Thysanopoda monacantha Ortmann, 1893
Etymology: Thysanopoda - tassel foot; monacantha - single spined
Eye: (T. monacantha eye & rostrum) The eye is medium in size. The eye diameter : carapace length is about 0.16. Small immatures have a proportionally much larger eye with a very small lobe appended to the top. This lobe disappears during further development.
Peduncle of 1st Antenna: A setose lobe on the upper distal margin of the 1st segment extends anteriorly as a long spiniform process directed somewhat outward and downward, reaching to or beyond the midpoint of the 2nd segment. The 2nd segment also extends anteriorly as a slender, acute, dorsal process, overlying the basal half of the 3rd segment (T. monacantha,). A notched keel is situated on the distal dorsal surface of the 3rd segment (T. monacantha lappet).
Rostrum: There is an acute, forward directed, triangular plate, reaching almost to the anterior limit of the eye. The apex narrows and projects slightly upward (T. monacantha eye & rostrum) (T. monacantha dorsal head).
Carapace: A longitudinal furrow extends the length of carapace just above the lateral margin. The margin of the carapace bears a postero-lateral denticle (T. monacantha carapace denticle).
Abdomen: The 3rd segment bears a mid-dorsal posterior spine. The 4th and 5th segments each sometimes have a small mid-dorsal spine or are slightly acuminate (T. monacantha abdominal spine).
Length: Adults are 25-32 mm.
Petasma: The terminal process curves somewhat outward above a broad base, then tapers symmetrically to an acute tip. The proximal process is of similar length and thickness, but above its stout basal portion it bends sharply inward at <45°, toward the terminal process. The lateral process is nearly straight. The additional process is half as long and slightly curved (T. monacantha petasma). In specimens from the Atlantic, at least, there is usually a 2nd small, slender additional process.
Thelycum: Described by Sebastian, 1966; Guglielmo and Costanzo, 1977.
Comments: The spiniform extensions of the 1st two segments of the peduncle of the 1st antenna, and the tapering forward- then slightly upwardly directed rostral process are characteristic.
ECOLOGY
T. monacantha is a mesopelagic species centered near 300-400 m and does not migrate above the thermocline.
HORIZONTAL DISTRIBUTION
T. monacantha occurs between the 35° parallels, north and south, in all three ocean basins. It is lacking in the O2-deficient Indian Ocean north of 10°N and Eastern Tropical Pacific north of 10°N. In the California and Peru-Chile regions it is found only far seaward of the coastal currents (T. monacantha distribution).
VERTICAL DISTRIBUTION
Adults appear to be in the 100-500 m layer, and are more concentrated in the upper part of it at night.
LARVAE
See the development summary (T. monacantha Table) for the stage descriptors and length in stage.
Metanauplius - undescribed
Calyptopis 2-3 (3 stages) (T. monacantha A)
Carapace:
Shape - The carapace is large and pointed posteriorly. The lateral margin is indented a little below the eye.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - A small pair of denticles is present in C3.
Furcilia - (number of stages unknown) (T. monacantha B), (T. monacantha C)
Eye: The eye is large and round with pigmented area slightly lobed anteriorly.
Carapace:
Frontal plate / rostrum - The rostrum tapers to a point.
Marginal spines - absent
Postero-dorsal spines - 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
Segment 2 - There is a dorsal hump which projects posteriorly.
Pleopods: The 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 medial spine elongates during the furcilia phase.
Comments: The very large carapace in the calyptopis phase, and the round black eye and dorsal hump on abdomen segment 2 in the furcilia stages are characteristic of the species. The upper eye lobe, which is incipient in the furcilia phase, becomes distinct in the juvenile, before disappearing in the adult.
Development
(T. monacantha Table), development summary for the stage descriptors and length in stage.
Pictures
T. monacantha, selected stages
T. monacantha A [calyptopis 2-3]
T. monacantha B [furcilia]
T. monacantha C [furcilia, juvenile]
key to larval illustrations