A new amphipod species and genus, Chevreuxiopsis franki, found in a pelagic sediment trap southwest of Tasmania is described. The new species can be recognized by its unique antenna 2, which consists of a narrow peduncle, and a 4-articulate flagellum, which has a massively developed, article 1, large, posteriorly drawn out articles 2 and 3, and an elongate lanceolate 4^th article. The pereopod 1 basis surrounds large maxillipedal plates. Pereopod 3 to 6 are equipped with subchelate propodus dactylus arrangements. The bases of pereopods 5–7 are narrow.

Aristioidea, Australia, Indian Ocean, new species, pelagic sediment trap, Tasmania, taxonomy

An unusual amphipod was found in a pelagic sediment trap in the Indian sector of the Southern Ocean, southwest of Tasmania, Australia. Careful examination allowed us to identify it as a member of the family Thoriellidae Lowry & Stoddart, 2011. The Thoriellidae consists of four genera: Chevreuxiella Stephensen, 1915, Danaella Stephensen, 1925, Parachevreuxiella Andres, 1987, and Thoriella Stephensen, 1915. The morphological diversity in this family is very high. The two Danaella species, Danaella mimonectes Stephensen, 1925 and Danaella obensis (Birstein & Vinogradov, 1962) (initially described as Chevreuxiella obensis) have inflated bodies that are more reminescent of hyperiid amphipods than of other Aristioidea Lowry & Stoddart, 1997. In contrast to Danaella, the genus Thoriella, represented by the slender Thoriella islandica Stephensen, 1915, has rather small coxal plates. Chevreuxiella and Parachevreuxiella are very similar, only differing by the length of uropods 1 and 2, their shape and the presence/absence of an inner ramus on both appendages. As the new species does not fit in any of the known thoriellid genera we are erecting the new genus Chevreuxiopsis herein.

The material was collected by a McLane 21-cup sediment trap at 1,000 m depth in the subantarctic Southern Ocean, southwest of Tasmania. The conical sediment trap has a surface of 0.5 m² and is filled with unfiltered water from the region (49°S, 153°E at 1,200 m), which was treated with sodium chloride (5 g/L) to increase the solution density, sodium tetracarborate (1 g/L) as a pH buffer, and mercuric chloride (3 g/L) for preservation (Roberts et al. 2008). The sample was filtered through a 1 mm screen and the specimen was found in the fraction > 1 mm, which contains plankton and micronekton organisms (Roberts et al. 2008).

For taxonomic study, we transferred the material in a graded series of ethanol-glycerol mixes into pure glycerol and then mounted the specimen or dissected parts on slides for the preparation of the drawings. Pencil drawings of the habitus were made with on a Leica M 205c dissecting microscope and details of the appendages and mouthparts on a Leica DMLB compound microscope. Both microscopes were equipped with a camera lucida. The line drawings were made following the technique described in Coleman (2003, 2009). Measurements were made along the dorsal outline of the animals, from the rostrum to the end of the urosome.

The material is held in the collections of the Leibniz Institute for Evolution and Biodiversity Science, Museum für Naturkunde Berlin (ZMB).

We classified the new species in a new genus, Chevreuxiopsis. This genus is related to Chevreuxiella (represented by C. metopoides, the only species) and both genera share the following: pereon not inflated (cf. Danaella); coxa 4 enlarged; similarities in the mouthparts: rather underived maxilla 1 and 2 and the morphology of the maxilliped; urosome and both uropods are very similar in the lanceolate shape of the rami and the dimensions of the inner rami.

However, there are also strong differences between the new species and C. metopoides. Chevreuxiopsis franki gen. n. sp. n. has a differently shaped, slender antenna 1 (vs expanded and elongate first flagellar article. This is perhaps a sexually dimorphic character, as C. metopoides was described on a male specimen) and especially antenna 2 flagellum, which has an enlarged article 1, large, posteriorly drawn out articles 2 and 3 and a lanceolate article 4 (vs normally shaped and multiarticulate); maxilla 1 with inflated palp article 2 (vs normal), inner plate with 2 plumose setae (vs 4 setae); basis of pereopod 1 ovoid expanded with anteromarginal nose-like process (vs weakly expanded and without nose-like process); coxa 3 slightly longer than coxa 2 (vs much longer and wider); coxa 4 posteromarginally straight (vs posteroventally lobate); basis of pereopods 5–7 slender (vs expanded); pereopod 3–6 probably prehensile due to their subchelate arrangement of propodus and falcate dactylus (vs simple).

The maxilliped is of a unique shape in thoriellids. Large plates surround the mouthparts and leave a small slit anteriorly and ventrally. However, due to its derived morphology it is very difficult to homologize the parts. For example, Stephensen (1915: 41, fig. 24) labelled the massive maxillipedal plates, that surround the mouthparts, as the first palp articles, but we think they are the outer plates of the maxilliped. These plates are overlapped by the huge bases of the first pereopod, which has dark purple/black pigmentation and may act as a shutter (see below).

Due to the few records in literature, knowledge about the biology of the Thoriellidae is limited (Lowry and Stoddart 2011). Stephensen (1915) suggested a semiparasitic lifestyle for Chevreuxiella and Thoriella, and Andres (1987) found Parachevreuxiella lobata attached to a wound of a bathypelagic fish. However, it is difficult to draw conclusions about the lifestyle of Chevreuxiopsis franki sp. n. Due to the relatively good preservation of the body, an active entering of the specimen into the sediment trap is assumed. The long-term deployment of the sediment trap prevents analyses of potential differences in day vs night distribution due to diurnal vertical migration. Hence, no additional information on the vertical distribution of this species can be concluded.

The specimen is of transparent appearance apart from the dark purple gnathopod 1, which covers the maxilliped (Fig. 6). Herring (1981: 171) already noted a blue-green bioluminescence in the genera Chevreuxiella and Danaella in the thoriellid family while handling, which “almost extinguished when the maxilliped plate was withdrawn between the two densely pigmented expanded basal articles of the first pair of gnathopods”. Also, Parker (1999) studied the luminescence of an unidentified thoriellid juvenile and found the expanded fifth articles of the second antennae to act as reflectors of the luminescent maxilliped. He assumed that rather than having a communication or defensive function, light flashes could be used to catch prey. Similarly, this could be the case in our specimen. Additional to the dark purple/black shutter, we also note enlarged articles of the second antenna, which could function as reflector of the emitted bioluminescence and lead potential prey towards the maxillipeds. However, this has to be further investigated in behavioural studies.

Figure 6. 

Chevreuxiopsis franki gen. n., sp. n., holotype 12 mm. Photo of head, antennae, maxillipeds and anterior pereopods. Note the dark purple/black colour of the 1^st pereopods. Scale bar: 500 µm.

This research was supported by the Australian Research Council through a Laureate awarded to Philip Wallace Boyd, IMAS, UTAS, Australia (FL160100131), the PhD supervisor of the first author.