Mesamphisopidae Nicholls 1943

Gouws, Gavin, 2008, New species of Mesamphisopus, an endemic South African freshwater isopod genus (Isopoda: Phreatoicidea: Mesamphisopidae), Zootaxa 1690, pp. 1-62 : 6-9

Family Mesamphisopidae Nicholls 1943

Type genus. By monotypy, Mesamphisopus Nicholls 1943: 29–30

Diagnosis. Body small, fusiform, sub-cylindrical; surface smooth. Head small, with cervical and genal grooves, mandibular, clypeal and antennal notches. Eyes small, prominent. Coxal articulation of pereonites 2– 7 free and exposed. Sternal processes absent. Gut round in cross-section. Pleonites with large pleurae, obscuring basal region of pleopods. Pleotelson shorter than deep; posterolateral margins entire, without processes, lobes, crenulations or inflections differentiating apex; posterior apex projecting medially in dorsal view, visible laterally, free (not strongly reflexed or flattened against dorsal surface with ventral surface exposed); ventral surface posterior to anal opening smooth, lacking setae. Antennula short; penultimate article longer than other articles; terminal article minute, vestigial. Both mandibles with lacinia mobilis, right with two dentate plates; spine row with bifurcate spines on projecting ridge, forming strongly convex arc in ventral view, protruding mesially. Pereopod I subchelate; propodus broadly ovate, dorsal margin proximal region not produced beyond carpus-propodus joint, palm with stout bifid and serrate denticulate setae and basally-inflated robust setae. Pereopod IV prehensile in adult male, with hinge between dactylus and propodus. Pereopods II–VII with distal margin of propodus lateral/posterior to dactylus produced as triangular articular plate. Pereopod VII basis dorsal margin expanded to form distally-indented plate. Pleopods II–V exopods with lateral and mesial proximal lobes; minimally pleopod I endopod with setae on margins; minimally pleopods I–III protopod mesial margins with coupling hooks; pleopods II–V protopods with mesial epipods; pleopods III–V protopods only with lateral (lobe-like) epipods. Appendix masculina broadly concave in cross-section proximally, not forming tube; setae on smooth lateral and mesial margins; tip broadly rounded. Uropod protopod dorsomesial margin produced and plate-like ridge; distoventral margin with three robust setae; rami in cross-section flattened dorsally, distal tips rounded, with robust setae apically and along length.

Remarks. Considering the importance of certain character suites distinguishing the families as arranged by Poore et al. (2002) or suggested by cladistic analyses ( Wilson & Keable 2001, 2002b; Wilson & Edgecombe 2003), key differences between Mesamphisopus and Eophreatoicus warrant the latter’s removal from the Mesamphisopidae . This renders the family monotypic.

In Eophreatoicus , inflections in the posterolateral margins of the pleotelson distinguish the broad and rounded terminal apex from a vertical plate formed by the posterolateral telsonic pleural margin, which obscures the apex in lateral view ( Nicholls 1926, 1943). Inflections also differentiate the dorsal terminal region in the Phreatoicidae , Ponderellidae, Poore et al. ’s (2002) Amphisopidae , as well as Wilson and Keable’s (2002a) new genera ( Platypyga and Eremisopus ) ( Nicholls 1943, 1944; Wilson & Keable 2002a, 2004). In the Amphisopidae and Ponderellidae , the posterolateral margin forms a similar vertical plate ( Amphisopus , Eremisopus , Paramphisopus Nicholls 1943 , Phreatomerus Sheppard 1927 and Synamphisopus ), while in the Phreatoicidae and some Amphisopidae this forms a rounded or angular process, with several positions on the margin defined by large setae ( Nicholls 1943, 1944; Wilson & Keable 2002a, b, 2004).

