Haplomunnidae Wilson, 1976

Cunha, Marina R. & Wilson, George D. F., 2003, Haplomunnidae (Crustacea: Isopoda) reviewed, with a description of an intact specimen of Thylakogaster Wilson & Hessler, 1974, Zootaxa 326, pp. 1-16 : 5-8

Family Haplomunnidae Wilson, 1976

Haplomunnidae Wilson, 1976: 572 ; Kussakin, 1988: 355; Roman & Dalens, 1999: 250. Dendrotionidae Wolff, 1962: 64 (part).

Munnidae Gamô 1983: 11 (part).

Type genus. Haplomunna Richardson, 1908 .

Diagnosis (modified from Wilson 1976): Head without eyes, anteriorly rounded, dorsal surface sloping anteriorly, lacking anterior margin. Body with spines, dorsally vaulted, posterior pereonites axially compressed and reduced. Female spermathecal duct emerging anterodorsally near margin of pereonite 5. Anus covered by opercular pleopods. Antennula inserting dorsally on head, sexually dimorphic, with many flagellar articles and aesthetascs in male, and fewer and thinner articles in female. Antenna inserting anterodorsally, basal articles robust, article 3 without scale. Mandible palp (when present) elongate, longer than body of mandible, article 3 elongate with elongate setae only on distal tip. Pereopod I with major subchelate hinge between carpus and propodus; carpus palm (ventral margin) robust, with elongate robust setae. Pereopods II– VI and VII, if present, longer than body. Male pleopod II protopod strongly tapering and narrow distally; exopod lacking setose posterior projection. Pleopod III exopod biarticulate, narrower and longer than endopod, without plumose setae. Uropods emerging at pleotelson ventral margin; uniramous; tiny, not extending beyond terminal margin of pleotelson.

Relationships: The family Haplomunnidae was argued by Wilson (1976) to be the sister group of the Dendrotiidae , and not closely related to Munnidae , Santiidae or Paramunnidae . The primary similarities between the former two families include general body spinosity, vaulted thoracic region, compact anteriorly­sloping emarginate head capsule with antennae and antennulae placed on anterodorsal surface, and elongate pereopods II– VII that exceed the body length. While not fully consistent among species in both families, the ischium and merus of the walking legs are often subequal and shorter than the basis. Notably, the opposite situation occurs among the Munnidae , where the basis is often shorter than the ischium. Although Acanthomunna Beddard, 1884 is somewhat similar in body form to Haplomunna , male dendrotiids have a setose posteriorly­directed appendage on exopod of pleopod II, an apomorphy of the family. Tiny marginal uropods are an apomorphy of the Haplomunnidae , although reductions of the uropods are common among other deep­sea isopod taxa. Some, but not all, members of both families share the heterochronic non­expression of the last pereonite. This oddity also occurs among other deep­sea families (e.g., Munnopsidae , Ischnomesidae ), so its phylogenetic significance is limited to those clades that reach adulthood without the last pereonite.

Regarding higher­level relationships, Wägele (1989), partially following Wolff (1962), suggested that the asellotan families Munnidae , Santiidae (as Pleurocopidae ), Dendrotiidae and Haplomunnidae were related in a group referred to as ‘munnoiden’, or as having a ‘munnoid habitus’. The sister group relationship between the Dendrotiidae and the Haplomunnidae , as discussed above, was not in dispute, but the principal difficulty with Wägele’s (1989) hypothesis lies in the underlying assumption of similarity of body form ( Wilson 1996). In Wägele’s (1989: 67) concept, the munnoid habitus includes the following features: an approximately oval body outline, in which the middle pereonites are somewhat broader than the anterior or posterior segments, head broader than long, eyes laterally projecting and often clearly stalked, pereonites 5 to 7 not only narrower but also shorter than the anterior pereonites, pleotelson dorsally convex, and at its origin somewhat narrower, distally rounded and often spindle­like. Other than the stalked eyes, most of these features are seen throughout the Janiroidea, especially among the deep­sea families. Acanthomunna is the only genus with eyes among the Dendrotiidae or the Haplomunnidae , but only has rounded lateral projections, somewhat similar to those seen among some Janiridae and clearly unlike the stalked eyes of the presumptive ‘munnoiden’ clade. Although not fully relevant here, the form of the stalked eyes (where present) varies consistently between the eyed families: short, thick and generally with many ocelli among the Munnidae and Santiidae , elongate and often with few ocelli among the Paramunnidae and extremely elongate and thin, with few ocelli in Pleurocope , which belongs to its own separate family, Pleurocopidae . The ‘munnoiden’ are unlikely to be a cohesive phylogenetic group because different taxa have different positions for the spermathecal duct and configurations of pereopod I (Wilson 1987; personal observation). Additionally, antennae in Paramunnidae , Munnidae and Santiidae , and Pleurocopidae all insert anteroventrally, but with differing positions among the three groups; Haplomunnidae and Dendrotiidae have antennae inserting anterodorsally, often on projections of varying sizes. Other features, such as the above­mentioned comparative lengths of the pereopodal podomeres, also differ considerably among these families. Clarification of the haplomunnid classification will require detailed morphological study and phylogenetic analysis of all families of Janiroidea.

