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 Author Bruce, Niel L. Author Buxton, Chad text Zootaxa 2013 3664 4 445 478 journal article 10.11646/zootaxa.3664.4.3 50bcb629-492f-43bf-b365-56caac37748a 1175-5326 220066 2535EF79-7191-4805-946D-0668E3FD0C52 Genus Hansenium Serov and Wilson, 1995 Hansenium Serov and Wilson, 1995: 72 .—Bolstad and Kensley 1999: 164.—Kensley and Schotte 2002: 1443.—Martin, Heard and Wetzer 2003: 975. Type species. Stenetrium hanseni Nobili, 1906 ; by original designation (Serov & Wilson 1995). Species included. Hansenium hanseni ( type species), Hansenium remocarpus Kensley and Schotte, 2002 . Species retained in combination with Hansenium but here regarded as incertae sedis , species inquirenda, or transferred to other extant genera are discussed in the ‘Remarks’ section below. Hansenium tropex Bolstad and Kensley, 1999 is here transferred to Machatrium gen. nov. and Stenetrium bowmani Kensley, 1984 (pseudorostrum rounded; merus and carpus without evident lobes or process; propodus with transverse palm) is returned to the original combination. Table 1 lists all species in the original and new combinations. Diagnosis (male). Cephalon lateral and antennal spines acute, sub-equal in length. Pseudorostrum quadrate, wider than long. Eyes large (more than 12 ommatidia), reniform. Male pereopod 1 merus subquadrate, inferodistal process small and rounded; carpus inferodistal process blade-like, serrate; propodus robust, propodal palm oblique with 2 teeth, without a terminal seta or spine; dactylus length 1.0–1.5 times propodal palm length. Male pleopod 2 appendix masculina distally broad, terminally blunt without setae, lateral margin with or without a distal proximally directed spine. 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. TABLE 1. Species of Hansenium Serov and Wilson 1995 and Machatrium gen. nov. mentioned in this work (alphabetical by species).

Original combination	Type locality	Current combination
Hansenium aldabrae Kensley and Schotte, 2002	Aldabra Atoll, Seychelles	Machatrium
Stenetrium antillense Hansen, 1905	West Indies	Incertae sedis
Stenetrium bowmani Kensley, 1984	Belize	Stenetrium
Stenetrium caicosense Kensley and Heard, 1991	Turks and Caicos Island	Incertae sedis
Stenetrium chiltoni Stebbing, 1905	Sri Lanka	Species inquirenda
Stenetrium dodo Müller, 1991	Réunion Island	Incertae sedis
Stenetrium echiurum Nobili, 1906	Tuamotu Islands	Incertae sedis
Stenetrium entale Nordenstam, 1946	Tapeteuea, Gilbert Islands	Incertae sedis
Hansenium expansum Kensley and Schotte, 2002	Nosy Bé, Madagascar	Incertae sedis
Machatrium falcensis sp. nov.	Lizard Island, GBR	Present work
Stenetrium gilbertense Nordenstam, 1946	Ararai, Gilbert Islands	Species inquirenda
Stenetrium hanseni Nobili, 1906	Tuamotu Islands, French Polynesia	Hansenium
Stenetrium medipacificum Miller, 1941	Hanauma Bay, Hawaii	Species inquirenda
Stenetrium monodi Nordenstam, 1946	Gulf of Suez	Machatrium
Stenetrium occidentale Hansen, 1905	St. Thomas, West Indies	Incertae sedis
Hansenium remocarpus Kensley and Schotte, 2002	Mahé Island, Seychelles	Hansenium
Stenetrium spathulicarpus Kensley, 1984	Belize	Machatrium
Stenetrium stebbingi Richardson, 1902	Bermuda	Incertae sedis
Hansenium thomasi Bolstad and Kensley, 1999	Madang, Papua New Guinea	Incertae sedis
Machatrium thungku sp. nov.	Lizard Island, GBR	Present work
Hansenium tropex Bolstad and Kensley, 1999	Madang, Papua New Guinea	Machatrium
Stenetrium wilsoni Müller, 1991	Moorea, Society Islands	Incertae sedis
Machatrium wongat sp. nov.	Madang, Papua New Guinea	Present work
Stenetrium zanzabaricum Kensley and Schotte, 2002	Murogo Reef, Zanzibar, Tanzania	Machatrium

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 without setae; carpus inferodistal margin with well-developed process; propodus with convex superior margin, propodal palm with teeth along palm margin. Pleopod 1 protopod rectangular, lateral margin setae present or absent; rami lateral margins evenly convex. Pleopod 2 protopod longer than wide, distal apex sharply pointed; appendix masculina lateral margin groove absent, mediodistal margin hardened. Pleopod 5 distal apex with 3–5 plumose setae. Female. Generally similar to male but for primary sexual characters in the pleopods and dimorphism of pereopod 1. Pereopod 1 in females is smaller, with a simple sub-chelate propodus and less setose. Remarks . Serov and Wilson (1995), when establishing the genus Hansenium , acknowledged that a number of characters remained variable in Stenetrium , including rostral shape, going on to say that ‘we suspect the genus may yet be further divided as the species are better illustrated’. In addition, the thirteen species included in Hansenium presented a great diversity in the male pereopod 1 morphology, some of which did not conform with the genus diagnosis. Consequently, there has been uncertainty as to what characters unambiguously define Hansenium and also what characters distinguish Hansenium from Stenetrium . All subsequent accounts of the genus (Bolstad & Kensley 1999; Kensley & Schotte 2002; Martin et al . 