The terminal article of the antennula is a small knob. A reduced terminal article approaching the vestigial condition of Mesamphisopus is seen only in certain hypsimetopid genera ( Hyperoedesipus , Phreatoicoides and Pilbarophreatoicus ) and the Ponderellidae ( Sayce 1900; Nicholls & Milner 1923; Nicholls 1944; Knott & Halse 1999; Wilson & Keable 2004).

The dorsal margin of the propodus of pereopod I is proximally produced towards the distodorsal region of the carpus in Eophreatoicus , as illustrated by Nicholls (1926, 1943). Such protrusions are found in the Ponderellidae and certain genera in the Amphisopidae and Hypsimetopidae ( Sayce 1900; Nicholls 1943; Knott & Halse 1999; Wilson & Keable 2002a, 2004), the most extreme example being found in Hyperoedesipus ( Nicholls & Milner 1923; Nicholls 1943). The dorsal margins of the bases of all three hinder pereopods are produced to form large rounded plates in Eophreatoicus and the ischium and merus of these pereopods are similarly expanded ( Nicholls 1926, 1943). Large, greatly expanded basis plates are characteristic of the Amphisopidae , with pereopods V–VII bearing basis plates in Amphisopus , Eremisopus , Paramphisopus and Phreatomerus ( Chilton 1922; Glauert 1924; Nicholls 1924, 1926, 1943; Wilson & Keable 2002a). Synamphisopus and Platypyga , like most Mesamphisopus species , have plates on pereopods VI and VII approaching this condition ( Sheard 1936; Nicholls 1943; Wilson & Keable 2002a, b). The Ponderellidae and certain Mesamphisopus species have plates only on pereopod VII ( Wilson & Keable 2004).

Like the Phreatoicopsidae Nicholls 1943 , the exopods of pleopods I–V have both lateral and medial proximal lobes in Eophreatoicus ( Nicholls 1926, 1943; Wilson & Keable 2002b). By contrast, these are only apparent on pleopods II–V in Mesamphisopus , the Phreatoicidae and Amphisopidae (including Eremisopus and Platypgya) ( Nicholls 1943, 1944; Wilson & Keable 2002a, b). The pleopodal endopods of Eophreatoicus lack setae ( Nicholls 1926, 1943). Setae are found on the margins of the endopods in only Hypsimetopus , Paraphreatoicus Nicholls 1944 and Notamphisopus Nicholls 1944 (first pleopods only), Mesamphisopus (minimally pleopod I), Eremisopus (pleopods I–V) and Pilbarophreatoicus ( Sayce 1902; Nicholls 1943, 1944; Knott & Halse 1999; Wilson & Keable 2002a). Similarly, pleopodal protopods in Eophreatoicus lack coupling hooks ( Nicholls 1943). Coupling hooks only occur in Mesamphisopus , Hyperoedesipus and in the Amphisopidae (including Eremisopus and Platypyga , but excluding Synamphisopus ) ( Nicholls & Milner 1923; Nicholls 1943; Wilson & Keable 2002a, b). Eophreatoicus and Eremisopus each bear a lobe-like, lateral epipod on the propotod of pleopod II ( Nicholls 1926; Wilson & Keable 2002a). This is in addition to those on pleopods III–V, typical of the Amphisopidae , the Phreatoicidae , Hypsimetopus and Mesamphisopus ( Sayce 1902; Nicholls 1943, 1944; Wilson & Keable 2002a, b). Only the Phreatoicopsidae have lateral epipods (and mesial epipods) on all five pleopods ( Nicholls 1943; Wilson & Keable 2002b).

