Thysanopoda cristata G.O. Sars, 1883
Etymology: Thysanopoda - tassel foot; cristata - crested
Eye: The eye is conspicuously large (T. cristata eye & rostrum). The eye diameter : carapace length is about 0.22.
Peduncle of 1st Antenna: The dorsal anterior margin of the basal segment is raised as a hood over the base of the 2nd segment. This hood is relatively high and short (antero-posteriorly) compared with similar structures in other Thysanopoda species (T. cristata lappet). The 2nd segment is not produced anteriorly as an elevated process, but as a low triangular cover above about 1/4 of the dorsal surface of the 3rd segment. The 3rd segment is without a keel, but the dorsal-distal part is somewhat elevated (T. cristata,).
Rostrum: It is slender and acute, reaching to the anterior limit of the eye. The frontal plate has a conspicuous keel which extends forward, but not quite to the tip of the rostrum, and ends abruptly as a forward-directed tooth, elevated well above the rostrum (T. cristata eye & rostrum).
Carapace: The keel on the frontal plate becomes abruptly higher in the gastric region, extending posteriorly to the midpoint of the thorax. There is a sub-marginal lateral furrow on each side of the carapace, above a small postero-lateral denticle (T. cristata carapace denticle).
Abdomen: The 4th and 5th segments bear mid-dorsal posterior spines (T. cristata abdominal spine). The 6th segment is short, scarcely longer than the 5th. There are seven pairs of lateral spines on the telson.
Length: Adults are 32-65 mm.
Petasma: Terminal and proximal processes are slender and curving, tapering to sub-acute flattened ends. The lateral process is long, tapering to a hooked end. A plate-like outgrowth of the median lobe extends toward the distal hook of the lateral process (T. cristata petasma).
Thelycum: Described by Guglielmo and Costanzo, 1977.
Comments: The high vaulted lobe of the 1st segment of the peduncle of 1st antenna, the high, anteriorly notched keel of the frontal plate, and the mid-dorsal spines on the 4th and 5th abdominal segments are distinctive for T. cristata . The short 6th segment is also a helpful character (T. cristata ).
ECOLOGY
T. cristata is considered a mesopelagic species. However, the large eye and the robust musculature of the large body are consistent with a shallower euphotic habitat and a strong net-avoidance capability.
HORIZONTAL DISTRIBUTION
T. cristata appears to be sparsely distributed between about 30°N and 30°S in the Atlantic and Pacific ocean basins, excluding the broad (ca 2000 km wide) eastern boundary regions on the eastern tropical Pacific. In the Indian Ocean it has been found south of the equator to 30°S, with one record in the Bay of Bengal at 10°N (T. cristata distribution).
VERTICAL DISTRIBUTION
Adults have been caught only below 280 m; larvae and juveniles appear centered in the 150-300 m layer, probably in the thermocline.
LARVAE
See the development summary (T. cristata Table) for the stage descriptors and length in stage.
Metanauplius - undescribed
Calyptopis - undescribed
Furcilia - (number of stages unknown) (T. cristata A)
Eye: The eye is bilobed.
Carapace:
Frontal plate / rostrum - The rostrum is long and acute.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A relatively high dorsal keel is present which drops down sharply posterior to its peak.
Thoracic legs: There is sequential development without elongate leg (s).
Abdomen: In the oldest larva illustrated, there are posterior extensions of the mid-dorsal parts of the 3rd and 4th abdominal segments.
Pleopods: A 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.
Terminal spines - The medial spine elongates in the last two furcilia forms illustrated.
Comments: The older furcilia larvae are characterized by the long acute rostrum, the bilobed eye, the high dorsal keel, and the extension of abdominal segments 3 and 4. The young of T. cristata resemble T. pectinata but are larger and have a more conspicuous dorsal carapace keel. Gurney (1947) describes the furcilia with 3' pleopods as having a body that is vivid red in life, the color is seated in numerous small scattered chromatophores which contract rapidly in death. The first antennae have stout, profusely hairy yellow setae.
Development
(T. cristata Table), development summary for the stage descriptors and length in stage.
Pictures
T. cristata, selected stages
T. cristata A [furcilia]
key to larval illustrations