A new genus and species of axiid shrimp (Crustacea, Decapoda) from a southwestern Indian Ocean seamount

A new genus and species of axiid shrimp, Montanaxius mediumquod gen. et sp. nov., is described and illustrated based on three specimens collected from hexactinellid sponges from a seamount in the southwest Indian Ocean. The new genus is characterized by a laterally denticulate rostrum, short lateral carina, absence of submedian carina, a prominent toothed median carina, round pleomere pleura 2–5, pleurobranchs on second to fourth pereopods, and the presence of a male first pleopod and appendix interna on pleopods 3–5. It most closely resembles Levantocaris Galil & Clark, 1993 and Planaxius Komai & Tachikawa, 2008, but differs from the former by being gonochoristic, having a strongly elevated gastric region and well-developed eyes, and from the latter by its toothed median carina and the presence of a median telson spine.


Introduction
During the IUCN Seamounts expedition, extensive samples were taken on board the RRS 'James Cook' from 7 November to 21 December 2011 from underwater mountains in the southwestern Indian Ocean (Rogers & Taylor 2012;IUCN 2013). Sponges were one of the most frequent benthic organisms recorded on these seamounts. Several hexactinellid sponges were inhabited by associated caridean, stenopodidean and axiidean shrimps. One of these shrimps has been described recently as a new species of the hippolytid genus Paralebbeus Bruce & Chace, 1986 by Komai (2013). The "glass shrimps" (Rogers & Taylor 2012: figs 53-54) belong to the genus Spongiocaris Bruce & Baba, 1973 and have recently been described as a new species . The axiid shrimps from the seamount 'Coral' (12 ♂♂, 5 ♀♀, 4 ovigerous ♀♀) are similar to Eiconaxius indicus (de Man, 1907) (Rogers & Taylor 2012: fig. 48). Three specimens (1 ♂ and 2 ♀♀) collected at the seamount 'Middle of What' could not be assigned to any of the axiid genera. The present paper describes them as a new species and erects a new genus for it. European Journal of Taxonomy 229: 1-11 ISSN 21181-11 ISSN -9773 http://dx.doi.org/10.5852/ejt.2016 www.europeanjournaloftaxonomy.eu 2016 · Dworschak P.C.
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R e s e a r c h a r t i c l e
urn:lsid:zoobank.org:pub:181AB3EE-0B35-47AD-B06A-8A327D4B0BA7

Material and methods
Host sponges were collected with a manipulator installed on the ROV 'KIEL 6000' during a dive on 1 Dec. 2011 at the 'Middle of What' seamount in the southwestern Indian Ocean during the 66th voyage of the RRS 'James Cook'. Shrimps were photographed alive on board in cold-water aquaria and later fixed in ethanol or formaldehyde (Rogers & Taylor 2012: 60, 106).
All drawings were made using a camera lucida mounted on a stereo microscope, digitized and then inked and composed in Adobe Illustrator (Coleman 2003(Coleman , 2009. Digital photographs were taken with a Nikon 995 camera mounted on a stereo microscope or a compound microscope. Stacks of several frames of different focal planes were fused using CombineZ5 (Haug et al. 2011).
Terminology for the orientation of the limbs follows Poore (1997), where "upper" is used to describe the extensor (or anatomically anterior) margin and "lower" the flexor (or posterior) margin.

Type species
Montanaxius mediumquod gen. et sp. nov. by monotypy and present designation.

Etymology
The name is a combination of the stem of montanus (Latin for 'mountain', referring to the seamount where it was found) combined with Axius. The gender is masculine, as in Axius.

