Themisto gaudichaudi

Themisto gaudichaudi Guérin, 1825

Languages: English

Overview

Biology

Notes: 

Two forms -- compressa and bispinosa -- have been identified within the species, which are distinguished by the degree of development of the dorsal denticles on somites VI-VII of the pereon and somites I-II of the pleon, the shape and ornamentation of the segments of the pereopods, and by the relative length of pereopods V and VI. [Guérin, 1825]
Author(s): Dunn , Adam
Rights holder(s): Dunn , Adam

Description

Diagnostic Description

Somites V and VII of the pereon and somites I and II of the pleon bear dorsal denticles developed to a various degree. In females and young males the flagellum of antennae I is conical, with a curved and narrow distal part not divided into segments. In sexually mature males the proximal segment of the flagellum is oval-conical and twice the length of the peduncle; the remaining 12-15 segments are thin, virgate. Antennae II are thin and much longer than antennae I; as males mature the flagellum of antennae II becomes segmented, elongates, and in sexually mature specimens consists of 15-20 segments; in females the flagellum is much shorter and not divided into segments. The structure of the mouthparts does not differ significantly from other species of the genus. Maxillae I have a small, poorly developed inner lobe. The 5th segment of pereopods I is elongated-oval, the 6th conical and slightly curved; the distal part of the 6th segment is finely denticulate; the strong curved claw is half the length of the 6th segment and the proximal part of its posterior margin is finely denticulate. Pereopods II are longer than pereopods I; the 5th segment is distally broadened and its distal process extends to 1/2-4/5 the length of the slightly tapering 6th segment; both margins of this process bear long strong setae and a much stronger seta occurs at the apex; in the 6th segment the posterior margin is very finely denticulate; the strong, almost straight claw is half the length of the 6th segment and finely denticulate in the proximal part of its posterior margin. the 4th segment of pereopods III is notably narrower and about half the length of the broadly oval 5th segment; the maximum width of the 5th segment is 1/2 its length in males and 2/3 in females, and the posterior margin more convex and armed with strong setae; ornamentation of the 5th segment is weaker in females than in males; the 6th segment is much narrower, slightly curved and approximately the same length or somewhat longer than the 5th segment; the claw is almost straight, smooth, and 1/3 to 2/5 the length of the 6th segment. Pereopods IV are similar in structure but the 5th segment is relatively broader and in males its maximum width may be more than half its length. Pereopods V are longer than all the other legs; the 2nd segment is broad and shorter than the narrowly lanceolate 5th segment or almost equal to it; the thin, straight, or slightly curved 6th segment is longer than the 5th segment, and armed along its anterior margin with sparse submarginal setae with a row of dense short and tender setae between them (in f. bispinosa these setae, especially in the distal part of the segment, are notably longer than in f. compressa); the claw is almost straight, its anterior margin neither armed nor denticulate, and 2/3 the length of the 6th segment is young specimens but only 1/10-1/6 in sexually mature specimens. Pereopods VI and VII are identical in structure but pereopods VII somewhat longer than VI; the 6th segment is somewhat (1.2-1.5 times) longer than the 5th segment and equal to or somewhat longer than the 2nd; the claw is smooth and almost straight. The posterior margin of the exopodites and the anterior margin of the endopodites are finely denticulate in all the uropods. The inner distal angle of the basipodite of uropods III is stretched into a denticle. The inner margin of both rami of uropods II has a sharp concavity in the proximal part; in this concavity both rami bear numerous minute tender setae on the ventral surface. The exopodites of the uropods are shorter than the basipodites, constituting in the young about 3/4, but in sexually mature individuals about 1/2 their length. The triangular-oval telson has an obtuse apex and extends to 1/5-1/3 the length of the basipodite of uropods III. [Guérin, 1825]
Author(s): Dunn , Adam
Rights holder(s): Dunn , Adam

Look Alikes

Guérin, 1825: 744 (Themisto); Bovallius, 1889: 299; Barnard, 1930: 420; 1932: 280; Stephensen, 1933: 63; 1944: 10; 1947: 76 (Themisto); Pirlot, 1939: 39; Hurley, 1955: 161; Bowman, 1960: 379; Kane, 1964: 35; Sheader and Evans, 1974: 915. --gracilipes, (?) obliva Norman, 1869: 287 (Hyperia). --gracilipes Stephensen, 1924: 97 (Themisto). --compressa Stephensen, 1924: 103 (Themisto); Chevreux, 1935: 191 (Euthemisto). --bispinosa Chevreux, 1935: 191 (Euthemisto). --antarctica Ealey and Chittleborough, 1956: 22 (Euthemisto).
Author(s): Dunn , Adam
Rights holder(s): Dunn , Adam

Size

A highly variable species. Length of sexually mature specimens ranges from 4 to 28 mm. [Guérin, 1825]
Author(s): Dunn , Adam
Rights holder(s): Dunn , Adam

Ecology and Distribution

Dispersal

During the day the main mass of crustaceans remains deeper of 25-50 m, including layers at 100-200 and 200-500 m, but at night the major part of the population ascends to lesser depths, right up to the very surface. [Guérin, 1825]
Author(s): Dunn , Adam
Rights holder(s): Dunn , Adam

Distribution

A bipolar, circum-Antarctic, moderately cold-water species. In the Northern Hemisphere it inhabits cold-water regions of the Atlantic Ocean from 76° to 40° N. It penetrates farther south in small numbers and is found in the Bermuda and Canary Islands and in the Mediterranean Sea. It is not found in the Central Polar Basil. In the northern part of the Pacific Ocean it is reported only from the Yellow and East China seas (identification doubtful), but is absent in farther northern regions. In the Southern Hemisphere it inhabits the entire South Ocean from the coastal pack ice to the zone of the Antarctic Convergence. Together with the cold Antarctic currents it also penetrates farther north, where it has been found in Cape Town (37° S) western Australia (18-25° S), (107-108° E), southern coastal areas of Australia (41° S), and in the Pacific oceanic coastal areas of South America (the Juan Fernandez Islands). In the Southern hemisphere, P. gaudichaudi is one of the numerous species of the Antarctic plankton, forming at places vast concentrations in the surfacial layer of the water, often together with concentrations of Euphausia superba. Farther north of the Antarctic Convergence it is confined mainly to cold Antarctic intermediate waters. [Guérin, 1825]
Author(s): Dunn , Adam
Rights holder(s): Dunn , Adam