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Nematoscelis difficilis Hansen, 1911

Etymology: Nematoscelis - thread leg; difficilis - troublesome, difficult

Eye: The eye is bilobed, large and about 3/4 as broad high. The transverse constriction is above the middle. The upper lobe is almost as wide as the lower lobe (N. difficilis eye photo).

Peduncle of 1st Antenna: The segments are shorter and thicker in the male than in the female. The dorsal distal margin of the 1st segment is higher than the 2nd segment and is setose, but there are no lappets or protuberances (N. difficilis lappet lacking).

Rostrum: In the female the rostrum is very long and thin and tapers in a slight downward arc to an acute point. In the males there is variation in the shape and length; in large adults it is usually short and triangular, but can be as long as in females. The rostrum bears a keel that extends backward over the vaulted gastric area of the carapace (N. difficilis rostrum) (N. difficilis eye & rostrum).

Carapace: There are no lateral denticles on the margin of the carapace in the "typical" form (N. difficilis). A "denticled form" has been described from the northeastern Pacific, 47-33°N, in which the pair of denticles regularly present in juveniles persists in the adult (McLaughlin, 1965).

Thoracic legs: The penultimate segment (propodus) of the 1st thoracic leg has 5 setae in a row along its inner side and 6 on or near the posterior margin. (cf. N. megalops). The 2nd leg is extremely elongate with straight distal spines. Legs 3 to 7 are short. The 8th leg is rudimentary in both sexes (N. difficilis thoracic leg) (Thoracic leg 2 (N. difficilis)).

Abdomen: There are no dorsal spines or keels (N. difficilis).

Length: Adults are 16-24 mm.

Petasma: The spine-shaped process is about half the length of the terminal process. The terminal process has a basal part that is bent outward, and a longer, more slender, distal part that is set off at an angle. The distal part has a strongly curved tip and about twenty-seven serrations along the concave outer margin. The proximal process is about 3/4 the length of the terminal process, and its distal part is straight, slender, and delicately serrated along the outer margin. The distal end of the spine-shaped process reaches considerably beyond the angular bend in the terminal process. The lateral process, is about the same length as the spine-shaped process and is curved (N. difficilis petasma).

Thelycum: Described by Costanzo and Guglielmo, 1980.

Comments: N. difficilis and N. megalops are closely related, but can be differentiated by the relative lengths of the spine-shaped process, the terminal process and the proximal process of the petasma, and by the number of marginal setae on the propodus of the 1st thoracic leg. The geographical distributions of these two species are widely separated.

ECOLOGY
N. difficilis is an important food source for whales, fish and birds. As an example, a 12 ft. oarfish trapped by a gill net at 120 m depth off Southern California was engorged with this species.

HORIZONTAL DISTRIBUTION
N. difficilis is endemic to the North Pacific, occupying the North Pacific Drift (ca 37-45°N), the California Current and the Gulf of California (N. difficilis distribution).

VERTICAL DISTRIBUTION
N. difficilis is most often found above about 200 m, but within the thermocline; there is no clear indication of diel vertical migration. Adults are difficult to catch in the daytime, presumably due to strong net-avoidance ability.

LARVAE
See the development summary (N. difficilis Table) for the stage descriptors and length in stage.

Metanauplius - (2 forms) (N. difficilis A)

Marginal spines - There are 6 small spines spaced along each antero-lateral margin.

Calyptopis - (3 stages) (N. difficilis B)

Carapace:
Shape - The frontal hood tapers anteriorly and the posterior margin becomes pointed by C3.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent
Dorsal keel - A small keel is present in C2-C3.

Leg 1 (maxilliped) - There are five setae space along the inner margin of the basis, and 2 submarginal setae. The second seta from the distal end is stouter than the others. (larval maxilliped basis)

Telson:
Shape - The telson is broad, especially in C2.
Postero-lateral spines - PL3 is longer than PL2.
Terminal spines - The terminal spines lack the strong subspines present in the atlantica-gracilis-microps-tenella species complex in Nematoscelis .

Furcilia - (3 stages) (N. difficilis C), (N. difficilis D), (N. difficilis rostrum & keel)

Eye: The pigmented area of the eye is pear-shaped in F1-F2 and the eye is bilobed in F3.

Carapace:
Frontal plate / rostrum - The rostrum is triangular and relatively slender.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A keel is present and large in F2-F3.

Thoracic legs: There is precocious development of Leg 2, the elongate leg of the adult. During the furcilia phase the dactylus and propodus develop spines.

Abdomen:
Mid-dorsal spines - absent

Pleopods: The developmental pathway is 2'- 2"3'- 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.
Terminal spines - The terminal spines lack the strong subspines present in the atlantica-gracilis-microps-tenella species complex in Nematoscelis .

Comments: The first two juveniles stages are included in the development summary to illustrate the timing of the change in the second antenna from natatory to juvenile form.

N. difficilis and N. megalops are very closely related but are separated geographically.

The characters used to identify the furcilia larvae of Nematoscelis species are carapace frontal plate and rostrum, carapace dorsal keel, pleopod developmental pathway, and armature of terminal telson spines. See Nematoscelis larval characters (Nematoscelis A) and (Nematoscelis B).

In calyptopis stages, the medial setation of the basis of the maxilliped may be use to separate N. difficilis from some species of other genera, i.e. Euphausia pacifica , Thysanoessa gregaria and T. spinifera (larval maxilliped basis).

Development
(N. difficilis Table), development summary for the stage descriptors and length in stage.

Pictures
N. difficilis, selected stages
N. difficilis A [pseudometanauplius, metanauplius]
N. difficilis B [calyptopis 1-3]
N. difficilis C [furcilia 1]
N. difficilis D [furcilia 2-3]
N. difficilis E [second antenna, telson]
larval maxilliped basis [species characters]
N. difficilis F [thoracic leg 2]
Nematoscelis A [species characters]
Nematoscelis B [species characters]
N. difficilis photo [calyptopis 2]
N. difficilis photo [furcilia 1]
N. difficilis dorsal view photo [calyptopis 3]
N. difficilis rostrum & keel [furcilia 1 photo]
N. difficilis telson photo [calyptopis 2]
Furcilia 1 (dorsal scan) [N. difficilis]
key to larval illustrations

Nematoscelis difficilis