The Isopods of Abyssal Depths in the Atlantic Ocean

The systematic arrangement of this monograph follows
the scheme set forth by Menzies (in press). In outline
this is:
Order: Isopoda
Suborder: Gnathiidea
Suborder: Quatuordecapoda (Isopoda, sensu stricto)
Tribe 1: Asellota
Tribe 2: Valvifera
SYSTEMATIGS
Tribe 3: Flabellifera
Tribe 4: Epicaridea
Tribe 5: Oniscoidea
Tribe 6: Phreatoicidea
The Phreatoicidea and Oniscoidea, with terrestrial
and fresh water species, lack abyssal representatives
and are, therefore, outside of the scope of this
work.
Suborder: GNATHIIDEA MONOD, 1926a
This major category of the Isopoda deserves mention
in a treatise on abyssal organisms mainly because
it is so poorly represented in the abyss. Only two
species are known from below 2000 meters; these are
Gnathia stygia (G. O. Sars) and Gnathia caeca Richardson.
The majority of the species live in shelf depths
(viz., down to 200 meters). The animals, by virtue
of their parasitic mode of life, are intimately tied to a
fish host. This is probably the major reason why
they have not been successful in penetration of the
deep sea.
The main zoogeographic conclusions that can be
drawn from the great work by Monod (1926a) are
that the gnathiid isopods are cosmopolitan in their
distribution; that there are more species in the north
and south temperate zones than elsewhere; and that
the Antarctic has three times the number of species
found in the Arctic. Bipolarity is not known in this
group.
Diagnosis: Isopoda with five pairs of peraeopods.
Mandibles of male project beyond cephalon as a pair
of pincers. Last pair of appendages lost and seventh
somite much reduced. Appendages of first peraeonal
somite united into the cephalon as a second pair of
maxillipeds (the pylopods). Adult male is the gnathia
stage, adult female the praniza, and larval form the
anceus.
Affinities: The gnathiidea have obviously been
derived from a cirolanid-type ancestor, and as early
as the Jurassic the probable precursor Urda was
developed. The nearest relative to Urda living today,
however, is probably Gnatholana, a shallow water
genus from Africa.
I tend to favor the consideration that the gnathiids
represent a group which has evolved since the Mesozoic
from an Urda-like ancestor. Since then they have
become highly specialized fish parasites. Gnatholana,
on the other hand, probably represents a relict
descendant from the Urda type.
Two new species of Gnathia were found in the
Vema collections from bathyal depths off the South
African coast. These are described here together with
the only two known abyssal species, G. stygia and
G. caeca.
A KEY TO THE FOURTEEN DESCRIBED
SPECIES OF GNATHIIDEA KNOWN FROM
DEPTHS OF 500 METERS AND GREATER
(Modified from Monod, 1926a, pp. 282-339)
1. Pylopods with five articles 2
1. Pylopods with three articles 5
2. Frons produced as a triangulate process as far as
the apex of the mandibles . Bathygnathia ... 3
2. Frons not produced . . . . Akidognathia ... 4
3. Apex of rostrum with a
patch of setae bathybia (Beddard)
(1638 meters, Challenger, 38° 11' N., 27° 9' W.)
3. Apex of rostrum nude . . . . curvirostris Richardson
(709-1232 meters, Albatross, N. Atlantic, south of
Martha's Vineyard and east of Georges Bank)
4. Pylopods operculate cristatipes (Stebbing)
(980 meters, Porcupine, 48° 6' N., 9° 18' W.)
4. Pylopods pediform poteriophora Monod
(914 meters, Ingolf, St. Croix, Antilles)
5. Frons produced 7
5. Frons not produced 6
6. Frontal process bifid abyssorum (G. O. Sars)
(N. Atlantic, Norwegian fjords, 128-887 meters)
6. Frontal process trifid oxyuraea (Lilljeborg)
(Norway, England, Mediterranean, 0-533 meters)
7. With eyes 8
7. Without eyes 11
8. Eyes produced on immovable
swellings elongata (Kreyer)
(0-890 meters, circumpolar, Norway, etc.)
8. Eyes not produced, sessile 9
9. Supraocular lobe produced . . . hirsuta (G. O. Sars)
(208-1755 meters, Norway, Davis Strait)
9. Supraocular lobe not produced 10