The Isopods of Abyssal Depths in the Atlantic Ocean

Type locality: North Atlantic, Albatross Station
2547, south of Martha's Vineyard, 713 meters, 41
females, four males, cat. no. 38969 U.S.N.M.,
Washington (Richardson, op. cit.).
Distribution: Known also from Albatross Station
2572, southeast of Georges Bank, 3235 meters, two
males, one female; Albatross Station 2571, southeast of
Georges Bank, 2479 meters, one male; Albatross
Station 2208, south of Block Island, 2155 meters,
one male, six females; Albatross Station 2078, off
Georges Bank, 912 meters, one female (Richardson,
op. cit.).
Affinities: This species appears closely related to
H. schmidtii Hansen, but has much shorter uropods.
Heteromesus spinescens Richardson
Figure 22 E
Synonyms: Heteromesus spinescens Richardson, 1908a,
pp. 83-84, Fig. 18.
Diagnosis: Heteromesus with pleon lacking lateral
spines, apex rounded, entire, lacking spine-like projections.
Uropoda less than five times as long as wide,
not bent toward midline. Lateral border of first
peraeonal somite with two spines. Last peduncular
article of first antenna with a prominent spine at distal
extremity (Richardson, op. cit. illustration.).
Measurements: None available.
Type locality: North Atlantic, Albatross Station
2105, off Virginia, 2557 meters, one male, cat. no.
38970 U.S.N.M., Washington (Richardson, op. cit.).
Distribution: Also taken from Albatross Station
2714, south of Martha's Vineyard, 3337 meters, two
females; Albatross Station 2208, south of Block Island,
2155 meters, two females Albatross Station 2084, off
Georges Bank, 2361 meters, one female; Albatross
Station 2571, southeast of Georges Bank, 2479 meters,
five females.
Affinities: This species seems related to H. greeni
(Tattersall) and differs from it in having the long spine
in the inner angle of the last peduncular article of the
second antenna.
Heteromesus similis Richardson
Figure: none available
Synonyms: Heteromesus similis Richardson, 1911, pp.
531-532.
Diagnosis: Heteromesus with spines at lateral border
of pleon, posterior border entire, rounded, without
spine-like projections or excisions. First peraeonal
somite laterally with two spines.
Measurements: None available.
Type locality: Talisman Station 31, northeast of
San Miguel in the Azores, 22 August 1883, 2995
meters, one fragment.
Distribution: Known only from type locality.
ABYSSAL ATLANTIC ISOPODS 123
Affinities: This species is perhaps related to
H. dentatus Hansen, from which it differs in having two
spines instead of one at the lateral margin of the first
peraeonal somite.
Genus: STYLOMESUS Wolff
Synonyms: Stylomesus Wolff, 1956, pp. 87-88, 97.
Type species: Rhabdomesus inermis Vanhoffen, Wolff,
1956, pp. 87-88, 97.
Diagnosis: Ischnomesidae having uropods with
two articles. Seventh peraeonal somite fused with
pleon. Third article of first antenna more than twice
as long as fourth.
Composition: Wolff (1956) put only inermis in this
genus. The species S. inermis was collected by the
German South Polar Expedition (1901-1903) in the
Antarctic Indian Ocean northwest of the Gauss
station from a depth of 2450 meters. Four additional
new species are described in this paper. Birstein
(1960) described four abyssal species from the North
Pacific, and Gurjanova (1946) described one from the
Arctic.
A KEY TO THE SPECIES OF
STYLOMESUS a
1. Pleon composed of three fused somites 2
1. Pleon composed of two fused somites 4
2. Apex of male first pleopod with lateral projections
6
2. Apex of male first pleopod without lateral projections
3
3. Inner margin of uropodal peduncle
spinulate spinulosus, n. sp.
3. Inner margin of uropodal peduncle
smooth simplex, n. sp.
4. Uropodal peduncle just extending to end of
pleon inermis (Vanhoffen)
4. Uropodal peduncle extending beyond end of pleon
by about one-half its length 5
5. Dorsum of pleon strongly granulate, without
central depression granulosus, n. sp.
5. Dorsum of pleon smooth with central transverse
depression elegans, n. sp.
6. Apex of pleon between uropods
triangulate regularis, n. sp.
6. Apex of pleon between uropods evenly rounded 7
7. Lateral margin of male first pleopods with
two distinct lobes at distal half . . . . productus, n. sp.
7. Lateral margin of male first pleopods with one
distinct lobe at distal half simulans, n. sp.
a Following species not included:
1. gorbunovi (Gurjanova)
2. wolffi Birstein
3. menziesi Birstein
4. gracilis Birstein
5. pacificus Birstein.
Stylomesus inermis (Vanhoffen)
Figure 23
Synonyms: Rhabdomesus inermis Vanhoffen, 1914,
pp. 560-561, Fig. 88. Stylomesus inermis (Vanhoffen),
Wolff, 1956, pp. 87, 97-99.