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Thysanopoda aequalis Hansen, 1905

Etymology: Thysanopoda - tassel foot; aequalis - equal

Eye: The eye is medium in size (T. aequalis eye & rostrum). The eye diameter : carapace length is about 0.18.

Peduncle of 1st Antenna: As in T. astylata, the lobe on the basal segment is heavy, and posteriorly it is rounded, directed upward, and somewhat backward or reflected. Anteriorly it flaps forward over the 2nd segment about as far as the midpoint of the segment, to which it adheres closely. The outer part of this flap extends slightly downward and forward, of the dorsal part, ending as an acute triangle over the outer side of the 2nd segment, beyond its midpoint (T. aequalis lappet). The 2nd and 3rd segments are without processes, but the 2nd has an upper inner end produced into a rather short lobe covering a small part of 3rd joint, as in T. obtusifrons, T. astylata, and some other species.

Rostrum: The tip of the frontal plate sometimes bears a minute vertical tooth which, in dorsal view (T. aequalis dorsal head), does not project anteriorly beyond the smoothly rounded anterior margin of the frontal plate. This tooth is frequently lacking. Behind the tip of the frontal plate there is a shallow longitudinal trough bordered by low, broad, poorly defined ridges. The posterior part of the trough contains, medially, a low keel. Viewed from the side (T. aequalis eye & rostrum), the frontal plate appears as a thickened lobe along its full length, the dorsal and ventral surfaces of which are almost parallel, scarcely converging anteriorly until the bluntly rounded front end. In dorsal view, the frontal plate appears narrow, its lateral margins tending to coverage at less than 90°angle. (cf. T. astylata).

Carapace: The head region is broad, dorsally flattened behind the anterior trough, and only slightly elevated as a short keel in the gastric region. There is a postero-lateral denticle (T. aequalis carapace denticle).

3rd thoracic leg: In the adult male, greater than about 12 mm in length, the propodus is stouter than in the other thoracic legs, and the dactylus is reduced but bears a long, strong styliform spine (T. aequalis thoracic leg). In the female, the distal segments are not modified and similar to the 4th-7th legs.

Abdomen: There are no spines or keels.

Length: Adults are 12-19 mm.

Petasma: The spine-shaped process is small, slender, almost straight, and occasionally lacking. The terminal process is tapering, directed somewhat outward, then curving inward before again reflexing distally. The distal portion is flattened but the tip appears acute. The proximal process is longer than the terminal. Configuration of the proximal process is S-shaped; near the tip, the anterior-outer surface bears 1-3 minute papillae. The lateral process is strong, almost as long as the terminal, and is distally curved; in outline the inner margin is concave, providing the basal portion with a heel-like aspect. The additional process is a strongly curved hook (T. aequalis petasma).

Thelycum: Described by Sebastian, 1966; Costanzo and Guglielmo, 1976b.

Comments: T. aequalis (Hansen, 1905b) is the species distinguished as T. subaequalis by Boden (1954) on the basis of the long styliform process on the adult male 3rd leg. Hansen's omission of mention of the male's styliform process led to the false assumption by Boden (1954), Boden and Brinton (1957), Brinton (1962b) and others that, of the two species here recognized as constituting a sibling pair (T. aequalis and T. astylata), Hansen had described the one which possesses an unmodified 3rd leg.

The type material from Monaco used for the original description of T. aequalis has now proved to possess the styliform 3rd leg in the adult male. That character therefore belongs to T. aequalis and distinguishes it from T. astylata. T. subaequalis Boden then became a synonym of T. aequalis and a new name was required for the species with the non-styliform 3rd leg (T. astylata).

The frontal plate frequently, but not always, provides useful characters for separating the two species in the two ocean regions where they co-occur: in specimens of T. 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.

ECOLOGY
T. aequalis is a food source for fishes.

HORIZONTAL DISTRIBUTION
In the Atlantic T. aequalis occurs from the Gulf Stream extension (40-50°N) southward to 35°S, but is sparse or lacking in much of the equatorial belt. It occupies the southern Indian Ocean, 0-30°S, but seasonly occurs north of the equator in the Somali Current and south of 30°S in the Agulhas Current. In the Pacific it occurs in the mid- and western North Pacific, and in the South Pacific it has been found along an east-west belt (ca 10-15°S), co-occurring there with T. astylata .(T. aequalis distribution).

VERTICAL DISTRIBUTION
T. aequalis adults migrate between the upper part of the thermocline (nighttime) and the depth of the deep scattering layer, about 300-600 m (daytime).

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

Metanauplius - undescribed

Calyptopis - (3 stages) (T. aequalis A)

Eye: By C3, there is pigment in the developing eye and some ommatidia are visible.

Carapace:
Shape - The anterior and posterior margins appear slightly pointed by C3.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present in C3.
Dorsal keel - a small keel is present in C3 slightly posterior to the midpoint of the carapace.

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

Furcilia - (5 stages) (T. aequalis B), (T. aequalis C), (T. aequalis D)

Eye: The eye is bilobed. The facets are distinct and slightly raised individually giving a crenulate appearance.

Carapace:
Frontal plate / rostrum - The frontal plate is broadly triangular with a small rostral spine.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A small keel is situated slightly posterior to the midpoint of the carapace in early furciliae and near the midpoint in later stages.

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

Abdomen:
Mid-dorsal spines - absent
Segment 1 - The tergal collar of segment 1 may overlap the posterior margin of the carapace.

Pleopods: The common developmental pathway is 0 - 2'- 2"2'- 4"1'- 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.
Terminal spines - The form with 1t + 3pl spines may have 2 very small spines on either side at the base of the long terminal spine.

Comments: The broad frontal plate and structure of the eye are characteristic of the species in the furcilia phase. The larvae are described by Lebour (1950) and Casanova (1974) as large and having, in dorsal view, a heavy and clumsy or awkward appearance. The mid-late furcilia stages have characteristic coloring in life. There is red pigment on the first antenna, behind the eye, on the thorax, and on the abdomen as a broad red band on segment 3 which extends a little into segments 2 and 4.

Brinton (1975) notes that the larvae of T. obtusifrons are similar to those of T. aequalis . The eye of the furcilia stages has very distinct facets, as inT. aequalis , but it is only very lightly pigmented, almost yellowish in preserved larvae. The frontal plate may also be narrower.

The larvae of T. astylata are undescribed but they may be almost identical to those of T . aequalis . The larvae described by Lebour and Casanova are from regions where only T. aequalis occurs.


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

(T. aequalis Graph 1) , a histogram of developmental form frequency, from the Mediterranean.

(T. aequalis Graph 2), a histogram of length freqency by stage, from the Mediterranean.

Pictures
T. aequalis, selected stages
T. aequalis A [calyptopis 1-3]
T. aequalis B [furcilia 1-2]
T. aequalis C [furcilia 3-4]
T. aequalis D [furcilia 4]
key to larval illustrations

Thysanopoda aequalis