Review of the Haploniscidae

Author: George D.F. (Buz) Wilson

Introduction

The Haploniscidae is an archtypical deep-sea isopod family that occurs in practically all samples of deep-sea benthos (Wolff, 1962; Hessler et al., 1979; Harrison, 1988; Thistle and Wilson, 1987, 1996). Although they do not dominate the fauna like Desmosomatidae or Munnopsidae (Hessler, 1970; Wilson, 1989), they are also not the rarest component (Hessler et al., 1979; Harrison, 1988). We know little of their ecology although the best evidence indicates that they live epifaunally (Thistle and Wilson, 1987, 1996) and are general detritovores. Their legs are completely unspecialised walking legs and, as a result, are not useful for taxonomy within the family. Their exoskeleton is incongruously heavily calcified, despite that most of the regions that they inhabit is well below the oceanic calcium carbonate compensation depth. As Lincoln (1985a) pointed out, they must therefore maintain this carbonate load against a substantial negative equilibrium, a stunning physiological feat in itself. Morphologically, the species of this group are fairly conservative, and appear surprisingly similar to terrestrial isopods in some instances. Some clades of species have acquired the ability to fold up or enroll, although the mechanisms involved are different than those seen for the terrestrial Armadillididae or Armadillidae. The articulations between somites of the posterior body in all Haploniscidae are poorly developed or not expressed at all. As a result, some species cannot complely close the gap between the head and tip of the pleon, while others, such as species of Hydroniscus, can close into a reasonably tight ball.

TAXONOMY

Family Haploniscidae Hansen, 1916

Type Genus. Haploniscus Richardson, 1908

Composition.— Abyssoniscus Birstein, 1971; Aspidoniscus Menzies & Schultz, 1968; Chauliodoniscus Lincoln, 1985A; Haploniscus Richardson, 1908; Hydroniscus Hansen, 1916; Mastigoniscus Lincoln, 1985A.

Not Included.—Chandraniscus George, 2003, nomen dubium.

Diagnosis. Body strongly vaulted, cuticle indurate, heavily calcified; without dorsal spination or large setae. Head freely articulated to pereonite 1; eyes absent; antennae and antennulae inserting anteriorly, frons not produced between antennae, with distinct anterior margin between frons and head dorsal surface; clypeus indurate, projecting anteriorly over labrum. Pereonal tergites extending over pereopodal coxae, laterally quadrate, coxae inserting ventrally on sternum on either side of vaulted medial surface, medial to tergal lateral extensions. Spermathecal pore on anterodorsal margin of pereonite 5; penes of male adjacent, external, emerging from posteromedial margin of pereonite 7 sternite; not elongate, approximately as long as wide at proximal margin; inserting directly into anterior funnel of pleopod I. Pleotelson including pleonite 1 fused to pereonite 7; anus separated from pleopodal chamber by distinct bar, subcircular, ventral. Antennula article 4 shorter than articles 3 and 5; flagellum (starting at article 5) multiarticulate. Antenna longer than antennula, shorter than body length; article 3 elongate compared to articles 1-2 and 4, scale absent (possibly expressed as spine in many species); flagellum (starting at article 7) multiarticulate, sometimes sexually dimorphic, expressed in male with enlarged proximal segments and more aesthetascs compared to female. Mandible molar process slender but truncate distally; palp longer than body of mandible, article 2 elongate, article 3 with row of setulose setae. Maxilliped endite broad, distal margin concave medially,  medially setose, with broadly rounded lobe produced distal and lateral to medial margin; epipod elongate, near length of endite, distal tip acute; palp narrow, much narrower than endite, without enditic lobes. Pereopod I not prehensile, I-VII slender and ambulatory; dactylar claw lengths strongly unequal; dorsal claw curved, elongate; ventral claw tiny flattened blade. Pleopod I of male broader proximally than distally, with small proximal hood covering penes, lateral margins concave, medial lobe produced posteriorly past medial lobe. Pleopod II of male endopodal stylet with sperm tube proximal end large, trough-shaped, opening anteriorly. Pleopod III exopod shorter than endopod, lateral and medial margins convex, with fringe of spinules and setae, distal tip terminating in rounded angle; endopod with 3 pappose setae, medial seta separated from lateral pair of setae by distinct gap subsuming endopod tip. Uropods uniramous, cylindrical, distally rounded, inserting terminally on pleotelson adjacent to anus.

