Comprehensive Description

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Description

Vermectias nelladanae, new species

 

Asellota gen. sp. No. 2.-Kussakin, 1967: 317- 318, figs. 56-57.

 

Material Examined.- Holotype. Male, 2.3 mm, Mac-quarie Island, Green Gorge, large rock platform 500 m south of hut, intertidal pool (54?38'S, 158?55'E), filamentous red algae, collected by D. S. Homing, 8 January1 978( MacquarieIs landE xpedition1 977-1978, station MA-257), Museum of Victoria (NMV) J21588.

 

Paratypes. 209 specimens from type locality, including male, ovigerous and post breeding female, 1 preparatory female with spermathecae, young females, mancas, and broken specimens: NMVJ21589 (1 female), J22075 (preparatory female), J21590 (manca), J21591 (1 male and 2 females), J21592 (1 male), J21593 (60 male, 46 female, 40 mancas), J21594 (9 specimens cleared in KOH or lactic acid); Australian Museum, Sydney (P40615), Zoological Museum, Copenhagen, National Museum of Natural History (USNM), Washington (5 males and 8 female each).

 

Ten male, 5 ovigerous female, 3 young female, Macquarie Island, Handspike Point, intertidal kelp zone (54◦30'S, 158◦53'E), holdfasts of Durvillea antarctica, collected by D. S. Homing and J. K. Lowry, 21 December 1977 (Macquarie Island Expedition 1977-1978, station MA-135), NMVJ21595.

 

Two 66, 2 ovigerous 99, 1 young 2, Macquarie Island, Handspike Point, intertidal rock pool (54◦30'S, 158◦53'E), coralline algae, collected by G. C. B. Poore, 21 December 1977(Macquarie Island Expedition 1977- 1978, station MA-136), NMV J21596.

 

Description. -Body 6.5-7.0 times as long as wide. Cephalon approximately 1.3 times as wide as long, as wide as pereionite 1; an-terior margin protruding to end of article 1 of antennae 1; clypeus and upper lip form-ing protruding curve to about two-thirds depth of cephalon. Eyes with 4 ommatidia. Pereionite 1 slightly shorter than cephalon and pereionite 2; pereionites 2-5 and 7 of equal length and width, 6 approximately 10% longer than others and approximately 20% wider than 7; 5, 6, and 7 with pronounced posterolateral tergal lobes, each lobe with stout seta; pereionites 2-4 with similar stout seta on anterolateral margin. Cephalon and pereionites with small groups of short simple setae. Pleon little shorter than wide, approximately 70% length of pereionite 7 (midline); pleonites 1-3 as wide as pleotelson, combined dorsal length ranging from 25% of entire pleon in mancas to approximately 10% in some large males; pleotelson with weakly convex lateral margins, weakly defined distolateral angles, distal margin convex in middle with faint concavity on each side.

 

Antennae similar in both sexes. Antenna 1 with 6 articles, reaching to about middle of sixth article of antenna 2; article 1 wider than remainder; article 2 approximately 1.25 times length of 1; articles 3, 5, and 6 all approximately 40% length of 2; article 4 approximately half length of 3; article 6 with 1 short terminal aesthetasc. Antenna 2 as long as cephalon and pereionites 1 and 2 combined; peduncular articles 1 and 2 shorter than wide; article 3 slightly longer than combined 1 and 2, with tiny antennal scale carrying 2 setae; article 4 shorter than wide, forming geniculate articulation with 3 directing rest of antenna laterad; article 5 as long and wide as 3; article 6 approximately 40% longer than 5; flagellum approximately 30% longer than peduncle, with up to 18 articles in males, 15 in females.

 

Upper lip about as long as wide, apex broadly rounded, with small setulose bump.

 

Mandibular molar process cylindrical, grinding surface with transverse groove and few marginal setae; palp article 2 with 3 stout bilaterally pectinate setae, article 3 with 1 stout terminal seta and 1 or 2 smaller setae; well-developed spine row on rounded projection; 3-toothed lacinia mobilis on left mandible only; incisors 5-toothed.

