Marine Biology 101, 195-203 (1989) Marine :--Biology: 9 S~ringer-VedagIg~g Functional feeding morphology of the euphausiid Nyctiphanes australis D.D. Dalley and S. McClatchie * Portobello Marine Laboratory, P.O. Box 8, Portobello, New Zealand Abstract The structure of the feeding basket, mandible and stomach armature of the krill Nyctiphanes australis (G. O. Sars, 1883) was examined by scanning electron microscope with the aim of predicting its diet. N. australis were collected during Feb- mary 1986 and October 1986 to February 1987 in Otago Harbour, New Zealand (45 ~ 50' S; 170 ~ 37' E). Predictions based on the functional morphology were tested by examin- ing stomach contents with SEM. Intcrsetule distances of the feeding basket (1 to 7.5/rm) are freer than in other krill species, suggesting that N. australis can efficiently collect nanoplankton-sized particles (2 to 20/xm). The mandibular edge index (0.74) matched the edge index of Meganycti- phanes norvegica. This indicates, in contrast to the free feeding-basket setulation, that N. australis has a mandible resembling that of predominantly carnivorous krill species. The ratio of mandibular palp length to mandible width is 3.2 + 0.2, or relatively longer than the elongate palp thought to be indicative of herbivorous habit in Euphausia superba. The t'me structure of the molar shows specialized surfaces which differ from those of other kill1 species. The internal armature of the stomach is heavily spinose, as is common in krill of herbivorous habit. Layers of various-shaped spines at differing densities were observed. The functional mor- phology suggests that N. australis is an opportunistic om- nlvore. The stomach contents tentatively support this pre- diction, containing fragments ofphytoplankton and detrital material. However, stomach contents were generally amor- phous, making identification of dietary components in the field difficult. Introduction Studies of the functional morphology of euphausiids have described the structure of their feeding appendages (Sars * Please address all correspondence to Dr. S. McClatchie 1885, Barkley 1940, Nemoto 1967, 1977, Mauchline and Fisher 1969, Lomakina 1973, Mauchline 1980a) and diges- tive organs (Nemoto 1966, 1967, 1977, Ikeda et al. 1984). Recent work with the scanning electron microscope (McClatchie and Boyd 1983, Ikeda et al. 1984, Marschall 1985, Suh and Nemoto 1988) provided the first information on the microstructure of the feeding appendages, mandibu- lax surfaces, and stomach armature. The studies of Barkley (1949), Nemoto (1966, 1967), Mauchline (1967), Ponomareva (1976), Artiges et al. (1978), Dzik and Jazdzewski (1978), Kittel and Ligowski (1980), and others established the relationship between the feeding patterns of euphausiids and differentiation of their body characters. The structures most commonly examined are length and setation of the thoracic legs forming the feeding basket, setation of the maxillae, relative proportions of the molar and incisor on the mandibles, development of the stomach spines and shape of the eyes. Using these features, Nemoto (1967) divided euphausiid species into herbivorous, omnivorous and carnivorous feeding types. Briefly, more herbivorous krill have highly setose maxillae and feeding basket, nearly equal length pereiopods, well-developed stomach spines, spherical eyes, and a disproportionately large pars molaris on the mandible. More carnivorous spe- cies often have the first and/or second pair of pereiopods elongated to form grasping legs, transversely divided eyes, a small pars molaris relative to the incisor, and a less freely setose feeding basket. These characters are useful to categorise broadly the modal feeding type for any given species, but the method gives only an indication of principal tendency. Euphausiids are known to change their feeding modes both diurnally (during vertical migrations) and seasonally. Krill are also likely to switch prey items as availability changes, feeding mainly herbivorously during the peak phytoplankton bloom, but becoming more carnivorous as the bloom de- clines (Pavlov 1971). Euphausia superba was considered an archetypal herbivore but will also take animal food (Pavlov 1971, McWhinnie and Denys 1978, Boyd et al. 1984, Price