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Thysanopoda tricuspidata Milne-Edwards, 1837

Etymology: Thysanopoda - tassel foot; tricuspidata - three-pointed

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

Peduncle of 1st Antenna: The anterior dorsal part of the basal segment differs from other Thysanopoda species by not extending upward and forward as a lobe, but simply extends forward as a spine (T. tricuspidata lappet). The 2nd segment is about twice as long as the 3rd, terminating dorsally as an acute tooth which overlies about 1/4 of the 3rd segment. The 3rd segment is without a keel.

Rostrum: The rostrum is long and acute, extending to the anterior limit of the eye (T. tricuspidata eye & rostrum).

Carapace: A long, straight (forward-directed) spine extends from the highest point on the carapace, approximately to the posterior limit of eye and resembles a 2nd rostrum, above the frontal plate. The lateral margin of the carapace bears two widely-separated denticles (T. tricuspidata ).

Abdomen: The pleuron of the 1st segment is deeply cleft at its lateral limit, appearing almost as two plates. The 3rd-6th segments bear mid-dorsal posterior spines (T. tricuspidata abdominal spine), which give this species the name tricuspidata.

Petasma: The terminal process is barely curved and slender, tapering to a flattened end. The proximal process is conspicuously expanded distally with that portion serrate or crenulated along its inner margin. The median lobe bears a small strongly-curved additional process, distal to the larger lateral process (T. tricuspidata petasma).

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

Length: Adult size is 15-25 mm.

Comments: The long dorsal forward-directed carapace spine is unique to T. tricuspidata. However, it may be homologous to the much shorter such spine on the dorsal keel in T. cristata.

ECOLOGY
T. tricusipidata is an oceanic species and a strong vertical migrator.

HORIZONTAL DISTRIBUTION
The range is tropical in all ocean basins, roughly 20°N-20°S. However T. tricuspidata extends northward to 40°N in the Gulf Stream extension in the mid- and western Atlantic, to about 35°N in the Kuroshio extension in the western Pacific, and southward to South Africa in the Agulhas Current. It is lacking in the eastern Pacific to about 2000 km from the coast, and in the northern Arabian Sea and Bay of Bengal (T. tricuspidata distribution).

VERTICAL DISTRIBUTION
Adults are above the thermocline at night and below 300 m by day, probably associated with the scattering layer.

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

Metanauplius - (1 stage) (T. tricuspidata A)

Carapace:
Shape - The carapace is relatively long with flared frontal hood and large lateral lobes.
Marginal spines - There are spines around the frontal hood, the lateral lobes and the posterior margin in a characteristic pattern. The anterior spines are relatively long.
Abdomen - There are lateral as well as distal spines on the relatively long abdomen.

Calyptopis - (3 stages) (T. tricuspidata B)

Carapace:
Shape - The carapace is pointed posteriorly in C1-C2 with a dorsal lift. In C3 the unique eyes are visible beneath the carapace.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - A pair of denticles is present in C3.
Dorsal crest - absent

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

Furcilia - (number of stages unknown) (T. tricuspidata C), (T. tricuspidata D ), (T. tricuspidata E)

Eye: The eye is unique. It is pear-shaped with 7 large ommatidia in a distal cluster. In a juvenile illustrated by Sars (1895), the eye is broader and the cluster of 7 ommatidia is on a small, round lateral projection.

Carapace:
Frontal plate / rostrum - The rostrum is acute.
Marginal spines - absent
Dorsal spine - In the 6.7-7.0 mm furcilia stage illustrated there is a rudiment of the forwardly- directed dorsal anterior spine of the adult.
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - There is a small keel.


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

Abdomen:
Mid-dorsal spines - Dorsal spines have developed on segments 3, 4, and 5 in the 6.7-7.0 mm furcilia stage illustrated.

Pleopods: A variety of forms with differing levels of pleopod development have been reported. The common developmental pathways are unknown.

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.
Terminal spines - The telson retains 7 terminal spines throughout the furcilia phase, in late stages the medial spine lengthens. In the juvenile illustrated by Sars (1885) the central spine becomes strong and dominant while the 3 spines on either side diminish in length and move to a more proximal position.

Comments: The larvae are characterized by their unique eye and long, slender body. Gurney (1947) notes that the inner setae on the first antenna of the furcilia with 0 and 5" pleopods were orange colored in life.

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

Pictures
T. tricuspidata,selected stages
T. tricuspidata A [nauplius, metanauplius]
T. tricuspidata B [calyptopis 1-3]
T. tricuspidata C [furcilia]
T. tricuspidata D [furcilia]
T. tricuspidata E [furcilia, juvenile]
key to larval illustrations

Thysanopoda tricuspidata