"Euphausia gibba" group (Euphausia pseudogibba Ortmann, 1893; Euphausia hemigibba Hansen, 1910; Euphausia paragibba Hansen, 1910; Euphausia gibba G.O. Sars, 1883)
Etymology: Euphausia - brightly shining; gibba - bent, hunched
Eye: The eye is round and small (E. gibba eye & rostrum). The eye diameter : carapace length is about 0.15 in all four species of the "Euphausia gibba" group.
Peduncle of 1st Antenna: The lappet on the 1st segment is an acute tooth, upward and forward-directed extending above the proximal part of the second segment (E. gibba lappet). The shape of the lappet shows no clear species specificity within this species group (E. gibba group,). The 2nd segment bears no processes. The 3rd segment has a low, dorsally-convex keel that, anteriorly, is either rounded or bears a minute tooth-like projection (not species specific) (E. gibba,). In E. pseudogibba this keel drops sharply away posteriorly, tending almost to truncate the posterior margin of the keel; this distinguishesE. pseudogibba from the three other species, in which the posterior margin of the keel more gradually merges with the dorsal surface of the 3rd segment, in lateral view.
Rostrum: This structure shows species specificity though specimens may depart from the following diagnoses (E. gibba group rostrum).
E. pseudogibba : The rostrum is distinctly upturned, rarely extending to the anterior margin of the eye. Viewed laterally, the frontal plate is horizontal, with a low keel behind the rostrum.
E. hemigibba : The rostrum is slightly upturned, rarely extending to the anterior margin of the eye. Viewed laterally, the frontal plate is not keeled behind the rostrum.
E. paragibba : The rostrum is nearly horizontal, usually extending to or beyond the anterior margin of the eye. Viewed laterally, the frontal plate appears strongly humped with a convex keel behind the rostrum.
E. gibba : The rostrum is barely upturned, usually extending to the anterior margin of the eye. Viewed laterally, the frontal plate is not humped behind rostrum.
Carapace: The gastric region is a low dome in all four species. There is a single pair of lateral denticles (E. gibba carapace denticle).
Abdomen: The 3rd segment bears a conspicuous mid-dorsal spine, not species specific in shape (E. gibba abdominal spine).
Length: All four species are 9-14 mm as adults. E. hemigibba and E. paragibba tend to grow larger than E. gibba and E. pseudogibba.
Petasma: This structure shows clear differences among the four species. E. gibba differs extremely from the other three.
E. pseudogibba : The tip of the terminal process is notched, the longer of the two resulting teeth are slightly curved. The proximal process is considerably longer than the terminal and the tip is strongly curved or reflexed as a hook. The distal part of the median lobe is long, straight and slender compared with the base of the lobe. The lateral process has two minute sub-distal spines and a vestige of a third (E. pseudogibba petasma).
E. hemigibba : The tip of the terminal process is hooked, with an opposing, distally-directed tooth at the base of the hook. The proximal process is closely paired with the terminal process, and is barely longer, ending distally as an oblong, obliquely-twisted plate, with a finely serrated inner margin. The median lobe gradually narrows distally and bears a simple lateral process (without spines) (E. hemigibba petasma).
E. paragibba : The terminal process is long and slender, with a short distally-directed tooth-like process at the base of the curved tip. The proximal process is longer, and curved through 35-45° distally. The median lobe narrows sharply above its base but broadens distally into a lobe-like ending. The lateral process is not an evenly curving hook, but bears a pair of short spines arising from a depression in the convex margin of the base of the process (E. paragibba petasma).
E. gibba : The terminal process is extremely short and stout, tapering to a blunt, slightly-curving tip. The proximal process is stout, long, and pointed, and the distal half curves in gentle arc. The median lobe is much reduced, terminating in a weak, distally-curving finger. The lateral process is simple, stout and sharply curved (E. gibba petasma).
Thelycum: Described by James, 1977; Guglielmo and Costanzo, 1978.
Comments: The petasma is the most reliable character. James (1977) has found specific differences among the thelyca, thus aiding in identification of females. The dorsal keel on the 3rd segment of the peduncle of the 1st antenna is particularly useful in distinguishing E. pseudogibba. The humped frontal plate is characteristic of E. paragibba.
E. gibba is a food of whales and fish.
E. gibba is limited to the subtropical South Pacific. E. hemigibba is subtropical in the North Pacific, Indian Ocean and North and South Atlantic Ocean. E. pseudogibba andE. paragibba are both tropical but with separate distributions: E. pseudogibba is in the tropical Atlantic and Gulf Stream, the basins of the Indo-Australian Archipelago and South China Sea, the Kuroshio Current, a narrow zone (2-10°N) in the Indian Ocean, and an east-west belt of 10-20°S across the South Pacific. E. paragibba is found from 20°N-10°S in the eastern tropical Pacific (excluding the O2-deficient regions), and in the western Indian Ocean from the Arabian Coast (15°N) to the cape of Africa in the Agulhas Current; in the eastern Indian Ocean E. paragibba is restricted to 0-10°S (E. gibba distribution).
In the eastern Pacific E. paragibba adults were above 100 m at night and, like E. pseudogibba , near 400 m in the daytime. E. hemigibba , in the North Pacific, was above 100 m at night and at 400-550 m in the daytime.
According to Brinton (1975), larvae of the "E. gibba group", E. hemigibba , E. pseudogibba , E. paragibba , and E. gibba , are distinguished by 1) smooth anterior margin of carapace, 2) absence of postero-dorsal carapace spine, and 3) small eye with the facets of the upper eye forming a broader area than in the lower portion (contrasting with E. tenera which also has a small eye but in which the lower faceted portion is broader).