 

Lower lip deeply and narrowly cleft, lobes apically rounded with marginal row of stout curved setae among scattered setules. Maxilla 1 outer lobe with 11 mostly dentiferous spinelike setae; inner lobe slender with distal simple setae.

 

Maxilla 2 lobes of equal length; inner lobe with rounded apex and apical and medial plumose setae; middle and outer lobes with broadly truncate apices, with 4 and 3 strong comb-shaped setae, respectively.

 

Maxillipedal palp with articles 1-3 wider than endite, 4 and 5 slender; article 2 about twice length of 1, wider than long, medial margin strongly convex; article 3 about as long as 1, forming an almost even curve from very broad base; endite reaching to apex of palp article 3, distal margin with double row of stout setae, marginal row unilaterally plumose, submarginal row strongly pectinate; epipod tapering in distal half to-wards bluntly pointed apex, reaching to apex of palp article 3.

 

Pereiopods subequal, pereiopod 1 slightly smaller; each with ovate basis approxi-mately 1.7 times as long as wide; sparsely setose along margins of all articles; propodus with posterodistal spiniform seta; all dactyli with curved, terminal unguis and 1 (pereiopod 1) or 2 (all others) strong, curved, bifid claws. Pereiopod 4 differing in male only with distal articles hooking under pereion, propodus curved.

 

Male pleopods one-half length of pleon, length 85-90% width at base; apex of each ramus produced medially, rounded; distal margin oblique, weakly concave with few setae; distolateral corer rounded with tuft of setae; dorsolateral margin at distal third with conical, rounded projection carrying few lateral setae; ventral surface with 1 seta; dorsal surface with distomedial oval field of fine setules.

 

Male pleopods 2 covered by pleopods 1 except laterally; peduncle regularly triangular, width at base subequal to length of lateral margin; apex rounded, medial mar-gin at approximately 45? to midline; exopod article 1 cylindrical, approximately twice as long as 2, poorly delimited article 2 slightly flattened dorsoventrally, apex irregularly rounded with small field of tiny setules (visible only under oil immersion); endopod article 1 cylindrical, smooth, approximately twice as long as wide, article 2 approximately 1.7 times as long as 1, tapering to-wards narrow, barbed tip, with broadly con-vex ventrolateral flange covering shallow open lateral groove; posterior entrance to groove (insertion point of penis) broad ventral furrow at junction of articles 1 and 2.

 

Pleopod 3 approximately half length of 4, oval; pleopod 4 elongate, exceeding posterior margin of pleotelson by up to half its length, inserted distinctly posterior to 3.

 

Uropods inserted lateral to ventroterminal anus, peduncle approximately 75% length of pleotelson with 1 long midmedial and several long distal setae, exopod as long as peduncle, inserted dorsolaterally, endopod 1.3 times as long as exopod, both rami gently tapering towards rounded apex, with apical tuft of long setae.

 

Oostegites broadly oval, with radiating lines and dense lateral cover of tiny microsetules in short rows (both structures visible under dissecting microscope with light manipulation).

 

Males slightly larger than females. Largest male, 2.4 mm.

 

Etymology. -This species is named after the Danish polar ship Nella Dan, which for many years faithfully moved personnel and supplies to and from Australia and Macquarie Island and bases in Antarctica. The Nella Dan foundered on the rocky shores of Macquarie Island on 24 December 1987.

 

Remarks. -Our specimens differ from Kussakin's (1967) description only in that he said his single specimen of "asellote 2" was blind and had only two free pleonites in front of the pleotelson; all of ours have eyes and three free pleonites. Although we did not examine Kussakin's specimen, it seems improbable that he had another species, given that this is the only species discovered in shallow water fauna during the thorough surveys of the 1977-1978 Macquarie Island Expedition.

