Rhachotropis

Boury-Esnault, Nicole, Bellan, Gerard, Bellan-Santini, Denise, Boudouresque, Charles-Francois, Chevaldonné, Pierre, Dias, Alrick, Faget, Daniel, Harmelin, Jean-Georges, Harmelin-Vivien, Mireille, Lejeusne, Christophe, Perez, Thierry, Vacelet, Jean & Verlaque, Marc, 2023, The Station Marine d’Endoume, Marseille: 150 years of natural history, Zootaxa 5249 (2), pp. 213-252 : 240

The Rhachotropis species ( Fig. 12 ) of Mid-Atlantic Ridge deep-sea hydrothermal vents: R. flamina , R. licornia , R. pilosa Bellan-Santini, 2006

SME taxonomists also have taken part in the major adventure started with the 1977 discovery of deep-sea chemosynthetic animal communities. In the vicinity of hydrothermal vents of the Mid-Atlantic Ridge, 3 new species of Rhachotropis (Crustacea: Amphipoda : Eusiridae ) were collected by sediment traps at 1700–2750 m depth: Rhachotropis flamina Bellan-Santini, 2006 , Rhachotropis licornia Bellan-Santini, 2006 , and Rhachotropis pilosa Bellan-Santini, 2006 .

With 63 species ( Horton et al. 2021) this genus is found in all oceans with a large bathymetric distribution (0–7160 m) (L̂rz et al. 2018). It is the most common amphipod genus in bathyal and abyssal zones.

Morphologically, Rhachotropis have a delicate body with slender pereiopods, long antennae and sometimes dorsal processes. However, some of them display antennae bearing complex and puzzling structures called calceoli (present in R. pilosa and R. licornia ) that are believed to be part of sensory organs. Cuplike receptacles, arranged serially on the antennae, would act as non-visual sensory organs, ensuring the perception of sound and vibration stimuli by the amphipods. These likely mechanoreceptors are found in several amphipod species ( Hurley 1980, Lincoln & Hurley 1981, Bellan-Santini 2015); some could be involved in the detection of mates, others to detect preys. However, at hydrothermal vents, they could also be a good way to locate active fluid emissions.

Figures

FIGURE 12. Rhachotropis licornia female holotype (10 mm): Left:habitus, antenna 1 and 2,mandibles, maxilla, maxilliped and gnathopods. Right: Scanning Electron Micrographs of antenna 1 with calceoli, and surface of antenna 1, different magnifications and views of calceoli. Modified from Bellan-Santini (2006).