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G.O. Sars, 1883

Description
Eye bilobed with a transverse constriction. The width of the upper lobe is about half the width of the lower lobe [T.gregaria eye & rostrum ]. The eye of the fully adult male is often larger than in the female [T.gregaria male & female ].
Peduncle of first antenna: the first segment is about as long as the second and third segments combined. The second and third segments are slender and cylindrical. The third is slightly longer than the second. There is no elevated lappet on the first segment, only a forward directed angle.
Rostrum acute but short, extending beyond the midpoint of the eye [T.gregaria eye & rostrum ].
Carapace with a well-developed lateral denticle [T.gregaria-m-drawing ]. At its post-ocular edge, the base of the frontal plate bends slightly upward and outward as a narrow flange, appearing to slightly shield the upper part of the eye. (Occurs in most species of Thysanoessa).
The second pair is elongated [T.gregaria 2nd thoracic legs ]. The setae along the upper and lower margins of the fifth and sixth (carpal and propodal) segments of the second to fourth thoracic legs are near-equal in length to the terminal setae on those legs.
Abdomen without keels or spines [T.gregaria-m-drawing ]. The length of segment 6 is markedly less than the length of segments 5 and 4 together.
Petasma with a small, slender and strongly curved spine-shaped process. The terminal process has a broad base, curves slightly at the first one-third of its length, and then maintains the same breadth until it expands somewhat at its broad, truncate, delicately serrate edge. The proximal process is longer and more slender than the terminal process and somewhat curved. Distally, it ends as a triangular, wing like expansion, which is serrate along the distal margin. Beyond this extension is a short, slender, somewhat curved, subacute process with teeth along the outer margin. The lateral process is long and slightly curved. Its level of insertion is considerably distal to that of the proximal process. The median lobe is narrow and truncate [T.gregaria petasma ].
Reproduction: sexual mature at a length of >8 mm to breed at one year of age, probably in spring and early summer; may not survive a second year. (After Mauchline, 1984).

Size
Adults are 8-17 mm.

Larval stages
T.gregaria-A
T.gregaria-B
T.gregaria-detail-photo
T.gregaria-eye-photo
T.gregaria-eye-rostrum-photo
[l][][f][/f]# T.gregaria-photo
T.gregaria-rostrum-photo

Ecology
T. gregaria is an offshore filter-feeding species. This species is consumed by fish, birds, and whales.

Depth range
Epi- mesopelagic. Diurnal vertical migration from 400-600 m during the day to 100-150 m during night is reported by Mauchline, 1984. Toward the offshore limit of its range it tends to submerge beneath the increasingly warm surface layer and follows the thermocline to 100-200 m depth.

Distribution in the North Sea
Regarding its depth range, T. gregaria is unlikely to occur in the North Sea. Also, its northern distribution limit in the Atlantic is at about 50°N. However, T. gregaria has been found regularly north of the North Sea, in the vicinity of the Orkney and Shetland Islands (S. Hay, pers. comm.).

World distribution
N and S Pacific, middle and S Atlantic, and Indian Ocean basins.

Remark
T. gregaria has been found infected with the parasitic Thalassomyces fagei (Ellobiopsidae), Notophryxux globularis (Crustacea: Isopoda: Dajidae), Distomum filiferum (Trematoda) and cysts of apostome Ciliata (Mauchline, 1984).

[After from Brinton et al., 1999]

Thysanoessa gregaria