are shown in Figure 4.
A KEY TO THE SUBTRIBES OF THE
TRIBE ASELLOTA
(After Menzies, in press)
1. Male first pair of pleopods fused along midline;
consisting of an elongate sympod, lacking rami.
Second male pleopods coupled loosely with first
pairs. First pair of female pleopods lacking; second
fused along midline to form a large operculum
covering the remaining pleopods. Pleon with one
or two somites Paraselloidea
1. Male first pair of pleopods consist of a short
sympod and a short ramus, neither coupled with
second pair. First pair of pleopods of female not
covering the remaining pleopods. Pleon with three
somites Stenetrioidea
1. Basal article of male first pleopods free, not joined
medially into a single piece. Neither the first pair
of pleopods of female nor the basal article of male
first pleopods fused together Aselloidea
It is perhaps well to add also that the Paraselloidea
(Fig. 4) never have more than two somites comprising
the pleon, whereas the Aselloidea and Stenetrioidea
always have more than two complete pleonal somites.
The structure of the pleon and pleopods shows
definitely that the Paraselloidea represent a specialized
advanced group, whereas the Stenetrioidea and Aselloidea
conserve the greatest number of primitive
characteristics. The Aselloidea contain only fresh
ROBERT J. MENZIES
dorsal medial sulcus. Flagellum of first antenna with
five articles, second with six articles.
Measurements: Holotype male length 5.4 mm.,
width pleon 0.4 mm., allotype length 5.5 mm.,
width pleon 0.5 mm.
Type locality: South Atlantic, L.G.O. Biotrawl
No. "54, types plus five male paratypes, cat. no. 1-42.
Distribution: Known only from type locality.
Affinities: This species is closely allied to Gnathia
albescens H. J. Hansen. It differs from that species
in having a narrower frons and in having six instead
of eight articles to the flagellum of the second antenna.
Otherwise the two are very similar.
Suborder : QUATUORDECAPODA (Isopoda, sensu stricto)
The quatuordecapods contain the majority of
the abyssal isopods, but even here the abundance of
abyssal species is strongly skewed toward the Asellota.
The generally detritus-feeding habits of asellotes and
the low incidence of parasitism (none known),
commensalism [Caecijera horvathi, Iais spp., Antias
Tribe: ASELLOTA
unirameus), and carnivorous feeding (none known)
are probably the most important factors related to the
ability of the asellotes to penetrate the abyss. Parasitic
isopods in general are rare in the abyss. The anthurids
perhaps constitute an exception.
water species, whereas the Stenetrioidea are exclusively
marine, most extensively developed in the
shallow water of the subtropics, and have only one
abyssal representative. To the Paraselloidea belongs
the majority of the abyssal species It is possible to
divide the Paraselloidea into two additional previously
unrecognized groups. The first category embraces
species in which the anal opening is separated
from the branchial cavity, and the second includes
species in which the anus is enclosed within the
branchial cavity. As obvious as this characteristic is
in some genera (viz., separated from the branchial
chamber in Haploniscus, and contained within the
branchial chamber in Pleurogonium), it is nevertheless
impossible to utilize it at this date, due to the fact that
it is not well enough described for the majority of
asellote genera. At first I thought there might be a
correlation between the separation of the anus from
the branchial chamber and abyssal habitat, but this
turns out not to be the case because in Abyssijaera,
a genus in which the anus is enclosed within the
branchial chamber, an abyssal habitat is typical,
whereas in Munna, an intertidal genus, the anus is
terminal and separated from the branchial chamber.
A separation of families in the number of pleonites
comprising the pleon is a more hopeful one, but here
again only in those cases where it is perfectly certain
and obvious—e.g., Haploniscus with one and Ilyarachna
with two pleonal somites.