 

The original material of Vermectias caudiculata consisted of one male, two ovigerous females, and two specimens of which no details were given. Sivertsen and Holthuis (1980) stated that the holotype, the only specimen from station 116, is a male. The "holotype" from station 116 examined by us is a young female. We have also examined the only other specimen available, a partly dissected paratype from station 94; its sex is unknown, since it lacks a urosome.

 

There are subtle differences between V. caudiculata and the material from Macquarie Island: the cephalon in V. caudiculata appears slightly narrower; the flagellum of antenna 2 is somewhat more robust and with 1 or 2 fewer articles than in similar-sized specimens of the new species; antero-lateral and middorsal setae on pereionites are distinctly longer in V. caudiculata.

 

The significance of these differences can-not be adequately explored from the material studied. Until more specimens from Tristan da Cunha become available, we pre-fer to treat the two populations as separate species.

 

Several details of Sivertsen and Holthuis's (1980) figures are incorrect. These include the mandibular palp, maxillae 1 and 2, and maxilliped, which are, in reality, very similar to V. nelladanae. We cannot interpret the supposed male pleopod 2 illustrated by Sivertsen and Holthuis (1980: fig. 40G).

 

Comparative Morphology

 

Pleon.- Harrison (1987), in remarks following his diagnosis of the suborder Asellota, stated that "no known asellote has more than three free pleonal segments (including the pleotelson) visible in dorsal view." The number and configuration of free or partly free pleonites is currently considered of doubtful taxonomic value in the higher classification of Asellota (brief summary of opinions in Wilson, 1987). In that paper Wilson cautioned that the interpretation, and thus the taxonomic application, of this character appears to be more complex than hitherto considered, and that detailed description, especially of the ventral side, is often lacking.

 

Vermectias has three completely free pleonites in front of the pleotelson, and thus shows the most plesiomorphic state for this character yet reported in asellotes. We conclude that the ancestral asellote had at least three free pleonites in front of the pleotelson. However, as reduction in the number of free pleonites may have occurred independently in many asellote lineages, this character alone permits more than one interpretation of the systematic position of Vermectias.

 

Pereiopod Protopod. -The basic eumalacostracan thoracopodal protopod consists of a coxa proximally articulating with the body wall and a basis articulating with the coxa (Hessler, 1982). In peracarids, the coxae of at least some pereiopods carry one or two medial branches: oostegites in brooding females, and, in Amphipoda, respiratory sacs (coxal gills).

 

We do not suggest that the protopod in Vermectias is fundamentally different, but in all life stages of V. nelladanae pereiopods 2-7 have a small, separate articulating sclerite between the body wall and the coxa (Figs. 6, 7, ps). The sclerite is free anteriorly and medially but appears to be continuous with the coxa laterally and posteriorly. The anterior and anterolateral parts are sclero-tized, while the medial portion seems to be more flexible.

 

We employ the neutral term proximal sclerite for this structure to avoid confusion with other terms, such as "precoxa," applied to the most proximal part of some arthropodan protopods. Its true nature (part of the coxa, or secondarily sclerotized arthrodial membrane, to mention the least controversial explanations) is uncertain. Homologous proximal sclerites carrying long penes are present on pereiopod 7 of Asellus sp. (Fig. 6E),P seudojanirain vestigatoris(F ig. 6F), and Australian Stenetrium spp. (per-sonal observation). Only Hoplocarida have 3-articulate thoracic protopods (Burnett and Hessler, 1973: 389, table III), but the homology of their articles with those of asellotes is uncertain. We draw attention to this sclerite because the penes and oostegites arise from it in Vermectias. The point of movement between limb and body wall is the coxa-prox-imal sclerite joint, and the function of the sclerite seems to be to provide a stable base for the oostegites and penes during walking. If this interpretation is correct, it is very different from the solution to the problem of protecting the marsupium against excessive movement hypothesized by Hessler (1982: 295) for many peracarids. That solution is to immobilize the coxa-body wall joint. According to Hessler, this trend is particularly pronounced in taxa with primarily promotion/remotion limb movement. Complete fusion between coxa and body wall (common in the Isopoda) represents the ultimate apomorphic possibility from which the Vermectias condition is un-likely to have been derived. The two solutions may have evolved independently from a simple coxa articulating with the body wall. Alternatively, the presence of a proximal sclerite may represent a primitive condition within the Isopoda. We cannot resolve these alternatives on the basis of the available evidence.

