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Nyctiphanes simplex Hansen, 1911

Etymology: Nyctiphanes - night visible/shining; simplex - simple

Eye: The eye is round, large, and tends to remain black under preservation(N. simplex eye & rostrum). The eye diameter : carapace length is about 0.22.

Peduncle of 1st Antenna: The dorsal surface of the 1st segment carries a large leaflet or lappet, which is directed upward and backward, is about twice as long as broad at the base, and has a hollow anterior surface. In females the outer margin of this leaflet is rounded, extending to a pointed apex. In the male the leaflet is longer, broadly truncate; distally it is directed more backward than in the female. The 2nd segment is longer than the 3rd and carries a small, sometimes divided or notched subacute tooth at its dorsal, distal, inner angle. In the male the distal tooth is broader and more vertical than in the female. Along the 2nd segment in both sexes there is a low dorsal keel. The 3rd segment in the female ends distally as a small dorsal tooth. In the male the 3rd segment is without a tooth, and, seen from above, the outer lateral margin of the entire segment is roundly convex (N. simplex,).

Rostrum: A rostrum is lacking; the frontal plate is short, and forms an obtuse trough-like triangle with raised margins (N. simplex eye & rostrum). In immatures (to 7 mm) the anterior edge of the frontal plate is truncate (squared-off rectangularly).

Carapace: There are no lateral carapace denticles (N. simplex).

Thoracic legs: Legs 1-6 (endopods) are similarly developed, the 7th is nearly as long but conist of only 2 segments. Exopods present on (legs 1-7) in males; in females 1-5. The 8th leg is rudimentary. In the female, paired eggs sacs are secreted adhering to the two posterior thoracic endopods (6th and 7th) (N. simplex female with eggs) (N. simplex female with eggs ).

Abdomen: The 6th segment has a small mid-dorsal posterior spine (N. simplex).

Length: Adults are 8-16 mm.

Petasma: The inner lobe is finely serrate along 2/3 of the straight edge of its outer margin; often visible only under >50x maginification. The inner lobe carries the long, bent, spine-shaped process. Proximal and terminal processes are lacking. The median lobe is extremely short and poorly developed but serves as a base for a long, slender, almost straight lateral process (N. simplex petasma).

Thelycum: Described by Guglielmo and Costanzo, 1983.

Comments: The leaflet on the 1st segment of the peduncle of 1st antenna can be used to readily separate N. simplex in the California Current from the frequently co-occurring similarly large-eyed species, Euphausia pacifica, which also is without a rostrum. E. recurva, which also has an upright leaflet on the 1st segment of the peduncle of 1st antenna, has a long narrowly acute rostrum, differing distinctly from N. simplex. The range of N. simplex does not overlap those of the 3 other species in this genus, N. capensis, N. australis and N. couchi.

ECOLOGY
N. simplex is a subtropical coastal species but disperses westward along the Pacific's eastern boundary current communities, locally dominating temperate Euphausia pacifica in southern parts of the California Current and E. mucronata in northern parts of the Peru-Chile Current off South America. It has been observed to form surface swarms (N. simplex surface aggregation) that exceed 32 g wet wt m3 biomass in the Gulf of California (Gendron, 1992). It is an important source of food for whales, fish, and birds along the continental margins and at the Galapagos Is.

HORIZONTAL DISTRIBUTION
N. simplex occurs in the eastern Pacific from central California (ca. 35°N) to central Chile (ca. 35°S), but is lacking in the Eastern Tropical Pacific from ca. 20°N to the Gulf of Panama, 7°N. It extends westward from coastal Peru along the equator beyond the Galapagos Is. In the northern sector, N. simplex is most abundant west of Baja California, in the Gulf of California and off southern California. It has been reported as far north as 46°N (Brodeur, 1986) during an extreme warming episode (N. simplex distribution).

N. simplex has been reported from the Gulf of Paria off Venezuela (Legare, 1961; see location marked with an "?" on the distribution map) but the specific characteristics of the first and second segments of the peduncle of the first antenna must be examined for positive confirmation of this record.

VERTICAL DISTRIBUTION
N. simplex occupies shelf, slope and continental borderland waters to about 250 m depth.

LARVAE
See the development summaries (N. simplex Table 1) and (N. simplex Table 2) for the stage descriptors, length in stage, and two systems of staging furcilia larvae.

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

Carapace:
Shape - The carapace is smoothly rounded.
Marginal spines - absent

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

Carapace:
Shape - The carapace is smoothly rounded. The developing large eyes may be visible beneath the carapace in C3.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - absent

Leg 1 (maxilliped) - There are five setae spaced 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:
Postero-lateral spines - PL3 is longer than PL2.

Furcilia - (6 stages) (N. simplex C), (N. simplex D), (N. simplex E)

Eye: The eye is large, round, and dark. (N. simplex eye photo)

Carapace:
Frontal plate/rostrum - The frontal plate is truncated and the anterior margin is flat to concave medially.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - A pair of denticles is present.

Thoracic legs: There is sequential development without elongate leg(s).

Abdomen:
Mid-dorsal spines - absent

Pleopods: There is a variety of forms in F1 and F2 with different levels of pleopod development. The developmental pathway varies, for example 2' - 2"3' - 5" and 0 - 4' - 4"1' - 5".

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Lateral spines - One pair of lateral spines is present.

Comments: In calyptopis stages, the medial setation of the basis of the maxilliped may be used to separate N. simplex from some species of other genera, i.e. Euphausia pacifica , Thysanoessa gregaria and T. spinifera . The furcilia stages are characterized by the large round eye and the shape of the carapace frontal plate.

N. simplex and E. pacifica co-occur along the coast of California; they are both round eye species and sometimes can be difficult to distinguish in the calyptopis stages. A comparison of the telsons of Euphausia pacifica and Nyctiphanes simplex in C3 (E. pacifica vs. N. simplex) shows that: 1) the telson of E. pacifica is more rounded distally and the inner pair of postero-lateral telson spines (PL3) is relatively long, 2) the telson of N. simplex has a more flattened distal margin and spine PL3 is shorter in relation to other telson spines.

N. simplex is separated geographically from the other three species of the genus, N. australis , N. capensis , and N. couchi . In two of the species, N. capensis and N. simplex , the carpace frontal plate is truncated, the anterior margin is usually concave, and the antero-lateral angles may be acute. In N . australis and N. couchi , the anterior margin of the truncate frontal plate appears usually to be flat, it may be slightly indented in F1.

Development
(N. simplex Table 1) and (N. simplex Table 2), development summaries for the stage descriptors and length in stage, and two systems of staging furcilia larvae.

Pictures
N. simplex, selected stages
N. simplex A [egg, pseudometanauplius, metanauplius]
N. simplex B [calyptopis 1-3]
N. simplex C [furcilia 1-2]
N. simplex D [furcilia 3-4]
N. simplex E [furcilia 5-6]
larval maxilliped basis [species characters]
N. simplex calyptopis 3 [photo]
N. simplex eye photo [furcilia 2]
N. simplex furcilia 2 photo [uropods, telson]
E. pacifica vs. N. simplex [calyptopis 3 telson photo]
E. pacifica vs. N. simplex [furcilia 1 telson photo]
Ontogeny of swimming
key to larval illustrations

Nyctiphanes simplex