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Thysanoessa spinifera Holmes, 1900

Etymology: Thysanoessa - small tassel; spinifera - bearing spines

Eye: The eye is large, essentially round, the upper-most part being slightly narrowed, but this is difficult to detect because there is no transverse constriction (T. spinifera eye & rostrum). The eye diameter : carapace length is about 0.22.

Peduncle of 1st Antenna: The dorsal edge of the 1st segment (viewed laterally) is almost straight, ending anteriorly as a blunt angle. The 2nd segment is a little longer than the 3rd; the 1st two segments are more slender in the female than in the male. In the male the dorsal, distal margin of the 2nd segment is produced into a lobe which carries a dense bunch of thick, curved setae. In the female this lobe and its setae are much smaller (T. spinifera,).

Rostrum: It is narrowly triangular, long, and very acute. Its tip reaches nearly to the end of the 1st segment of the peduncle of 1st antenna (T. spinifera dorsal head) (T. spinifera eye & rostrum).

Carapace: A prominent post-ocular spine is carried on either side of the base of the rostrum. The margins of the carapace are without denticles (T. spinifera).

Thoracic Legs: The 2nd pair is not clearly elongate, but slightly longer and thicker than the other legs. The dactylus and prododus are together about as long as the carpus, and the extremely short dactylus is about as long as broad (T. spinifera elongate 2nd leg).

Abdomen: The lateral angles of the pleura on the abdominal segments are acute. The 2nd and 3rd segments have small dorsal keels, and the 4th, 5th and 6th have heavy, dorsal, backward-directed spines (T. spinifera abdominal spines) (T. spinifera abdominal spines).

Length: Adults are 16-25 mm in the California Current; but are reported to reach 38 mm in more northern areas.

Petasma: There is a long, thin, curved spine-shaped process. The terminal process is rather short, with the proximal half very thick. The distal half is at an angle to the rest and tapers to an acute, outwardly-turned point. The proximal process is longer and thinner than the terminal. It is thick at the base, and tapers to an end resembling the tip of the the terminal process. The lateral process on the median lobe is slightly curved or straight and tapers to an acute point some distance short of the end of the lobe. There is no additional process (T. spinifera petasma).

Thelycum: Not yet described.

ECOLOGY
T. spinifera is neritic and is most abundant at near-shore stations; swarms have been observed off San Diego, Channel Islands, Monterey, San Francisco (Gulf of the Farallones), Tomales Bay, and Oregon. Larvae may be dispersed far seaward. It is an important food for fish, birds and whales.

Horizontal Distribution
T. spinifera occurs along the western coast of North America from the southeastern Bering Sea, to as far as mid-Baja California (28°N) during particularly cool springtimes (T. spinifera distribution). It is not known from the western side of the North Pacific.

Vertical Distribution
T. spinifera appears to be restricted to depths of less than 100 meters.

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

Metanauplius - (1 stage) (T. spinifera A)

Carapace:
Shape - The carapace appears loose even on live larvae.
Marginal spines - There are small spines along the anterior and lateral margins.

Calyptopis - (3 stages) (T. spinifera B), (T. spinifera C)

Carapace:
Shape - The carapace is short in relation to abdomen length.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - usually absent

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

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

Furcilia - (5 stages, 6 instars) (T. spinifera D), (T. spinifera E), (T. spinifera F), (T. spinifera G), (T. spinifera H)

Eye: The eye is fully pigmented by F5 and becomes nearly round.

Carapace:
Frontal plate / rostrum - The frontal plate is rounded with a small median spine in F1. Its lengthens and tapers in later stages and becomes acute.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - absent

Abdomen:
Mid-dorsal spines - Segments 4-5 usually have rudimentary spines in F3 instar 2. Strong dorsal spines appear on segments 4 and 5 in F4 and a small spine may be present on segment 6. In F5 there are well developed spines on segments 4-6.

Pleopods: The developmental pathway is 0'- 5'- 5".

Telson:
Postero-lateral spines - In F3 to F5 either PL2 or PL3 may be slightly longer.
Lateral spines - One pair of lateral spines is present.

Comments: In the metanauplius stage of T. spinifera the carapace is loose while in T. raschii and E. pacifica , species which occur with T. spinifera , the carapace fits snugly around the body.

In calyptopis 3, the carapace in T. spinifera is reduced and small in relation to abdomen length exposing the eyes and much of the abdomen. In the other species the carapace is relatively larger, covering more of the eyes and abdomen.

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

The distinguishing feature of T. spinifera in Furcilia 1 is the shape of the frontal plate, it is rounded with a tiny spine at the tip. T. raschii also has a "U" shaped frontal plate but without the point at the tip. T. longipes has a triangular frontal plate/rostrum and E. pacifica has a short blunt frontal plate. From furcilia 2 T. spinifera differs from T. raschii and T. longipes in lacking a lateral carapace denticle. Both the other species develop a lateral denticle in F2.

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

Pictures
T. spinifera, selected stages
T. spinifera A [nauplius, metanauplius]
T. spinifera B [calyptopis 1-2]
T. spinifera C [calyptopis 3]
T. spinifera D [furcilia 1-2]
T. spinifera E [furcilia 3, 3a]
T. spinifera F [furcilia 4, 4a]
T. spinifera G [furcilia 5, 5a]
T. spinifera H [furcilia 6, juvenile]
larval maxilliped basis [species characters]
key to larval illustrations

Thysanoessa spinifera