 

Female Oostegites. -Oostegites in Vermectias arise from the posteromedial part of the proximal sclerite (see above). They develop gradually from a small longitudinal medial ridge in the manca. In young females without spermathecae and stylet receptacles, the developing oostegite extends along the entire posterior surface of the proximal sclerite, but with moderate or little medial expansion. In the spermathecae-bearing (preparatory) female, the oostegites are approximately as long as they are wide at the base with the interior filled with densely folded matter. During the subsequent molt to breeding female, fully developed oostegites appear.

 

We have observed gradual development of oostegites in a wide range of janiroidean families, in Asellus, Stenetrium, and Pseudojanira, and Waigele (1989: 9) reported it in most nonasellote marine Isopoda and in most other Peracarida. It seems reasonable to assume that gradual development of oostegites is plesiomorphic, while appearance of fully developed oostegites in one molt, as in the janiroidean Munnidae and Paramunnidae (see Wolff, 1962), is apomorphic.

 

Male Penes and the Loops of the Vasa Deferentia.- The penes in Vermectias are long powerful organs that arise as cuticular outgrowths from the proximal sclerite immediately medial to the functional coxa of pereiopod 7. They point mediad but their tips are curved and fit into the ventral proximal groove between the rami of pleopods 1.

 

Janiroid asellotes have medial short penile papillae (often fused at the bases) or vas deferens openings, either free or covered, occasionally secondarily displaced to pleonite 1. All other asellotes have long penes placed distinctly away from the midline. In some Protojaniridae, e.g., Enckella, the distance between the penes is small (Wilson, 1987, fig. 2A). Wilson (1987: 263) stated that "the form of the penile papillae correlates with the presence of the sperm tube [of the fused pleopods 1] ..." For that reason the form and position of the penes were not used as separate characters in his analysis of asellote phylogeny. While we agree with Wilson's statement, it is useful to at-tempt to polarize the position of the penes, because taxa with similar pleopods 1 exhibit a range of positions of the penes.

 

Within the Asellota lateral penes have been reported only in Angeliera (Janiroidea, Microparasellidae; e.g., Chappuis and Delamare Deboutteville, 1960: fig. 136b; Coineau and Rao, 1972: 70; Stock, 1985: fig. 4) and possibly Metastenasellus powelli (Aselloidea, Stenasellidae; Magniez, 1979: 269 "The male genital papillae are borne on the medial angle of the coxopodial areas of the last pereonal sternite."). Of the other isopod suborders only Phreatoicidea have lateral penes arising from the base (in casu, the coxa) of pereiopod 7. In other peracarid orders, openings of lateral penes or vasa deferentia are present in Amphipoda and Mysidacea (personal observations), Mictacea (Bowman et al., 1985), Spelaeogriphacea and Thermosbaenacea (Pires, 1987). Openings of lateral penes or vasa deferentia at the base of thoracopod 8 or on its coxa are found in most other Malacostraca, including Leptostraca, Stomatopoda, and Decapoda (Kaestner, 1980). The evidence strongly suggests that a lateral opening of the vas deferens on the proximal sclerite or coxa, with or without penial extension, is plesiomorphic in Isopoda, and that Vermectias represents the most plesiomorphic condition so far reported in Asellota.

