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Thysanopoda obtusifrons G.O. Sars, 1883

Etymology: Thysanopoda - tassel foot; obtusifrons - with blunt forehead

Eye: The eye is small (T. obtusifrons eye & rostrum). The eye diameter : carapace length is about 0.12.

Peduncle of 1st Antenna: The lobe at the distal end of the 1st segment is an abruptly elevated thickened flap that overlies less than half of the 2nd segment, appearing to adhere to it. This flap is dorsally almost flat in lateral view (T. obtusifrons, ), and narrow, not flaring outward over the side of the 2nd segment (c.f. T. aequalis (T. aequalis,) and T. astylata (T. astylata,)). In fresh specimens, setae covering the lobe are reddish. The 2nd and 3rd segments are without projections.

Rostrum: The frontal plate is thick and rounded anteriorly, with a small distal, upwardly directed, conical tooth (T. obtusifrons eye & rostrum).

Carapace: The gastric region is only slightly elevated. A postero-lateral carapace denticle is present (T. obtusifrons ).

Abdomen: There are no spines or keels.

Length: Adults are 18-23 mm.

Petasma: The terminal process is nearly straight; the distal one-third is flattened with the outer margin serrate and the inner margin convex. The proximal process has an acute heel just beyond the base where the process bends at a right angle; it is slightly curving and flattened distally. The lateral and additional processes are of nearly equal length, slender and curving distally (T. obtusifrons petasma).

Thelycum: Described by Guglielmo and Costanzo, 1977.

Comments: The frontal plate and short rostral spine are sometimes indistinguishable from those of T. aequalis-T. astylata or immature T. orientalis . The distally narrow peduncular lobe, not flaring outward, as it extends over the 2nd segment, separates obtusifrons from the former pair, while the low or negligible elevation (above the upper surface of the 2nd peduncular segment) of the most distal flap of the lobe separates it fromT. orientalis.

T. obtusifrons is part of the oligotrophic central water mass community.

The known range is approximately 40°N-30°S in the Atlantic, including the Gulf of Mexico and off northwest Africa but not the Gulf of Guinea. In the Indian Ocean it is found from the equator to about 30°S. In the Pacific it is from 35°N to 35°S, but is lacking in the Eastern Tropical Pacific, eastern boundary currents, and much of the western tropical Pacific (T. obtusifrons distribution).

Most adults are within the thermocline, below 140 m, at night. Daytime occurrences are mainly below about 300 m.


Brinton (1975) notes that the larvae of T. obtusifrons are similar to those of T. aequalis . The eye of the furcilia stages has very distinct facets, as inT. aequalis , but it is only very lightly pigmented, almost yellowish in preserved larvae.

Thysanopoda obtusifrons