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Holt and Tattersall, 1905

Description
Eye small and spherical [T.acutifrons eye ]. The ratio of the eye diameter and carapace length is about 0.13.
Peduncle of first antenna: the first segment with a distal dorsal margin that extends forward as a roughly triangular extension of a setose lobe; this extends about one-third of the way along the second segment [T.acutifrons ped 1st ].
Rostrum a forward- and slightly upward-pointing process (tooth) [T.acutifrons eye ]. The frontal plate is triangular and the apical angle is greater than a right angle; it extends to or beyond the eye, depending on the eye stalk's direction.
Carapace without denticles on the lateral margins in the adult [T.acutifrons-drawing ], but a pair is present in immature specimens.
Thoracic legs: the eighth thoracic leg is extremely minute, with the endopodite reduced and the exopodite normally developed. Leg 7 is smaller than the sixth, but of the same appearance and with the same number of segments (6).
Abdomen without spines on the abdominal segments. The median posterior margins of all segments are smoothly curved; none are pointed.
Photophores present in the eyes, thorax and abdominal segments 1-4.
Petasma: the spine-shaped process is almost straight. The terminal process is distinctly shorter than the proximal process and tapers to an acute, curved end. The proximal process is almost straight along most of its distal half and is moderately curved toward its base; it is trifid at the tip and coarsely toothed before the tip. The lateral process is slender and slightly hooked and is about as long as the terminal process. The first additional process is a broad lobe and subacute, whereas a second additional process is slender, straight and acute [T.acutifrons petasma ].
Breeding: females mature sexually at about 25 mm length and at 2 years of age; probably is the reproduction restricted to the spring (Mauchline, 1984).

Size
Adults are 35-50 mm.

Larval stages
T.acutifrons-A
T.acutifrons-B
T.acutifrons-C
T.acutifrons-D

Ecology
T. acutifrons is a food source for whales and demersal fish.

Remark
The short acute rostrum of T. acutifrons (in dorsal view) is not longer than the more upward directed rostral tooth in T. microphthalma. In lateral view the rostral process of T. acutifrons is not distinguishable from the similarly forward- directed frontal plate. T. acutifrons and T. microphtalma are difficult to distinguished and are best identified as adults on the petasma or thelycum.

Depth range
Adults are mostly above 140 m at night but are not found in the mixed surface layer. They live at about 700-900 m by day. Most larvae are above 300 m.

Distribution in the North Sea
Regarding the depth range of the species, T. acutifrons is not likely to occur in the shallow North Sea. It is known from the NW North Sea off Scotland (Mauchline, 1984) and in the vicinity of the Orkney and Shetland Islands (S. Hay, pers. com.).

World distribution
T. acutifrons is subarctic and subantarctic. It occurs in two zones of about 35-50°N and 40-60°S in the Pacific. In the Atlantic it has been reported from 35-70°N and is known from S African waters near 30-40°S in both the Atlantic and Indian Ocean basins. This probably reflects a bipolar distribution (Mauchline, 1984).

[After Brinton et al., 1999]

Thysanopoda acutifrons