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Thysanoessa gregaria G.O. Sars, 1883

Etymology: Thysanoessa - small tassel; gregaria - pertaining to a flock

Eye: The eye is bilobed with a transverse constriction. The width of the upper lobe is about half the width of the lower lobe (T. gregaria eye & rostrum). The eye of the fully adult male is often larger than in the female (T. gregaria-large male).

Peduncle of 1st Antenna: The 1st segment is about as long as the 2nd and 3rd segments combined. The 2nd and 3rd segments are slender and cylindrical. The 3rd is slightly longer than the 2nd. There is no elevated lappet on the first segment, only a forward directed angle (T. gregaria lappet lacking).

Rostrum: It is acute, extending beyond the midpoint of the eye (T. gregaria dorsal head) (T. gregaria eye & rostrum).

Carapace: There is a well-developed lateral denticle (T. gregaria carapace denticle). At its post-ocular edge, the base of the frontal plate bends slightly upward and outward as a narrow flange (flange photo), appearing to slightly shield the upper part of the eye. (This occurs in most species of Thysanoessa.)

Thoracic Legs: The 2nd pair is clearly elongate (T. gregaria 2nd thoracic legs). The setae along the upper and lower margins of the 5th and 6th (carpal and propodal) segments of the 2nd to 4th thoracic legs are near-equal in length to the terminal setae on those legs (T. gregaria thoracic leg).

Abdomen: There are no keels or spines (T. gregaria).

Length: Adults are 8-17 mm.

Petasma: There is a small, slender and strongly curved spine-shaped process. The terminal process has a broad base, curves slightly at the first one-third of its length, and then maintains the same breadth until it expands somewhat at its broad, truncate, delicately serrate edge. The proximal process is longer and more slender than the terminal process and somewhat curved. Distally, it ends as a triangular, wing like expansion, which is serrate along the distal margin. Beyond this extension is a short, slender, somewhat curved, subacute process with teeth along the outer margin. The lateral process is long and slightly curved. Its level of insertion is considerably distal to that of the proximal process. The median lobe is narrow and truncate (T. gregaria petasma). The auxiliary lobe is shorter than that of T. longipes .

Thelycum: Described by Costanzo and Guglielmo, 1976b, 1977.

Comments: The post-ocular flange at each anterior edge of the frontal plate, though present on other Thysanoessa spp., is useful in separating co-occurring T. gregaria and young of Nematoscelis difficilis of similar size (T. gregaria & N. difficilis) (especially when the elongate 2nd legs of N. difficilis are broken off).

ECOLOGY
T. gregaria is an offshore filter-feeding species. This species is consumed by fish, birds, and whales.

HORIZONTAL DISTRIBUTION
T. gregaria occurs in the North and South Pacific, middle and South Atlantic, and Indian ocean basins. It is considered a Transition Zone species, bordering the central and sub-polar water masses, and is common in the eastern boundary currents (T. gregaria distribution).

VERTICAL DISTRIBUTION
T. gregaria appears not to migrate but this is unconfirmed. It remains in the 0-150 m stratum day and night in the cooler part of the California Current. Toward the offshore limit of its range it tends to submerge beneath the increasingly warm surface layer and follows the thermocline to 100-200 m depth. It has been found as deep as 1200 m off Bermuda in the Atlantic, but maximum numbers were at 200 m.

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

Metanauplius - undescribed

Calyptopis - undescribed

Leg 1 (maxilliped) - There are three setae spaced along the inner margin of the basis with no submarginal setae. (larval maxilliped basis)

Furcilia - (7 stages) (T. gregaria A), (T. gregaria B) (T. gregaria eye photo)

Eye: The pigmented area of the eye is pear-shaped in young furcilia with a division between the upper and lower portions. The eye appears bilobed in late furcilia.

Carapace:
Frontal plate / rostrum - The rostrum is triangular, tapering to a point (T. gregaria rostrum photo) (T. gregaria eye & rostrum photo) .
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present from F2.

Thoracic legs - undescribed

Abdomen:
Mid-dorsal spines - absent

Pleopods: There is a variety of forms with different levels of pleopod development. The common developmental pathway is 0'- 5'- 5" (T. gregaria photo ).

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

Comments: In calyptopis stages, the medial setation of the basis of the maxilliped may be used to separate T. gregaria from some species of other genera, i.e. Euphausia pacifica , Nematoscelis difficilis , and Nyctiphanes simplex .

Size may prove useful in identification of T. gregaria . The larvae are slender and small compared to related species.

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

Pictures
T. gregaria, selected stages
T. gregaria A [furcilia 1-3, 5]
T. gregaria B [furcilia 6-7, juvenile]
T. gregaria photo [furcilia 1-3]
T. gregaria detail photo [furcilia 3]
T. gregaria eye photo [furcilia 4]
T. gregaria rostrum photo [furcilia 4]
T. gregaria eye & rostrum photo [furcilia 7]
larval maxilliped basis [species characters]
key to larval illustrations

Thysanoessa gregaria