Distribution: Although Kussakin (1988), in his useful compendium of the isopods of the Northern Hemisphere, characterised the Haplomunnidae as an equatorial and Southern Hemisphere group and only listed the equatorial Atlantic species Thylakogaster peterpauli Wilson & Hessler (1974) , recent records ( Table 1 , Figure 1 ) indicate that this family is much more widespread than previously suspected, and clearly not limited to equatorial regions. Species of Haplomunna are known from abyssal and hadal waters of the North Pacific, off southern and Baja California ( Wilson 1976; Beaulieu 2001a, 2001b) and the Japan Trench ( Gamô 1983) from depths of 3880 to 6450 m. Thylakogaster has the broadest distribution from depths of 1135 m to 5223 m, including the equatorial Atlantic, Argentine Basin, north of Surinam, Bermuda slope, off Ireland, in the Rockall Trough ( Harrison 1988) and in the east Pacific ( Thistle & Wilson 1987). New records include the Lucky Strike vent field on the Mid­Atlantic Ridge, and the east Pacific Ocean. Abyssaranea has been found at depths from 3459 to 4438 m in the equatorial Atlantic and East Pacific, as well as a new record from a Woods Hole Oceanographic Institution deep­benthic station in the northwestern Atlantic. This latter record is the first non­equatorial record for this genus. Munella has most records in the bathyal deep sea (950–2076 m) including the Bay of Biscay, the Bermuda Slope and north of Surinam in the Atlantic. Unconfirmed records of Munella in the central Mediterranean are from depths of 100–200 m (Bay of Napoli; Lo Bianco 1903). Munella also occurs at abyssal depths in the east Pacific (4797 m, Thistle & Wilson 1996). New records of Munella include Lucky Strike vent field on the Mid­Atlantic Ridge (1685m, TTR­10), the eastern Atlantic near the African coast (1175–1283 m, BALGIM stations), and the Gulf of Mexico (3732 m., Rowe 2003). A new species and new genus of Haplomunnidae collected by Poore et al. (1994) occurs in southeastern Australia at bathyal depths (J. Just, personal communication).

Habitat and Ecology: All Haplomunnidae have long­legged ambulatory morphologies, and are unlikely to be burrowers. Because they are primarily epibenthic and exposed to erosion, they are not likely to be found at high energy sites. This family has not been recorded from the HEBBLE (high energy benthic boundary layer experiment) sites ( Thistle & Wilson 1987, 1996), and haplomunnid species are a reasonably consistent member of benthic assemblages at quiescent sites. In the Clipperton­Clarion Fracture Zone region of the East Pacific, a region characterised by low abyssal hills, pelagic clay and manganese nodules, Thylakogaster has been found at the ECHO 1 site ( Thistle & Wilson 1987) and Munella at the PRA site ( Wilson 1990; Thistle & Wilson 1996). The recorded substrates for haplomunnids range from typical deep­sea sediments to rocky outcrops, manganese nodules and carbonate crusts. Several Atlantic records are from continental margins in muddy sediments ( Bonnier 1896), sand and volcanic gravel ( Lo Bianco 1903), mixed Globigerina and pteropod oozes ( Bermuda slope; Wilson & Hessler 1974). We now report them from hydrothermal vents (Lucky Strike) including a high temperature, diffuse venting area. In this case, the Thylakogaster specimens were collected on volcanic rocks (hyaloclastic breccia and pillow lavas) in the vicinity of active venting areas and in colonisation trays deployed near black smokers in the SE part of the field, while Munella specimens were collected on fragments of inactive chimneys in the NW part of the field. Biogenic structures appear to be favoured, because Haplomunna sp. was found highly aggregated on Hyalonema sponge stalks ( Beaulieu 2001a, 2001b).

Like most deep­sea isopods, the trophic habit of Haplomunnidae species appears to be mostly detritivory. Haplomunna sp. , found on the basal and middle portions of hexactinellid sponge stalks, may be feeding upon particles accumulating in pockets ( Beaulieu 2001a). Thylakogaster species have elongate and setose pereopods II–III that may be used to search food in the sediments or catching suspended particles. These legs on preserved specimens are generally held with the carpus and propodus curving under the mouth field and the first pereopods, like a basket.

Figures

FIGURE 1. Distribution of known Haplomunnidae species.