 

Medial migration of the penes (in concert with increasing specialization of pleopods 1 and 2) is more easily explained if seen as being the result of a medial extension of the proximal sclerite along and partly under the posterior margin of the sternum. This would allow the functional coxa to be unaffected by the changing position of the penis.

 

Medial migration of the penes has occurred independently in more than one asellote line, and in advanced forms with fully medial penes no external evidence remains of a proximal sclerite link with the limb base. The hypothesis implies the presence of a penis-bearing proximal sclerite in the ancestral Asellota, but it does not explain its true nature. Nor does it indicate whether such a structure was present in the immediate ancestor to the Asellota.

 

The concept of a gradual evolution from lateral to medial penes is corroborated by the fate of the vasa deferentia. The vasa deferentia are dorsal for most of their length. In pereionite 7 or posteriorly in pereionite 6, they dip ventrad and, in isopods with fully lateral penes, curve laterad. In isopods with median penes or pores, the vasa deferentia still dip ventrad and curve laterad to-wards the bases ofpereiopods 7, from which points they loop mediad ventral to the ventral longitudinal trunk muscles (VLTM). The lateral loops of the vasa deferentia are present in tanaidaceans and all isopods with median openings (e.g., Aega sp.; Serolis bromleyana; Asellus sp., Fig. 6E; Pseudojanira investigatoris, Fig. 6F; many janiroid families). They are not present in Phreatoicidea. The loops result from the vasa deferentia being pulled across the VLTM by the me-dially moving penes. They are least pronounced in species where pleonite 1 is very narrow relative to pereionite 7 and where the pereion and pleon articulation is poor. In such species the VLTM of pereionite 7 are weak and close to the midline of the body.

 

Male Pleopod 1. -We interpret the pleopod 1 complex in Vermectias as having very short, completely fused peduncles and separate rami fully fused to the peduncle. The basis for this interpretation is the similarity of the complex to that in the Pseudojaniridae (Poore and Just, 1990: fig. 3 PL1) and Stenetriidae( Wilson, 1987: fig. ID). In those taxa, the deep rounded groove allowing the penes access to pleopods 2 is in the middle of the distal margin of the fused peduncles between the rami and level with their articulation with the peduncle. We consider the position of the groove to be homologous in Vermectias, thus delimiting the extent of the peduncle and rami. A similar condition is found in Gnathostenetroididae and Protojaniridae. In Protojaniridae, as in Vermectias, the rami are completely fused with the peduncle. Middorsal ramal locking tabs and dorsal distolateral stylet guide grooves as seen in Pseudojaniridae and Janiroidea are not present in Vermectias. Instead, Vermectias has a small dorsolateral conical pro-jection on each ramus. In other Asellota the dorsal surface of the ramus is smooth.

 

Pleopod 1 in Vermectias is plesiomorphic compared with Janiroidea, but apomorphic compared with most Aselloidea (peduncles separate with medial coupling setae, rami free with densely setose margins). Some aselloids, however, have proximally fused peduncles [e.g., Psammasellus capitatus Braga, 1968, Synasellus mariae (Braga) (Birstein, 1951: fig. 242), S. barcelensis Noodt and Galhano, 1969].

 

Male Pleopod 2.-The male pleopod 2 of Vermectias is typically asellote. Primitively in isopods, the rami of pleopod 2 insert side by side on the transverse distal margin of the peduncle. This is the condition found in most nonasellote isopods. In aselloid Asellota, the rami are inserted on a more or less reduced distal margin, as is at least the exopod in stenetriids and gnathostenetroidids. All other asellotes, including Vermectias, lack a distinctive distal margin on the peduncle. The rami are inserted more or less serially along the expanded and axial medial margin. In Vermectias, however, the peduncle is unusually short (length: width approximately 1:1) with the medial margin at 45◦ to the body axis.