Remarks. Of all Asellota, the Haploniscidae resemble some members of the crinochaete oniscidean isopods, at least from our perceptual standpoint, owing to their broad indurate bodies and an ability in many species to roll up. This appearance, however, is convergent. The lateral margins of the body in the Oniscidea are large coxal plates whose articulation with the body is not expressed, and their tergite is not produced laterally. The haploniscids have normally articulated coxal rings that insert medially on the pereonal sternum, and the tergites extend laterally well beyond the coxal insertions: this is the primary synapomorphy that defines the family Haploniscidae. Despite some authors describing haploniscids as “dorsolaterally flattened” (e.g., Park, 2000:196), haploniscids are distinctly vaulted: the lateral tergites have a strong angle to the horizontal plane of the body. Probably related to the vaulted laterally-expanded tergite habitus, haploniscids have a calcium carbonate hardened body and, unlike many deep-sea isopods, lack dorsal spines or robust setae. In the following, I discuss some of the other useful identifying features of the Haploniscidae.

Fusion of the pereonites. Haploniscidae lack medial expression of the first pleonite or the free articulation between the pleon and the pereon. This compound posterior tagma also includes pereonites 5 and 6 in many species. The details of how the articulations of these pereonites are expressed ventrally and dorsally can assist in defining genera. Much confusion about the “fusion” of the pereonites has been introduced by consideration of weak sutures on the dorsal surface and Lincoln’s (1985a,b) diagnoses are somewhat confusing in this regard (see below under the discussion of “Chandraniscus”). In addition, several different combinations of articular expression in the pereonites can be found within the genus Haploniscus: some species appear to lack pereonal articulations posterior to that between pereonites 4 and 5, while others may have a weakly expressed articulation between pereonites 5 and 6. Mastigoniscus, Hydroniscus and Aspidoniscus are fully inarticulate in the posterior tagma, while Chaulidoniscus has a flexible articulation between pereonites 5 and 6, as part of its unique form of enrolling (cf. Lincoln, 1985a, fig.16c).

Antenna article 3 spine. This antennal spine, which occurs on many, but not all, Haploniscidae, may be homologous to the antennal scale, because it occurs in the same approximate condition as the scale of many other asellotans. The spine, however, is dorsal or dorsolateral while a plesiomorphic antennal scale is typically lateral. The antennae may be rotated along their axis relative to a more primitive condition, because the haploniscid antennae flex dorsally rather than laterally. Therefore the antennal spine is assumed to be homologous to the antennal scale. The spine occurs in many forms, either as a broad flattened blade, to a sharp curved spine. The spine may also have denticles and small setae on the anterior and posterior margin; these elaborations, although microscopic, are essential for identifying males with female of the same species when several sexually dimorphic species are present in the same sample.

Other apomorphies. The lack of any substantial modification of the pereopod I is unusual and occurs in only a few other janiroidean familes (e.g., some Janiridae and Joeropsidae). This limb is plesiomorphically propodosubchelate and the first pereopod of many janiroideans is prehensile in some way. Although not mentioned by previous authors, the ventral dactylar claw of the haploniscid pereopod is distinctive, being a tiny flat blade varying in shape from triangular to a thin spine. The small subtriangular exopod of pleopod III is distinctive and can serve to define the Haploniscidae. Most asellotans have enditic lobes or plates on the maxillipedal palp, but haploniscids do not. Although other families have a broad maxillipedal endite, the haploniscids have an unusual broad rounded expansion of the distolateral margin, which causes the distomedial margin to be concave and somewhat compressed appearing. Finally haploniscids have small tubular uniramous uropods. The uropods can be tiny and are concealed in some species (e.g., Abyssoniscus).

Rostrum and other structures on the head. Quite a few species of Haploniscidae are illustrated with a anteromedial structure on the head that has been refered to as a “rostrum ” (Lincoln, 1985a:9). Whether these structures are a true rostrum, i.e., an anterior projection of the dorsal surface of the head or whether they are the clypeus (“epistome” in Lincoln, 1985b), is uncertain. Lincoln’s (1985a) text lists species with both structures under the same name, “rostrum”, and much of the literature is unclear on which is being described. These two structures are different; the projecting clypeus should not be referred to as a “rostrum”, although Just (2001:314, fig.9) calls the enlarged clypeus a “pseudorostrum” in Rugojoeropsis. Unfortunately, the head is rarely illustrated in a lateral to frontal oblique aspect that would allow the assessment of this feature. The rostrum is discussed further under Antennuloniscus.

