Nematoscelis megalops G. O. Sars, 1883
Etymology: Nematoscelis - thread leg; megalops - great eyed
Eye: The eye is bilobed and large as in N. difficilis. The upper lobe is almost as wide as the lower lobe (N. megalops eye & rostrum).
Peduncle of 1st Antenna: There are no lappets or protuberances, but the distal part of the 1st segment is higher than the 2nd segment (N. megalops,).
Rostrum: The rostrum length is variable but in males it is more often short or lacking as in N. difficilis. In females it is usually long, slender, and downward curving (N. megalops rostrum) (N. megalops eye & rostrum).
Carapace: There are no lateral denticles on the margin of the carpace in adults, but a pair of denticles are present in immature specimens.
Thoracic legs: The penultimate segment (propodus) of the 1st thoracic leg has 6 setae in a row along its inner side and 7 on or near its posterior margin (cf. N. difficilis). As in N. difficilis, the 2nd leg is extremely long, with a group of long, straight, distal spines (N. megalops thoracic leg).
Abdomen: There are no dorsal spines or keels (N. megalops).
Length: Adults are 22-26 mm.
Petasma: The spine-shaped process is less than half the length of the terminal process and reaches to the angular bend in the terminal process. The terminal process has its basal part directed somewhat outwards, and then the process bends abruptly. The distal part of the terminal process is longer and more slender than the basal part and is flatly curved, with the concave outer margin serrated; there are about 44 dentate serrations on the outer margin. The proximal process is shorter than the terminal, with its distal part slender, straight and finely serrate along the outer margin reaching about the middle of the serrate margin of the terminal process (N. megalops petasma).
Thelycum: Described by Einarsson, 1942; Sebastian, 1966; Costanzo and Guglielmo, 1976a; 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 the two species are widely separated.
ECOLOGY
N. megalops is a food source for fishes.
HORIZONTAL DISTRIBUTION
N. megalops occurs in the warm-temperate belts of the North and South Atlantic, Indian Ocean, South Pacific, and is present in the Mediterranean Sea (N. megalops distribution). Isolated population segments have been monitored in the northwestern Atlantic in cold-core rings that have migrated across the Gulf Stream, away from the cool coastal zone.
VERTICAL DISTRIBUTION
N. megalops is most often found above about 140 m but within the thermocline, and is believed not to be a vertical migrator.
LARVAE
See the development summary (N. megalops Table) for the stage descriptors and length in stage.
Metanauplius - (2 forms), only pseudometanauplius illustrated (N. megalops A)
Calyptopis - (3 stages) (N. megalops B)
Carapace:
Shape - The frontal hood tapers anteriorly and the posterior margins becomes pointed.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent
Dorsal keel - A small keel is present in C3.
Telson:
Shape - The telson is broad.
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. megalops C), (N. megalops D)
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 broad.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A keel is present.
Thoracic legs: There is precocious growth 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. megalops and N. difficilis 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).
Development
(N. megalops Table), development summary for the stage descriptors and length in stage.
(N. megalops Graph 1) , a histogram of developmental form frequency, from the Mediterranean.
(N. megalops Graph 2), a histogram of length freqency by stage, from the Mediterranean.
Pictures
N. megalops, selected stages
N. megalops A [egg, pseudometanauplius]
N. megalops B [calyptopis 1-3]
N. megalops C [furcilia 1-2]
N. megalops D [furcilia 3, juvenile]
Nematoscelis A [species characters]
Nematoscelis B [species characters]
key to larval illustrations