 

Aselloidea, Protojaniridae, and Gnathostenetroididae have a biarticulate exopod; in Vermectias the exopod is indistinctly biarticulate as in Stenetrium and Pseudojanira; Janiroidea have a uniarticulate exopod (see Wilson, 1987: 265, for comments on the uncertain polarization of this character). In Aselloidea the exopod is flattened and normally marginally setose as in most nonasellote Isopoda, while in all other asellotes it is more or less cylindrical, curved, and with-out multiple long setae.

 

The short, conical endopod article 2 (sty-let) in Vermectias, with its simple, shallow lateral groove, is similar only to that in Pseudojanira (see Wilson, 1986a; Poore and Just, 1990). In Vermectias the groove is expanded by a ventrolateral flange. The proximal entrance to the groove is via a shallow ventral depression where articles 1 and 2 join (Fig. 5D, E). A similar entrance has been reported in Metastenasellus powelli by Magniez (1979: fig. 18a) and Pseudojanira by Poore and Just (1990: fig. 3). We suggest that this is the plesiomorphic condition in Asellota and is functionally linked to the primitive lateral, medially pointing penes.

 

Evolution of the stylet of the male pleopod 2 in asellotes appears to have progressed along different lines towards greater complexity. The homologous appendix masculina in nonasellote isopods is a simple rod, sometimes grooved and sometimes with the tip ornamented. It therefore seems un-likely that the stylet in Vermectias could have been derived from any of the more elaborate asellote forms.

 

Because the developing male article 2 of the endopod of pleopod 2 in juvenile Desmosoma and Eurycope (Janiroidea) is bluntly sausage-shaped, Wilson (1987: 266) suggested that article 2 of the primitive asellote endopod was bluntly club-shaped, as in Asellidae and Stenetriidae, and that the elongate, pointed stylet of "higher" asellotes is derived. We are not convinced that this is the plesiomorphic condition, since most appendages are blunt and poorly articulated when they first appear. Club-shaped stylets in asellotes are highly complex structures from which it is difficult to imagine the derivation of a simple laterally grooved stylet. Elongate stylets in Stenasellidae, Protojaniridae, and Janiroidea are similarly complex and diverse. It is more parsimonious to consider different lines of complex asellote stylet morphology to have been derived from a simple, pointed stylet with a shallow lateral groove. Following this hypothesis, the ancestral asellote would have had a sty-let very similar to Vermectias, but probably without its pronounced lateral flange. In deed, the flange in Vermectias suggests how, by further elongation and rotation around the axis, a complex stylet, such as the strongly convoluted one in Stenasellidae, could have evolved.

 

Wilson (1981: 291 and fig. 10) described the development of male pleopods 1 and 2 in the janiroid species Eurycope iphthima Wilson, 1981. Pleopod 1 first appears as a small bifurcate appendage. At that stage pleopod 2 is "an undifferentiated" opercular plate similar to that in mancas and females. A small "opercular" plate is also the precursor for male pleopods 2 in Stenetriidae and Pseudojaniridae (personal observations).

 

In male mancas of Vermectias, pleopods 2 first appear as two separate lobes (Fig. 3F) at a stage where there is no trace of developing pleopods 1. In the example shown, each budding pleopod has bilobed soft tissue inside. We consider this ontogenesis of pleopods 2 more primitive than that found in the higher Asellota, because free pleopods 2 are found in all nonasellote isopods. How-ever, ignorance of the development in several asellote super families prevents us from analyzing the implications of this characteristic of Vermectias any further.

 

Sperm Transfer. -We believe that transfer of sperm from the penes to pleopods 2 and onto the female spermathecae is achieved in the same way as that described (Poore and Just, 1990) for Pseudojanira investigatoris: a downward/forward movement of pleopods 1 and 2 in concert to bring the curved tips of the penes in contact with the entrance to the sperm groove on the endopods of pleopod 2; ejected sperm flows into the lateral sperm groove on article 2; the barbed tip of the stylet is positioned in the stylet receptacle of the female; a stroke of the exopod in the groove pushes the sperm mass towards the opening of the female cuticular organ.

