The Isopods of Abyssal Depths in the Atlantic Ocean

Diagnosis: Hydroniscus with a spearpoint-shaped
pleon lacking lateral projections. Uropoda absent.
Flagellum of first antenna with five articles, that of
second antenna with nine articles.
Measurements: Female holotype length 2.8 mm.
Type genus: Ischnomesas Richardson, 1908a.
Diagnosis: Paraselloidea with head and first
peraeonal somite joined. Eyes lacking. Mandibles
generally lacking palp, molar expanded, lacinia and
setae row present. Antennae shorter than body. All
peraeopods simple walking legs; dactyl with at least
one terminal claw, never three. Uropoda terminal
uniramous. Fifth peraeonal somite generally much
longer than wide. Last two articles of maxillipedal
palp narrow, others as wide as endite. Pleon with onetwo
distinct separated somites. Anus widely separated
from branchial chambers.
Composition: This family contains Ischnomesus,
Haplomesus, Heteromesus, and Stylomesus. The genus
Ischnosoma was replaced by Ischnomesus, and the genus
Rhabdomesus was subsequently found to be a synonym
of Ischnomesus (see Wolff, 1956). Like Haploniscidae,
the family is characteristically abyssal but contains
shallow water representatives as well. The absence of
a mandibular palp is not a constant characteristic of
the family: a triarticulate palp is present in /.
simplissimus, but only a rudimentary seta remains in
/. paucispinis and nothing at all in the remaining species.
A KEY TO THE GENERA OF
THE ISCHNOMESIDAE
(Modified after Wolff, 1956)
1. Uropoda with a single article 2
1. Uropoda with two articles 3
2. Pleon and three posterior peraeonal somites
fused into a single piece. Third article of first
antenna markedly elongate Haplomesus
2. Pleon and two posterior peraeonal somites
fused into a single piece. Third article of first
antenna minute Heteromesus
3. First pleonal somite separate from
pleotelson Ischnomesus
3. First pleonal somite fused with
pleotelson Stylomesus
Genus: ISCHNOMESUS Richardson, 1908a
Synonyms: Ischnosoma G. O. Sars, 1868. Rhabdomesus,
Richardson, 1908a, p. 81. Ischnomesus Richardson
1908a, p. 81.
Type species: Ischnosoma bispinosum G. O. Sars, 1868.
Diagnosis: Ischnomesidae with a distinctly twojointed
pleonal somite. Uropoda with two articles.
Remarks: The genus may be indistinctly split into
two groups on the basis of the fundamental structure
of the male first and second pleopods. Unfortunately,
ABYSSAL ATLANTIC ISOPODS 111
Family: ISCHNOMESIDAE
Type locality: North Atlantic, Ingolf Station 38,
latitude 59° 12' N., longitude 51° 05' W., 3521 meters.
One female specimen.
Distribution: An abyssal species known only from
type locality.
these characteristics are not known for all species and
therefore have not been used in the key. To the most
primitive group belong those species in which the
stylus of the male second pleopod is shorter than the
exopod and the lateral projections of each ramus at the
apex of the first pleopod are quite evident. To this
group belong the species /. bispinosum, I. paucispinis,
I. multispinis, I. sparcki, I. decemspinosus, I. bidens, etc.
A secondarily derived group has a long semi-coiled
stylus which inserts into depressions on the interior
surface of the sympod of the male first pleopod. The
lateral expansions of the sympod are generally coalesced
with the sympod, giving the apex a blunt
appearance under low magnification. To this group
belong the remaining species, /. caribbicus, I. bruuni,
I. bacilloides, I. wolffi, etc.
1.
2.
3.
4.
5.
6.
7.
8.
9.
bispinosum (G. O. Sars)
bacillopsis Barnard
armatus Hansen
bacilloides (Beddard)
profundus Hansen
sparcki Wolff
bruuni Wolff
bacillus (Beddard)
andriashevi Birstein
94-1100
1280-1280
2702-2702
2652-2652
3521-3521
6660-7000
6960-7000
3292-3292
4000-6560
Nine new species may be added to the list of known
species. All are considered in the following key.
A KEY TO THE SPECIES OF
ISCHNOMESUS
1. Dorsum of pleon with stout spines 2
1. Dorsum of pleon smooth, lacking spines . . . . 4
2. Postero-medial margin of pleon with
spines magnificus, n. sp.
2. Postero-medial margin of pleon entire, lacking
spines 3
3. Lateral borders of pleon each with only
three spines multispinis, n. sp.
3. Lateral borders of pleon each with more than ten
spines sparcki Wolff
4. Sixth peraeonal somite with a long lateral spine
on either side 5
4. Sixth peraeonal somite without a lateral spine on
either side 8
5. Lateral spine of fifth peraeonal somite directed
posteriorly at about 20° angle to peraeonal
axis 16
5. Lateral spine of fifth peraeonal somite directed
more laterally, about perpendicular from peraeonal
axis 6