Nematoscelis atlantica Hansen, 1910
Etymology: Nematoscelis - thread leg; atlantica - of the Atlantic
Eye: The eye is bilobed and medium in size (N. atlantica eye, rostrum photo) (N. atlantica eye & rostrum). The height of eye/carapace length = about 0.23. The upper lobe is distinctly smaller and narrower than the lower lobe.
Peduncle of 1st Antenna: There is no lappet (N. atlantica lappet lacking) but the dorsal part of the 1st segment ends as an angular tip, scarcely overlying the base of the 2nd segment.
Rostrum: It is long, slender, straight, and acute in both sexes extending to, or beyond the anterior limit of the eye (N. atlantica eye & rostrum) (N. atlantica rostrum).
Carapace: A dorsal keel is low but distinct. A low, keel-like ridge extends anteriorly from the dorsal keel to the rostral process. A lateral denticle is present at the carapace margin in both sexes (N. atlantica carapace denticle).
Thoracic legs: The 1st thoracic leg has a dactylus with the outer margin strongly convex and an inner setae-bearing margin that is concave. There is no dorsal seta. The propodus has a single row of 5 setae on inner margin. The 2nd thoracic leg is very elongate with a cluster on long apical bristles arising from both the dactylus and propodus (N. atlantica thoracic leg).
Abdomen: The abdomen is slender and there are no dorsal spines or keels (N. atlantica).
Length: Adults are 10.5-15 mm.
Petasma: The spine-shaped and terminal processes are slender, of equal length, both barely longer than the proximal process or about equal to it in length. The proximal process has a filmy appearance, described by Hansen (1910) as "diaphanous". The lateral process is equal in width to the proximal, but it is only about half as long. The median lobe is narrow and extends well beyond the processes of the inner lobe (N. atlantica petasma).
Thelycum: Described by Einarsson, 1942; Costanzo and Guglielmo, 1976a; Costanzo and Guglielmo, 1980.
Comments: The eye, together with that of N. lobata, is the smallest in the genus. The body of this species is particularly slender, the long, straight, acute rostrum enhancing this feature. The keel on the frontal plate is low compared with the characteristically domed keel in male N. lobata but is like the low keel in female N. lobata.
N. microps, N. gracilis and N. atlantica are difficult to separate. Because distributions of N. microps and N. atlantica overlap in the Subtropical zones, small differences in eye-size become important. (See Comments under N. microps which has the larger eye of the two species.)
ECOLOGY
N. atlantica is the food for planktivorous fish.
HORIZONTAL DISTRIBUTION
N. atlantica is antitropical in the Pacific, roughly 20°-40°, occupying the oligotrophic central water masses. Similarly, in the Indian Ocean it occurs about 10-35°S. In the Atlantic it occurs between the 40° parallels, or to a little farther north in the Gulf Stream extension; its recorded presence in the tropical Atlantic is uncertain. It is widely present in the Mediterranean Sea (N. atlantica distribution). The specific name atlantica is an unfortunate misnomer.
VERTICAL DISTRIBTION
Adults are mostly below about 250 m by day, and above that depth at night. Most immatures and larvae are above 50 m, day and night.
LARVAE
See the development summary (N. atlantica Table) for the stage descriptors and length in stage.
Metanauplius - undescribed
Calyptopis - (3 stages) (N. atlantica A)
Carapace:
Shape - The frontal hood tapers anteriorly and the posterior margin is somewhat pointed in C3.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent
Dorsal keel - A small keel is present in C3.
Telson:
Postero-lateral spines - PL3 is longer than PL2.
Terminal spines - The terminal spines each have a pair of conspicuous sub-spines and have setules proximal to the subspines.
Furcilia - (3 stages) (N. atlantica B)
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, elongate, slender, and acute.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.
Dorsal keel - A low keel is present.
Thoracic legs: There is precocious growth of Leg 2, the elongate leg of the adult. During the furcilia phase the propodus develops spines.
Abdomen:
Mid-dorsal spines - absent
Pleopods: The developmental pathway is 1'- 1"3'- 4"1'- 5".
Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.
Terminal spines - The terminal spines each have a pair of conspicuous sub-spines and have setules proximal to the subspines.
Comments: The first two juveniles stages (N. atlantica C) are included in the development summary to illustrate the timing of the change in the second antenna, from natatory to juvenile form.
Brinton (1975) notes that the larvae of the N. atlantica-N. microps-N. gracilis complex of subtropical-tropical species have proven difficult to identify. Of these species, only N. atlantica is known from the Mediterranean and hence the larval series described by Casanova (1974) as N. atlantica-microps and the larvae identified as N. microps by Lebour (1926c) may be referred to N. atlantica .
Casanova (1974) observed that in N. atlantica the rostrum is distinctly curved downward in front of the eyes in F1 only, this condition is not mentioned by Lebour (1926c) or Gopalakrishnan (1975). Gopalakrishnan describes the rostrum of N. tenella as usually curved downward in furcilia.
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. atlantica Table), development summary for the stage descriptors and length in stage.
(N. atlantica Graph 1) , a histogram of developmental form frequency, from the Mediterranean.
(N. atlantica Graph 2), a histogram of length freqency by stage, from the Mediterranean.
Pictures
N. atlantica, selected stages
N. atlantica A [calyptopis 1-3]
N. atlantica B [furcilia 1-3]
N. atlantica C [juvenile 1-2]
Nematoscelis A [species characters]
Nematoscelis B [species characters]
key to larval illustrations