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Thysanoessa parva Hansen, 1905

Etymology: Thysanoessa - small tassel; parva - small, little

Eye: The eye is bilobed with a transverse constriction. It is slightly smaller than in T. gregaria. The width of the upper lobe is smaller than the width of the lower lobe (T. parva eye & rostrum).

Peduncle of 1st Antenna: As inT. gregaria, the 1st segment is equal in length to the 2nd and 3rd combined. The 2nd and 3rd segments are cylindrical and of equal length. There is no lappet (T. parva lappet lacking).

Rostrum: The rostrum is slender and ends near the anterior margin of the eyes (T. parva eye & rostrum).

Carapace: There is a small denticle near the posterior angle of the lateral margin (T. parva carapace denticle).

Thoracic legs: The 2nd pair is clearly elongate. The setae along both the upper and lower margins of the carpal and propodal segments of the 2nd to 4th thoracic legs are short and slender compared with the terminal (distal) setae on those legs (T. parva thoracic leg).

Abdomen: There are no keels or spines (T. parva).

Length: Adults are 9-10.5 mm.

Petasma: The terminal process is stout, and ends in a smooth, rounded tip (T. parva petasma).

Thelycum: Described by Costanzo and Guglielmo, 1977.

Comments: T. parva , T. gregaria , and young specimens of T. longipes are sometimes difficult to separate. In the Pacific, there is little overlap in their distributions. T. parva has been found in deep water near the offshore limits of the submerging part of the population of T. gregaria . It has been suggested thatT. parva may be an ecotypical variant of T. gregaria . There is a difference between the relative lengths of the lateral and distal setae on the two distal segments ot the 2nd to 4th thoracic legs in T. parva, but not inT. gegaria.

In the North Pacific, T. parva has been taken in deep hauls ( to 300 m or more) at distances of 400-600 miles off California, 23-33°N; east of Japan it has been caught at 32-38°N between 135 and 165°E. In the South Pacific it has been caught east of New Zealand, between 40°S, 164°W and 26°S, 156°W. In the western Atlantic it has been taken 35-40°N, 67-71°W between Cape Cod and Bermuda (Leavitt, 1938). In the eastern Atlantic it has been taken off Portugal and northwest Africa, 15-40°N, and off southwest Africa 20-35°S. James (1987) found both adults and subadults at 40°N, 20°W (T. parva distribution). It is not known whether the known eastern and western distributions are connected in either ocean.

In the Pacific maximum numbers are below 500 m, possibly to 1000 m or more. In the North Atlantic it was found at all depths between 400 and 3000 m, with maxima at 800 m and 1600 m.


The larvae may be very similar to T. gregaria but smaller.

Thysanoessa parva