Composition of the Haploniscidae. Below, the genus Chandraniscus George, 2003 is rejected as a nomen dubium, and a new genus to subsume several species removed from Haploniscus. Almost certainly, Haploniscus can be further subdivided after previously described types are re-illustrated, with details of the structure of the head, clypeus and ventral surface of the pereon. The genera Aspidoniscus and Abyssoniscus may be found to be synonymous once better descriptions of Asp. perplexus Menzies & Schultz, 1968 and Ab. ovalis Birstein, 1971 are prepared from the types, or from material from near the type localities. The primary difference between these species is the shape of the tip of the pleotelson. RYG (p. 340; key copied mostly from Lincoln, 1985a (p.14) but without acknowledgement) indicates that they also differ in the setae on the female pleopod II. Menzies & Schultz (1968, fig. 7B) give a clear illustration of this limb but Birstein (1971, fig.11) does not, leaving some uncertainty of the latter’s accuracy. Notably, the Birstein’s illustration of pleopod II of the male shows elongate setae. The value of the length of setae as a useful character is dubious, as being particularly variable both in ontogeny and between species. These two genera are not treated further in this paper.

Taxonomic Status of Chandraniscus George,  2003. RYG creates a new genus of Haploniscidae, Chandraniscus, based solely on the lack of fusion of the posterior pereonites, without regard to other obvious and useful features of the taxa in this family (see Lincoln, 1985a,b). The other characters mentioned in the diagnosis are either uninformative or incorrect. For example, “uniramous uropod” (p. 342) is an apomorphy of the family and is uninformative and “[uropod] with a single segment” is inaccurate because some species may have two articles, the protopod and the endopod. “Antenna 2 lacking a spinous dorsal process on peduncular article 2” (p.342) may be a numbering error. No haploniscid has a spine on the second article of the antenna, but many have a large and variable spine on article 3, including the species RYG puts into his genus. The spine is clearly present in species where presumedly it should be absent, if RYG meant the spine on article 3.

His generic concept, based on the fusion of pereonites, is invalid because all Haploniscidae have pereonites 5-7 merged in some way; this tendancy is a synapomorphy of the family. The literature is not accurate with regard to how these somites are merged, and existing generic characters (cf. Lincoln, 1985a,b) address the relative degree of lateral expression of the somites, with Antennuloniscus, Chalidoniscus and some species of Haploniscus having the greatest expression and Hydroniscus having the least expression of the posterior pereonites and pleotelson articulations. Although RYG is correct in assuming that distinct clades within Haploniscus can be recognised, the use of the degree of somite "fusion" is unwarranted because the species in the genus Haploniscus show a great range in the apparent degree of fusion of the somites, at least medially. Moreover, RYG misinterprets his specimens: C. kussakini, one of the few relatively undamaged specimens (see below), does not have sutures in the dorsal surface of pereonites 5-7, despite his illustration (fig. 5A) clearly showing them. The same is true for C. chardyi (RYG fig. 6). This common error renders many recent illustrations of Asellota inaccurate. For example, Nannoniscus antennaspinis Brandt, 2002 is illustrated with distinctly separate pereonites 6 and 7, but an inspection of what remains of the holotype (ZMH K-40110) finds these segments to be fused. Divisions between somites may appear as partial separation between segments internally owing to muscle attachments, but articulation may be externally absent at the cuticular surface. In decalcified specimens, the condition of the cuticle can be difficult, but not impossible, to assess. In general, if the articulation of the somites is hard to see, then some degree of loss of the articulation is present. Freely articulated segments have several layers of cuticle and articular membrane that makes their junction obvious in a compound microscope.

RYG's (p.343) characterisation of Lincoln's (1985a) species, which RYG apportions to either Haploniscus or Chandraniscus, is inaccurate. These species do have unexpressed articulations at least medially on pereonites 5-7. Lincoln (1985a:14-15) diagnoses Haploniscus as "pereonites 5-7 free" but his text refers to the lateral extent of the pereonites, rather than the entire pereonite being free, because he (1985a,b) clearly illustrates species of Haploniscus with medially fused pereonites 5-7. For the species Haploniscus saphos, Lincoln (1985a:31) states "…pereonites 5-7 partially fused with pleotelson, suture line distinct laterally but very faint medially." This state is the typical for most species of Haploniscus, so Lincoln (1985a,b) was clearly referring to the lateral extent of the pereonites.

Thus, the concept of Chandraniscus fails as a useful concept for classifying the species in Haploniscus. Because the holotype of the type species is missing, or at least badly damaged and compromised (see below), we cannot reinterpret the generic concept, so Chandraniscus is a junior synonym and nomen dubium of Haploniscus. If Haploniscus were revised with several new genus-level groupings established, further use of Chandraniscus is not advised because we cannot establish the features of this genus and because the putative type species is a nomen dubium.

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