Etymology
The specific epithet mediumquod is an arbitrary combination of the Latin words "medium", meaning middle, and "quod", meaning what, derived from the type locality 'Middle of What'; the name is used as a noun in apposition.
Mouthparts similar to those of other species of the family, without specific characters (see Poore & Collins 2009). Third maxilliped (Fig. 2F-G) with ventral spine on coxa and basis, small spines on ischium lower border, prominent crista dentata; two strong spines on merus proximally, one spine on carpus lower border proximally; exopod as long as ischium-merus, flagellate distally.
First pereopods (chelipeds) significantly differentiated, major more swollen, with longer palm and shorter, more robust dactylus. Major cheliped on the right side in holotype (Fig. 1D) and paratype NHMW 25675, on the left side in the other female paratype (Fig. 3C); ischium as long as wide, one DWORSCHAK P.C., New genus and species of Axiidae spine proximally on lower border; merus three times as long as high, three spines increasing in size on lower border, one subterminal spine on upper border; carpus short, ⅓ length of merus, cup-shaped, unarmed; propodus with palm as long as merus, upper and lower borders smooth, curved, lower border with strong keel; small gap proximally between fingers when closed; fixed finger triangular, cutting edge with one strong triangular tooth at midlength and several smaller teeth distally, one large rounded tooth at 2/3 length mesially; dactylus with incision proximally, upper border strongly curved, cutting edge with low triangular blade at midlength, one corneous spine on upper border near tip (Fig. 3D). Minor cheliped (Figs 1C, 3E) slightly thinner and shorter than major one; ischium as long as wide, one spine proximally on lower border; merus with four spines on lower border and one subterminal spine on upper border; carpus short, cup-shaped; propodus with curved borders, unarmed, keel on lower border reaching to beginning of fixed finger, short keel on upper border distally; no gap between fingers when closed; fixed finger triangular, with almost straight cutting edge and corneous tip; dactylus slightly curved on upper border, almost straight on lower border, one corneous spine on upper border near tip (Fig. 3F).
Second pereopod (Fig. 3G) slender, 0.84 times as long as first pereopods; ischium short, merus long, five times as long as ischium, 8.3 times as long as wide; carpus 0.6 times length of merus; palm same length as carpus, fingers half as long as palm, cutting edges with tiny spiniform setae in distal ⅔ (Fig. 1E).
Telson longer than wide, posterior margin with median spine, lateral edges with two to three spines at midlength and group of three spiniform setae proximally, two pairs of spines on dorsal surface. Uropodal endopod with curved posterior border and straight anterior border, median row of spines on dorsal surface. Uropodal exopod with spines distally on anterior border, one to two strong spiniform setae at transition to transverse suture, the latter distally beset with spines, posterior border smooth (Fig. 3M).

Ecology
The shrimps were associated with an unidentified hexactinellid sponge.

Discussion
As the new species is gonochoristic, it clearly does not belong to the calocaridid group of axiid genera because hermaphroditism is a feature of all species of this group (Kensley 1989;Sakai 2011). In addition, the new species lacks the terminal enlarged appendix masculina on male Plp2 that is characteristic for the calocaridid group (Poore 1994).
When using the key in Sakai & de Saint Laurent (1989), the new species keys to the genus Dorphinaxius Sakai & de Saint Laurent, 1989. The only species of this genus, Dorphinaxius kermadecensis (Chilton, 1911) [senior synonym of D. appendiculis (Poore & Griffin, 1979)], differs from the new species by having a much shorter rostrum with no other lateral spines than the prominent supraocular spines (vs long rostrum with several lateral spines without supraocular spines), the presence of a submedian carina (vs absence), the absence of male Plp1 (vs presence) and the lack of a median telson spine (vs presence).
In the key to Axiidae of Poore (1994) the new species keys out to the genus Spongiaxius Sakai & de Saint Laurent, 1989. The only species of this genus, Spongiaxius brucei (Sakai, 1986), differs from the new species by the typical shape of its rostrum with erect spines, separated by a constriction from the carapace (vs rostrum continuous with carapace), the presence of a toothed submedian carina (vs absent submedian carina), acute pleomere pleura (vs rounded pleura) and a distinctive massive dactylus in the major cheliped (Poore & Collins 2009).
The key to Axiidae of Sakai (2011) leads to the monotypic genus Levantocaris Galil & Clark, 1993. The new species shows some similarities with Levantocaris hornungae Galil & Clark, 1993 in lacking the submedian carina and having rounded pleomere pleura, but differs by having a strongly elevated gastric region with a toothed median carina (vs slightly convex gastric region with unarmed median carina), fully pigmented, rounded eyes (vs weakly pigmented, anteriorly flattened eyes). Planaxius Komai & Tachikawa, 2008, a genus containing only the type species P. brevifrons Komai & Tachikawa, 2008, is also similar to the new genus, especially in the lack of a submedian carina on the anterior part of the carapace, a similar rostrum and a short lateral carina (Komai & Tachikawa 2008;Poore & Collins 2009). Planaxius was described originally as lacking a median carina. Poore & Collins (2009), however, reported that their individual from Australia referred to this species has a definite median gastric carina at the base of the rostrum extending as far back as the lateral gastric carinae. Nevertheless, the new genus can be differentiated from Planaxius by the presence of a prominent median toothed carina on the anterior carapace, the lack of supraocular spines on the carapace and the presence of a median telson spine. In addition the new species has a first male pleopod with mesially directed spine and appendix interna, not found in any other genus of the Axiidae. In summary, a new genus is warranted.