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Euphausia krohni (Brandt, 1851)

Etymology: Euphausia - brightly shining; krohni - after Mr. Krohn

Eye: The eye is round and large (E. krohni eye & rostrum). The eye diameter : carapace length is about 0.21.

Peduncle of 1st Antenna: The 1st segment has a pectinate lappet with 9 teeth projecting anteriorly over the 2nd segment (E. krohni lappet). The lappet's outer tooth is produced into an oblique, thin process which is distally trifurcate.The dorsal margin of the 2nd segment has two short but prominent, forward-projecting spine-like tubercles, one on the outer edge and one on the inner edge. These extend scarcely beyond the distal limit of the segment. The 3rd segment has a keel.

Rostrum: It is long, sharply pointed, reaching the anterior limit of the eyes. The gastric region is domed, with a small high keel at its apex E. krohni eye & rostrum.

Carapace: There are two pairs of lateral carapace denticles (E. krohni carapace denticles).

Abdomen: There are no dorsal spines or specific features (E. krohni).

Length: Adults are 12-19 mm.

Petasma: The terminal process is moderately long and conspicuously widened and rounded towards the end where there is a minute lateral tooth. The proximal process has three irregular, large teeth on the distal third of the inner margin. The end of the proximal process is flattened and rounded (E. krohni petasma).

Thelycum: Described by Costanzo and Guglielmo, 1976a.

Comments: E. krohni together with E. eximia and E. americana have conspicuous pectinate lappets. These species can be separated by the number of denticles in the lappet: E. krohni , 9 (the outer one being distally trifurcate); E. eximia , 11-12; E. americana , 5-6.

ECOLOGY
E. krohni aggregates and is the a food source for fish and decapods.

HORIZONTAL DISTRIBUTION
E. krohni occurs in the temperate and subtropical North Atlantic and in the North and tropical Atlantic and the Mediterranean. Northernmost records are in the Gulf Stream extension around Iceland (65°N), and off Norway (62°-63°N). Southernmost records are off the African coast at the equator (E. krohni distribution).

VERTICAL DISTRIBUTION
E. krohni lives at 400-600 m during the day but migrates vertically to 0-200 m at night.

LARVAE
See the development summary (E. krohni Table) for the stage descriptors and length in stage.

Metanauplius - (1 stage) (E. krohni A)

Carapace:
Shape - The frontal hood is expanded laterally.
Marginal spines - The frontal hood is fringed with small spines. There are three longer pairs interspersed on the anterior margin.
Dorsal crest - A crest is present with a pair of dorsal spines.

Calyptopis 1-2 (3 stages) (E. krohni B)

Carapace:
Shape - The frontal hood curves in medially and curves ventrally around the eyes in C2.
Marginal spines - The frontal hood is fringed with small spines.
Postero-dorsal spine - A postero-dorsal spine is present.
Lateral denticles - A pair of denticles is present in C3.

Telson:
Shape - The lateral margins curve in below the lateral spines.
Postero-lateral spines - PL2 is longer than PL3.

Furcilia - (6 stages) (E. krohni C), (E. krohni D)

Carapace:
Frontal plate / rostrum - The frontal plate is rectangular in F1, it becomes broadly trianguar in late furcilia, a rostal spine is present from F2. It is tiny in early furcilia.
Marginal spines - The frontal plate is fringed with spines in the furcilia phase, there are fewer spines in F6.
Postero-dorsal spines - A spine is present in F1 only.
Lateral denticles - A pair of denticles is present.

A1: The F5 larvae with 1t + 3pl telson spines may have 1 to 3 small knobs on the developing lappet of the 1st antenna (Frost 1934).

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

Abdomen:
Mid-dorsal spines - absent

Pleopods: The common developmental pathway is 1' - 1"4' - 5".

Telson:
Postero-lateral spines - PL2 is longer than, or equal to, PL3 through F5.
Lateral spines - There is one pair of lateral spines.

Comments: Larvae of Euphausia species group 1A (Brinton, 1975), E. brevis, E. diomedeae, E. mutica, E. recurva, E. eximia and E. krohni are very similar. E. americana is undescribed.

Development
(E. krohni Table), development summary for the stage descriptors and length in stage.

(E. krohni Graph 1) , a histogram of developmental form frequency, from the Mediterranean.

(E. krohni Graph 2), a histogram of length freqency by stage, from the Mediterranean.

Pictures
E. krohni, selected stages
E. krohni A [metanauplius]
E. krohni B [calyptopis 1-2]
E. krohni C [furcilia 1-3]
E. krohni D [furcilia 4-6]
key to larval illustrations

Euphausia krohni