G.O. Sars, 1883, 1885
Generic diagnosis:
The eye is bilobed, with a transverse constriction (eye shape).
The mandibular palp is very small.
The peduncles of the 1st antennae are more slender in the female than in the male (Nematoscelis- ).
The endopod of the 1st pair of thoracic legs (maxilliped) has a terminal segment that is triangular-shaped and furnished with brush-like bristles on the inner lateral margin.
The endopod of the 2nd pair of legs is very elongate with a cluster of long straight apical bristles arising from the terminal segment, or from both the terminal and preceding segment (Nematoscelis-thoracic legs). The 7th thoracic leg endopod is two-segmented in the female, and is lacking in the male. The 8th leg is a simple setose plate.
Eggs are carried externally by the female, attached to thoracic limbs by a glutinous adhesive (life cycle-egg brooder).
The petasma has a spine-shaped process that is straight and upright; the lateral process is not hooked.
The abdomen has no spines (Nematoscelis-abdomen).
Five of the seven species of Nematoscelis are tropical or subtropical. The sibling pair N. difficilis-N. megalops occupy 35-45° zones bordering the tropical-subtropical regime, N. difficilis in the North Pacific and N. megalops in the North Atlantic and circumglobal in the southern ocean. Both species extend toward the tropics in eastern boundary currents.
Most species of this genus are difficult to distinguish and lack specific structures on the peduncles of the 1st antennae. The species' eyes are similar in shape, except for N. tenella in which the upper lobe is distinctly larger than the lower. Differences in eye size and shape may be discerned, however, and these small differences, together with shape of rostrum, dorsal carapace keel and the setation on leg 1 provide the most useful means of identification when adult males with fully developed petasmas are not available.