Leucothoe vaderotti, a new Atlantic Leucothoe (Crustacea, Amphipoda) belonging to the “spinicarpa-clade” (Crustacea, Amphipoda)

Abstract Within the international IceAGE project (Icelandic marine Animals: Genetics and Ecology) some leucothoid amphipods (Crustacea) were collected, among them a rather small new species, belonging to the “Leucothoe spinicarpa-clade.”


Introduction
Within the IceAGE-collections (a follow-up of BIOICE, see for example Berge and Vader, 1997) some rather small specimens of the genus Leucothoe were collected, similar to the ones described in Krapp-Schickel and Vader (2012) from the Norwegian Sea, called "Leucothoe aff. spinicarpa". This genus is extremely easy to recognize as such, but its many species are differentiated from each other only by subtle characters, which has traditionally led to a wholesale lumping of many species under just a few names, especially Leucothoe spinicarpa (Abildgaard, 1789). In the last years, it has become clear that the genus, with many of its species living commensally, is a very speciose one and there is no doubt a considerable number of as-yet undescribed species to be discovered (cf. Thomas and Klebba 2007).

Material and methods
Samples were taken during IceAGE1 and IceAGE2 (with research vessels 'Meteor' and 'Poseidon'; see Brix et al. 2014); they were fixed in cold 96% un-denaturated ethanol, sorted on ice, and stored at 0-4 °C after sorting. They were identified, some mounted on slides with Faure's fluid, and drawn using a Leitz Laborlux microscope. "Inking" was done with a Wacom tablet, following Coleman, 2003. Acronyms used in the morphological descriptions are as follows: A   Diagnosis. Eyes oval, dark in ethanol. Mandibular palp long and narrow, art 3 more than half the length of art 2. Cx 1 inferior margin smooth, nearly as long as wide. Gn 1 carpus distal part approximately six times longer than wide, dactylus reaching approximately 1/3 of propodus length. P 3, P 4 with narrow basis, P 5 -P 7 basis oval and broadened, with regularly rounded and finely serrated hind margin. Ep 2 posterodistally with upturned tip, Ep 3 distoposterior corner with blunt, rounded angle.
Head. Anterior margin rounded, anterodistal margin rectangular with rounded corner. Mid-cephalic keel with acute projection. Rostrum small.
Eyes oval. Antennae short, nearly 1/3 of body length, A 1 peduncle art 1 inferodistally with acute tooth, flagellum up to 11 arts, accessory flagellum not seen. A 2 subequal in length to A 1, peduncle art 4 > art 5, flagellum around 6 arts.
Cx 2 nearly as long as wide, subquadrangular, much wider than Cx 3, smooth; facial setae absent.
Cx 3 length greater than its width, smooth, subrectangular with straight margins and rounded corners.
Cx 4 wider than Cx 3, posterior margin shorter than anterior one, somewhat excavate. P 3, 4 basis narrow, approx. the width of merus; dactylus reaching or surpassing half the length of propodus.
Etymology. Wim Vader completed eight decades in February 2017. He was born Dutch but having lived for much more than half of his life in Norway, he will easily guess that I used the Norwegian word åtti = eighty for dedicating this Atlantic species to him. Fifty years ago we began our long-lasting and harmonious collaboration, a "golden jubilee"-thank you, Wim! Geographical distribution. South Iceland-Farøer Channel and Ridge; depth 554-2531 m.
Remarks. Together with five specimens collected 1983 between Greenland and Iceland (see Krapp-Schickel and Vader 2012) the present material of the proposed new species consists of 23 animals belonging undoubtedly to the genus Leucothoe. All specimens are between 2 and 5 mm, rarely up to 6 mm long. There are four specimens larger than 10 mm with all characters fitting Leucothoe spinicarpa, sampled in similar depths as those of the animals 2-5 mm in size. The larger specimens show some differences which are not very conspicuous. It is most probable that they belong to two different species, and it seems also quite reasonable to presume that within the period of early June to end of September these 23 specimens are not all juveniles; however, no ovigerous females were found.
These differences are significant enough to distinguish these small specimens as a new species.
Other material examined. Two other Leucothoe species were sampled, L. spinicarpa and L. lilljeborgi, the latter clearly in lower depths than the new species.

Discussion
After Krapp-Schickel and Vader (2012) many Leucothoe species are known from the Atlantic Ocean, but mainly from warmer regions. Not much is known about their biology, but it is known that they prefer to live near, in, or with other species such as sponges, and this may also be the reason that they are often well hidden and thus overlooked when generalised ship sampling occurs.