On the balance of characters considered here, Eophreatoicus appears to be most closely allied to Eremisopus . This relationship has been stressed previously ( Wilson & Keable 2002a) and cladistic analyses ( Wilson & Keable 2002b; Wilson & Edgecombe, 2003) reveal them to be sister taxa. Both are also closely allied to Platypyga and the three genera form a monophyletic assemblage. Consideration of the familial placement of Eophreatoicus , Eremisopus and Platypyga is deferred at present, as is consideration of the phylogenetic relationships among families. More detailed analyses of generic relationships in the Phreatoicidea are currently in progress (G.D.F. Wilson, pers. comm.). These may better resolve relationships and suggest affinities for these and other “amphisopid” genera with uncertain placement (e.g., Crenisopus Wilson & Keable 1999 and Peludo Wilson & Keable 2002a ). However, recognizing Mesamphisopidae as a distinct monotypic family renders the Amphisopidae sensu stricto of Wilson and Edgecombe (2003) paraphyletic. This can be rectified by regarding the Eophreatoicus + Eremisopus + Platypyga clade as a unique family. The Amphisopidae would then comprise Nicholls’ (1943) Amphisopinae and Phreatomerinae, and would correspond to Poore et al. ’s (2002) Amphisopidae , if Synamphisopus were removed. These analyses and a re-evaluation of characters (below) advocate its removal.

A monotypic Mesamphisopidae is readily distinguished from currently constituted families ( Poore et al. 2002) or arrangements proposed by cladistic analyses ( Wilson & Keable 2001, 2002b; Wilson & Edgecombe 2003). The most fundamental differences between the Mesamphisopidae and the Hypsimetopidae regard the latter’s lack of eyes and shallow pleonites ( Sayce 1900, 1902; Nicholls & Milner 1923; Nicholls 1943; Chopra & Tiwari 1950; Tiwari 1955a; Knott & Halse 1999). These pleonites are no deeper than the pereonites and leave the proximal regions of the pleopods exposed. Various telson forms occur in the Hypsimetopidae , but all are longer than deep, truncate distally and lack the well-defined, projecting terminal apex and, often, the entire posterolateral margins of Mesamphisopus . This family and the majority of phreatoicids, with the exception of Mesamphisopus , have the coxal articulation of pereonites 2-4 fused. The Hyspimetopidae lack stout denticulate setae on the propodal palm of the gnathopod, as do the Phreatoicopsidae , Ponderellidae , Synamphisopus , Eremisopus and Platypyga ( Wilson & Keable 2002a, b, 2004). Serrate robust denticulate setae may occur in some Phreatoicidae , but bifid denticulate setae (e.g., Fig. 2F) appear to be unique to Mesamphisopus . In the Hypsimetopidae and Phreatoicopsidae , the anal opening adjoins the posterior telson margin ( Nicholls 1943; Knott & Halse, 1999; Wilson & Keable 2002b). In other groups, a ventral surface posterior to the anal opening is either smooth ( Mesamphisopus , Ponderellidae and most Phreatoicidae ) or adorned with ridges, grooves or setae ( Amphisopidae , Eophreatoicus , Eremisopus and Platypyga ) ( Nicholls 1943, 1944; Wilson & Keable 2002a, b, 2004). With the exception of Pilbarophreatoicus , hypsimetopids (and phreatoicopsids) lack the propodal articular plates (on pereopods II–VII) characteristic of Mesamphisopus and the Amphisopidae . The hypsimetopid uropodal protopod lacks distoventral setae and the dorsomesial ridge, although produced, does not form a plate-like projection.

Species of the Phreatoicopsidae lack a cervical groove on the head and the pleotelson is sharply truncate ( Spencer & Hall 1897; Nicholls 1943; Wilson & Keable 2002b). The typhlosole is well-developed. As in the Ponderellidae and Synamphisopus , the right lacinia mobilis consists of a single plate. The medial and lateral lobes of the maxillula bear, respectively, considerably more pappose and robust setae (none of which are denticulate) than in Mesamphisopus ( Nicholls 1943; Wilson & Keable 2002b). Pereopod I is fairly distinctive: the dactylus ventral margin has a proximal tooth, is otherwise smooth and lacks a distal accessory claw. The propodal palm bears a distinct distal spine, while the merus forms a spine-like distodorsal projection ( Wilson & Keable 2002b). The appendix masculina is solid and rod-like, proximally. The uropod has only two robust setae distoventrally, whereas Mesamphisopus has three.

