Thysanopoda astylata Brinton, 1975
Etymology: Thysanopoda - tassel foot; astylata - a pillar or support
Eye: The eye is medium in size (T. astylata eye & rostrum). The eye diameter : carapace length is about 0.18.
Peduncle of 1st Antenna: As in T. aequalis, the lobe at the distal part of the basal segment is heavy, posteriorly rounded and directed upward. Anteriorly it flaps forward over the 2nd segment about as far as the midpoint of that segment, to which it closely adheres. The outer part of the flap extends slightly downward and forward of the dorsal part, ending at an acute angle over the outer side of the 2nd segment, beyond its midpoint. The 2nd and 3rd segments are without processes (T. astylata,).
Rostrum: The tip of the frontal plate bears a minute upward-directed tooth that barely leans anteriorly beyond the otherwise rounded (in dorsal view) anterior margin of the frontal plate. Viewed from the side (T. astylata eye & rostrum), the anterior part of the frontal plate appears to narrow as an anteriorly-tapering lobe, the anterior-most upper surface of which bends slightly upward terminating in the small toothlike projection. The frontal plate is medially depressed and in dorsal view, appears broad, its lateral margins tending to converge obtusely (cf. T. aequalis).
Carapace: This is broad, flattened behind the anterior dorsal excavation (trough), and slightly elevated as a keel in the gastric region. A posterior-lateral denticle is present (T. astylata carapace denticle).
3rd thoracic leg: This is not modified in either sex. The dactylus is proportionate in length with the dactylus of other legs (T. astylata thoracic leg).
Abdomen: There are no spines or keels.
Length: Adults are 12-18 mm.
Petasma: This is like that of T. aequalis.
Thelycum: Described by Costanzo and Guglielmo, 1976a.
Comments: T. astylata is the renamed (Brinton, 1975) species long assumed, particularly by workers in the Pacific, to be that described from the Atlantic by Hansen (1905b) as T. aequalis. T. astylata males are characterized by a lack of a styliform process extending from the distal segment of the 3rd thoracic leg. T. aequalis ' males possess the styliform process but are otherwise very similar to T. astylata. It was accidental that Hansen failed to note the modified 3rd leg in his male specimens collected by the Prince of Monaco. Boden justifiably believed that his specimens from South Africa-Mozambique, the males of which had the modified 3rd leg and which he described as the new species, T. subaequalis (1954), therefore differed from the specimens upon which Hansen had based T. aequalis.. Re-examination of Hansen's specimens has shown that the males do possess the modified 3rd leg. Therefore the name T. aequalis must apply to the species in which the 3rd leg is styliform in the adult male.
The frontal plate frequently, but not always, provides useful characters for separating females of the two species in the two ocean regions where they co-occur: in specimens ofT. astylata from the Pacific and Indian ocean basins, the frontal plate, in lateral view, becomes increasingly thin distally, terminating with low ridges bordering a dorsal medial trough. In T. aequalis the distal part of the frontal plate appears more lobe-like and thick in lateral view and narrower in dorsal view.
T. astylata occurs in the Pacific from 40°N to about 35°S, excepting the Eastern Tropical Pacific and the eastern boundary currents. In the western North Pacific, 40°N-0°, it is replaced by T. aequalis , but the two species co-occur in the North Pacific along their mid-ocean limits and in the South Pacific along an east-west belt (ca. 10-15°S). T. astylata occurs in the northern Indian Ocean from about 0°-10°N. It appears to be lacking in the Atlantic (T. astylata distribution).
Adults occur at about 300-600 m in the daytime and at dusk, migrate up into the thermocline at depths of about 25-200 m (nighttime).
The larvae may be almost identical to those of T. aequalis .