2003) have had difficulties in generic placement of new species, regarding the boundaries between Stenetrium and Hansenium as being unclear. Serov and Wilson (1995) designated Stenetrium hanseni Nobili, 1906 as the type species for the genus Hansenium , a species for which there is no type material. The diagnosis presented by Serov and Wilson (1995) appeared to be based, in part, on Nobili’s (1907) few simple figures but all illustrations accompanying their diagnosis were taken from species other than the type species. Bolstad and Kensley (1999) and Kensley and Schotte (2002) both offered new diagnoses to the genus, but on the basis of contained species rather than the type species. Here the genus is again diagnosed, but on the basis of the type species. Although Nobili’s illustrations and description are brief (in the extreme), they correspond totally with the description given by Müller (1991a). Furthermore, we have not observed any similar species in the extensive material that we have examined for the genus. Nobili (1906) gave no indication as to where his type material might be housed. Some of Nobili’s decapod crustacean specimens are held at the Museo Regionale di Scienze Naturali in Torino, but there is no evidence that any of Nobili’s isopod type material is in existence. Several enquiries concerning Nobili’s isopod type material have been unsuccessful in locating the specimens. A neotype is therefore designated from Müller’s (1991a) specimens, the only existing material for the species, in order to stabilise the generic concept and use of the name Hansenium . In their diagnoses to the genera of Stenetriidae, Serov and Wilson (1995) use the term rostrum, although genera are figured with a rostrum (e.g. Stenetrium and Stenobermuda (figs 1A and 2E), with a pseudorostrum (e.g. Hansenium , fig. 1E) or without either (e.g. Mizothenar , fig 2A). Several descriptions (e.g. Bolstad & Kensley 1999; Kensley & Heard 1991; Kensley and Schotte 2002; Nordenstam 1946) and our own examination of freshly collected material from the CReefs expeditions, show clearly the presence of a pseudorostrum. A SEM taken by Bolstad and Kensley (1999) of Hansenium thomasi shows this morphology clearly. All species of Hansenium have a short, trapezoid pseudorostrum that is approximately twice as wide as long, while Stenetrium appears to have a broad-based, distally narrowly rounded rostrum or pseudorostrum, the lateral margins of which converge anteriorly and, in several of the species that we have examined, is serrate. The illustrations given by Serov and Wilson (1995) appear to show that there is a rostrum, although the illustration of the Stenetrium armatum Serov and Wilson, 1995 female (fig. 4E) has a fine line drawn across the rostrum base, suggestive of a pseudorostrum. In contrast, Hansen (1905), Kensley (1980), Kensley and Schotte (2002), Nicholls (1929) and Schultz (1982) all illustrate what appears to be a pseudorostrum. All species of Stenetrium collected from Ningaloo reef possess an anteriorly rounded pseudorostrum (personal observation). At present, whether Stenetrium has a rostrum or pseudorostrum remains unclear. We consider that the presence ( Stenetrium ) or absence (e.g. Hansenium , Machatrium gen. nov. ) of a broad-based, anteriorly rounded rostrum or pseudorostrum is a further distinguishing generic character. The revised concept for Hansenium includes only those species with a trapezoid pseudorostrum, reniform eyes, male pereopod 1 carpus with a well-developed and serrate inferodistal process, and lacking setae on the apex of the appendix masculina. This diagnosis restricts the genus to two species. The remaining species are assigned to the new genus described herein (three species and three new species), regarded as incertae cedis , species inquirend a or assigned to Stenetrium . Distribution. Hansenium is currently only known from tropical Indo-West Pacific coral reefs. Hansenium , incertae sedis The species listed here are retained in combination with Hansenium , but excluded from the genus sensu strictu as they either lack the diagnostic characters of Hansenium and Machatrium gen. nov. , or possess unique characters that preclude their inclusion in other genera. Further undescribed species of these groups are present in CReefs material and other collections. Some of these groups of species, particularly when considered in conjunction with the male pleopod 2 morphology, potentially warrant new genera: ‘ entale group’—male pereopod 1 without superior meral process; carpus inferodistal margin with ventrally directed, terminally acute process; inferodistal margin of merus with distinct (short to long) blade; propodus inferior margin with large, acute process; dactylus elongate, longer than propodus. Hansenium entale (Nordenstam, 1946) ; Hansenium thomasi Bolstad and Kensley, 1999 and Stenetrium echiurum Nobili, 1906 ; two species present on the Great Barrier Reef. ‘ stebbingi group’—male pereopod 1 carpus inferior margin with anteriorly directed acute process; propodus inferior margin concave, lacking distinct palm, terminating in small lobe. Hansenium stebbingi Richardson, 1902 ; Hansenium antillense Hansen, 1905 and Hansenium occidentale Hansen, 1905 . are regarded as junior synonyms of H. stebbingi (Menzies & Kruczynski 1983; Kensley 1984). Hansenium wilsoni Müller, 1991 a—male pereopod 1 with very large carpal process encompassing the propodus and extending beyond the distal margin of the propodus, the propodus articulating within the mesial surface of the carpal process rather than ‘end to end’; pereopod palm oblique. Hansenium wilsoni . Hansenium caicoense Kensley & Heard , 1991—male pereopod 1 with prominent, acute and anteriorly directed inferodistal meral process; propodus elongate, weakly inflected, oblique palm with spine at point of inflection. Hansenium expansum Kensley and Schotte, 2002 —shows few affinities to any other species or group of species; male pereopod 1 has a short semi-circular carpal process, propodus with large round lobe. Hansenium dodo Müller, 1991 b—shows little affinity to any other species or group of species; male pereopod 1 carpus with short process; propodal palm oblique with terminal tooth.