The principal distinction between the Mesamphisopidae and the Phreatoicidae , as proposed by Poore et al. (2002) and supported by cladistic analyses ( Wilson & Keable 2002b; Wilson & Edgecombe 2003), reflects the character used by Nicholls (1943, 1944) to distinguish his original Amphisopidae and Phreatoicidae . The lacinia mobilis of the right mandible is lacking or is vestigial and is not separated from the remaining spines between the incisor and molar in Phreatoicidae ( Nicholls 1944; Wilson & Ho 1996; Wilson & Fenwick 1999; Wilson & Keable 2002b). While a terminal pleotelson apex, projecting in dorsal view, appears typical of the Phreatoicidae ( Nicholls 1944; Wilson & Ho 1996; Wilson & Fenwick 1999; Wilson & Keable 2002b), the inflections, processes and lobes of the posterolateral pleotelson margin (above) distinguish the family from the Mesamphisopidae . The appendix masculina is much like that of the Phreatoicopsidae . The uropodal rami are distally pointed as in the Phreatoicopsidae , but are round in cross-section.

The more immediate differences between the Mesamphisopidae and the Ponderellidae include the latter’s lack of an antennal notch and pleotelson that is indented in dorsal view ( Wilson & Keable, 2004). Robust setae are also lacking from the propodal palm of pereopod I. Articular plates are not found on the propodus of pereopods II and III. The proximal articles of the exopods of pleopods II–V each have a large distolateral lobe. Mesamphisopus lacks such lobes. All five exopods also have lateral proximal lobes. The dorsomesial margin of the uropodal protopod does not form a produced ridge in the Ponderellidae ( Wilson & Keable 2004) .

In evaluating the differences between the Mesamphisopidae and Amphisopidae , the current composition of the Amphisopidae ( Poore et al. 2002) , as well as taxa ( Eremisopus and Platypyga ) shown to belong to the Amphisopidae sensu Wilson and Edgecombe (2003) were considered. On the basis of current taxonomy, Eophreatoicus was considered separately above, despite its suggested inclusion ( Wilson & Edgecombe 2003). The major difference among the Mesamphisopidae and Amphisopidae relates to the differentiation of the pleotelson terminal apex and the inflections and processes formed by the posterolateral pleotelson margin, as highlighted above. Mesamphisopus and the majority of the Amphisopidae have a projecting and visible terminal apex, the apical region is indented and strongly reflexed in Platypyga and indented with a small medial projection in Synamphisopus ( Wilson & Keable 2002a, b). The occurrence of setae on the margins of the pleopodal endopods serves to distinguish Mesamphisopus from most amphisopids, with the endopods of Eremisopus bearing setae proximally. Like the Phreatoicopsidae , the Amphisopidae and Platypyga lack a cervical groove. This is, however, present in Eremisopus . For most of the Amphisopidae , the gut is similar to Mesamphisopus , but a weakly developed typhlosole occurs in Eremisopus and is well-developed in Synamphisopus ( Wilson & Keable 2002a, b). Several other characters unite Mesamphisopus and the Amphisopidae , to the exclusion of Synamphisopus . These include the presence of articular plates on pereopods II–IV and coupling hooks on the mesial protopod margin of at least pleopod I, and the setation of the maxillula. The abundant setation of the latter, as well as the form of the appendix masculina and the presence of a mesial protopodal epipods on pleopods I–V suggest a relationship between Synamphisopus and the Phreatoicopsidae , as has been proposed elsewhere ( Nicholls 1943; Wilson & Keable 2002b; Wilson & Edgecombe 2003). The appendix masculina also distinguishes the remainder of Poore et al. ’s (2003) Amphisopidae from Mesamphisopus , Eremisopus and Platypyga . In the former, the stylet forms a tube distally and is terminally conical, but is concave, indented and rounded terminally in the latter genera.