Thysanopoda acutifrons Holt and Tattersall, 1905
Etymology: Thysanopoda - tassel foot; acutifrons - with sharp forehead
Eye: The eye is small (T. acutifrons eye & rostrum). The eye diameter : carapace length is about 0.13.
Peduncle of 1st Antenna: The distal dorsal margin extends forward as a roughly triangular extension of a setose lobe; this extends about one-third of the way along the 2nd segment (T. acutifrons,).
Rostrum: The rostrum is a forward- and slightly upward-pointing process (tooth) (T. acutifrons eye & rostrum). The frontal plate is triangular and the apical angle is greater than a right angle; it extends to or beyond the eye, depending on the eye stalk's direction.
Carapace: There are no denticles on the lateral margins of the carapace in the adult (T. acutifrons), but a pair is present in immature specimens.
Abdomen: There are no spines on the abdominal segments. The median posterior margins of all segments are smoothly curved; none are pointed.
Length: Adults are 35-50 mm.
Petasma: The spine-shaped process is almost straight. The terminal process is distinctly shorter than the proximal process and tapers to an acute, curved end. The proximal process is almost straight along most of its distal half and is moderately curved toward its base; it is trifid at the tip and coarsely toothed before the tip. The lateral process is slender and slightly hooked and is about as long as the terminal process. The 1st additional process is a broad lobe and subacute, whereas a 2nd additional process is slender, straight and acute (T. acutifrons petasma).
Thelycum: Described by Einarsson, 1942, 1945; Guglielmo and Costanzo, 1977.
Comments: The short acute rostrum of T. acutifrons (dorsal view) (T. acutifrons dorsal head) is not longer than the more upward directed rostral tooth in T. orientalis and T. microphthalma . In lateral view the rostral process of T. acutifrons is not distinguishable from the similarly forward- directed frontal plate.
ECOLOGY
T. acutifrons is a food source for whales and demersal fish.
HORIZONTAL DISTRIBUTION
T. acutifrons is subarctic and subantarctic. It occurs in two zones of about 35°-50°N and 40°-60°S in the Pacific. In the Atlantic it has been reported from 35°-70°N and is known from South African waters near 30°-40°S in both the Atlantic and Indian ocean basins (T. acutifrons distribution).
VERTICAL DISTRIBUTION
Adults are mostly above 140 m at night but are not found in the mixed surface layer. They are believed to descend below 500 m by day. Most larvae are above 300 m.
LARVAE
See the development summary (T. acutifrons Table) for the stage descriptors and length in stage.
Metanauplius - undescribed
Calyptopis 2-3 (3 stages) (T. acutifrons A)
Carapace:
Shape - The carapace is pointed posteriorly.
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.
Shape - The telson is wide distally.
Furcilia - (7 stages) (T. acutifrons B), (T. acutifrons C), (T. acutifrons D)
Eye: The faceted portion is cylindrical and higher than broad. It has a characteristic three-lobed appearance with distinct facets. The cones are arranged in three portions: an upper portion, where the cones face upwards and forward, a middle portion, where the cones point forward, and a lower portion, where the cones point forward and downwards.
Carapace:
Frontal plate / rostrum - The rostrum is acute.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A keel is present.
Thoracic legs: There is sequential development without elongate leg (s).
Abdomen:
Mid-dorsal spines - absent
Pleopods: The common developmental pathway is 3'- 3"2'- 5".
Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.
Comments: The larvae are relatively large. The shape of the carapace in calyptopis stages and the three-lobed eye in furcilia stages are characteristic of the species.
Brinton (1975) suggests that the description and figures of T. acutifrons may prove useful in recognizing larvae of the other members 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 analogous stages development.
Development
(T. acutifrons table), development summary for the stage descriptors and length in stage.
Pictures
T. acutifrons, selected stages
T. acutifrons A [calyptopis 2-3]
T. acutifrons B [furcilia 1-2]
T. acutifrons C [furcilia 3-4]
T. acutifrons D [furcilia 5-7, juvenile]
key to larval illustrations