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Thysanopoda orientalis Hansen, 1910

Etymology: Thysanopoda - tassel foot; orientalis - oriental

Eye: The eye is medium in size (T. orientalis eye & rostrum). The eye diameter : carapace length is about 0.16.

Peduncle of 1st Antenna: The dorsal lobe on the basal segment is abruptly elevated posteriorly, continuing to be high anteriorly over a short portion of the 2nd segment and ending as an acute angle, above 2nd segment. The inner dorsal part of the 2nd segment extends anteriorly over the base of the 3rd segment as a low cover (T. orientalis,).

Rostrum: There is a short forward- and upward-pointing tooth (T. orientalis eye & rostrum) situated at the apex of the thick, broadly triangular (in dorsal view) frontal plate. The distinctive tooth's angle with the horizontal is 45°. In immature specimens (about 15-23 mm) the rostal process, seen laterally, is long, spiniform, and more anteriorly directed than in the adult.

Carapace: The lateral margin is without denticles in the adult (T. orientalis), but a pair is present in immature specimens.

Abdomen: There are no dorsal spines, but the mid-dorsal posterior margins of the 4th and 5th segments are slightly pointed (acuminate) (T. acutifrons vs. T. orientalis ).

Length: Adults are 23-38 mm.

Petasma: The terminal process is strong and straight, ending as a bluntly rounded spoon-like lobe which has the same width as the stem of the process. The proximal process is evenly curved, almost in a semicircle, bending inward, then upward, and extending distinctly beyond the end of the terminal process. On its distal underside the proximal process appears serrated. The lateral process is tapered, distally hooked, and extends to the median lobe from which it arises. There is a lobe-like additional process (T. orientalis petasma). One or two short spiniform additional processes are sometime present, arising near the lateral process.

Thelycum: Described by Einarsson, 1942; Sebastian, 1966; Guglielmo and Costanzo, 1977.

Comments: T. orientalis is larger than T. obtusifrons with which it co-occurs in the subtropical regions. With respect to the frontal plate and rostral process, these two species are similar. In T. orientalis the lobe of the basal segment of the peduncle of the 1st antenna extends forward over the 2nd segment in a much more elevated and distally angular form than in T. obtusifrons or T. aequalis-T. astylata. T. acutifrons , a high-latitude sibling of T. orientalis , is best distinguished by the shapes of the rostrum and frontal plate. T. microphthalma is not known from the Pacific, and is very similar to T. orientalis with which it appears to co-occur in the Atlantic. In T. orientalis the proximal process of the petasma, extends beyond the tip of the terminal process, but in T. microphthalma the terminal process extends the farther.

ECOLOGY
The species of this group (T. orientalis, T. microphthalma, andT. acutifrons) are mesopelagic, living beneath the thermocline to about 500 m depth. T. orientalis is a food source for planktivorous fish.

HORIZONTAL DISTRIBUTION
T. orientalis is tropical-subtropical (40°N-40°S) in Pacific, but the Atlantic range is indistinct because of the difficulty of identifying all but fully adult males from T. microphthalma. T. orientalis is absent from eastern boundary currents in the Pacific and from the O2-deficient Bay of Bengal, Arabian Sea, and eastern Tropical Pacific, 10-22°N. It has been reported to 40°S in the Indian Ocean (T. orientalis distribution).

VERTICAL DISTRIBUTION
Most adults are in the range of 300-500 m, day and night, but individuals disperse beyond these limits. Most larvae are at 140-500 m.

LARVAE
See the development summary (T. orientalis Table) for the stage descriptors and length in stage.

Metanauplius - undescribed

Calyptopis - undescribed

Furcilia - intermediate stage (number of stages unknown) (T. orientalis A)

Eye: The eye is bilobed with distinct facets in the upper portion. Hansen (1910) notes that the upper portion is moderately developed while the lower part is still in a rudimentary state both as to cornea and visual units.

Carapace:
Frontal plate / rostrum - The rostrum is acute.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - There is a relatively large keel.

Thoracic legs: There is sequential development without elongate leg (s).

Abdomen:
Mid-dorsal spines - absent

Pleopods: A developmental pathway is 3'- 3"2'- 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.

Juvenile - (10 mm) (T. orientalis A)

Eye - The eye is light brown, considerably higher than broad, and divided into 2 areas by a constriction a little above the middle. In the adult the eye is round.

Comments: Lebour (1950) notes that in life, a furcilia with 3"2' pleopods and a later furcilia were bright red all over the body, on the first antenna, thorax, intestine, and all along the abdomen. The tips of the flagellae of the first and second antennae were also bright red.

Brinton (1975) suggests that the description with figures of T. acutifrons may prove useful in recognizing larvae of the acutifrons-orientalis-microphthalma group. Brinton (1962) notes that the larvae of T. acutifrons and T. orientalis can be separated most readily on the basis of the larger size of T. acutifrons at analagous stages of development.

Development
(T. orientalis Table), development summary for the stage descriptors and length in stage.

Pictures
T. orientalis, selected stages
T. orientalis A [furcilia, juvenile]
key to larval illustrations

Thysanopoda orientalis