Machatrium, Bruce, Niel L. & Buxton, Chad, 2013

Bruce, Niel L. & Buxton, Chad, 2013, Review of the marine isopod crustacean genus Hansenium Serov and Wilson, 1995 (Asellota: Stenetriidae) from tropical Australia and Papua New Guinea, with description of a new genus, Zootaxa 3664 (4), pp. 445-478 : 457

Machatrium gen. nov.

Type species: Machatrium falcensis sp. nov., here designated.

Species included. Machatrium falcensis sp. nov., type species, Great Barrier Reef; Machatrium spathulicarpus (Kensley, 1984) , comb. nov., Belize; Machatrium tropex (Bolstad & Kensley, 1999) , comb. nov., Papua New Guinea; Machatrium thunku sp. nov., Great Barrier Reef. All of these species have an long carpal blade (1.5–4.2 as long as wide), extending to half way along inferior margin of the propodus and up to the distal margin of the propodus.

The following species have a moderately long carpal blade (1.3–1.5 as long as wide) that extends up to half way along the propodus inferior margin: Machatrium aldabrae (Kensley & Schotte, 2002) , comb. nov., Aldabra Island, Western Indian Ocean; Machatrium monodi (Nordenstam, 1946) , comb. nov., Gulf of Suez, Egypt (Monod 1933); Machatrium monodi , of Kensley and Schotte, 2002, comb. nov., Mahé, Seychelles (not S. monodi Nordenstam ); Machatrium wongat sp. nov., Papua New Guinea; Machatrium zanzibarica (Kensley & Schotte, 2002) , comb. nov., Zanzibar, Tanazania, East Africa [this species keys to Hansenium in Kensley and Schotte (2002), and appears near identical to Hansenium aldabrae ].

Diagnosis (male). Cephalon lateral teeth acute, antennal teeth sub-equal in length to lateral, acute or rounded. Pseudorostrum truncate, trapezoid, wider than long. Eyes large (more than 12 ommatidia), reniform. Male pereopod 1 carpus inferodistal process smooth, spatulate; propodus elongate (length at least 2.5 times longer than wide), propodal palm transverse with 3–5 teeth, a terminal seta present. Male pleopod 2 appendix masculina distally broad, terminally concave with fringe of setae and interior pocket of setae, distolateral margin with 2–5 proximally directed spines.

Description (male). Body dorsal surface smooth or sparsely setose, widest at pereonite 1; pereonite 1 length greater than 1.5 times pereonite 2 length; pereonites 2–4 lateral margins convex, anteriorly acute. Pleotelson subequal to width; lateral margins smooth, sub-parallel, posterolateral spines prominent, margin posterior to spines rounded with weak apical point; dorsal surface smooth, or sparsely setose.

Cephalon lateral margins smooth. Antennae length equal or longer than total body length, article 1 lateral spine absent.

Pereopod 1 basis superior margin with regularly spaced setae along length, superior submarginal ridge with row of setae; merus angular; carpus inferodistal margin with well-developed process; propodal palm with teeth along palm margin,; dactylus length subequal to propodal palm length.

Pleopod 1 protopod rectangular, lateral margin setae present; rami lateral margins evenly convex. Pleopod 2 protopod longer than wide, distal apex sharply pointed; appendix masculina lateral margin groove absent. Pleopod 5 distal apex with 3–5 plumose setae.

Female. Generally similar to male but for primary sexual characters. The pereopod 1 in females is much smaller, less ornamented and less setose.

Remarks. Machatrium gen. nov. is characterised by the male pereopod 1 having an elongate propodus, spatulate and smooth carpal process and pleopod 2 appendix masculina apex being concave and fringed with setae. The principle differentiating and diagnostic characters that separate Machatrium from both Hansenium and Stenetrium are presented in Table 2 .

Distribution. The genus is known only from tropical coral reefs, with a worldwide distribution.

Etymology. Derived from the Latin ‘machaerium’ meaning bent sword and ‘–trium’ indicating family affinity; gender neuter.