 

Ventrolateral Groove. -All specimens, mancas, adult males, and best developed females have a longitudinal ventrolateral groove on pereionites 2-5; less distinct grooves are often present on 7. In pereionites 2-4, the grooves occupy approximately the middle third of the segment; in pereionite 5, approximately the anterior two-thirds. The groove in pereionite 5 is expanded anteriorly into a broad funnel. We suggest that the groove, which is thin-walled, represents the pleural space between the tergite and the sternite. Similar longitudinal grooves separating the calcified tergite and sternite are present in many Ischnomesidae (Janiroidea) and in many Tanaidacea (personal observations).

 

In females of Vermectias, the oopore, stylet receptacle, and opening of the cuticular organ are associated with the groove.

 

In many isopods, tergites and sternites are no longer recognizable as separate entities, and the sternum is often covered by ventral coxal plates meeting in the midline. How-ever, in asellotes and nonasellotes examined by us (e.g., Pseudojanira, Stenetrium, Notasellus, Eurycope, Serolis, Aega), the oopore is associated with a distinct ventrolateral cuticular line running posteriad (and often obliquely mediad) from a furrow on the ventroanterior margin of pereionite 5. In brooding females, the oopore opens in this cuticular line, and at least in Pseudojanira the stylet receptacle and opening of the cuticular organ are situated immediately anterior to the oopore in the common fur-row (Poore and Just, 1990: fig. 4C). We suggest that the cuticular line and its anteriorly directed furrow are homologous with the longitudinal pleural groove in Vermectias. We further suggest that the pleural groove could be the primitive site for the oopore and for the opening of the cuticular organ in Asellota. (We hesitate to extend this suggestion regarding the oopore to all Isopoda since Dr. G. Wilson has informed us, in correspondence that Phreatoicidea may differ.) If this were so, the situation in Vermectias is the most primitive yet reported in the Asellota. We have not studied Protojaniridae and nothing is known about the position of cuticular organs in that family (Wilson, 1986b). The presence, in that family, of a lateral groove similar to the one in Vermectias is , however, indicated for Enckella lucei major: "In all pereionites of Enckella the sternal side is somehow less sclerotized than the dorsal side, leaving even a narrow longitudinal strip of a pleura-like, largely unsclerotized, cuticula in the mid-line" (Sket, 1985: 201).

 

Oopore.-The oviduct opens (Fig. 9E, op) at the bottom of a cuticular fold set obliquely across the middle of the ventrolateral groove. As far as we can ascertain, the fold is derived from the tergal margin of the groove. The fold is present in young females before the development of the cuticular organ and spermatheca.

 

Stylet Receptacle. -The stylet receptacle appears to be an elaboration of the sternal margin of the groove opposite the oopore fold. It is a sclerotized, posteriorly pointing, cone-shaped pocket, the posterior part of which is hidden under the rim of the oopore fold and the sternal rim of the groove.

 

The stylet receptacle is present in young females before development of a cuticular organ and spermatheca. In the final molt from preparatory to brooding female the stylet receptacle is cast off.

 

The only other asellotes known to have a distinct, sclerotized stylet receptacle adjacent to the prospective oopore are species of Pseudojanira (see Wilson, 1986a; Poore and Just, 1990: fig. 4C).

 

Spermatheca and Cuticular Organ.-The cuticle-lined spermatheca is spherical and attached to a short, broad, conical cuticular organ. Its opening is in the floor of the ventrolateral groove immediately in front of the oopore and stylet receptacle. In the only preparatory female seen, both spermathecae hang outside the body wall, attached to it by a cylinder of thin tissue at a point which we believe is the opening of the cuticular organ. We could not observe the point of attachment of the spermatheca to the oviduct/ovary, but we suppose it opens into the proximal base of the oviduct through a thin-walled area in the spermatheca, as in Pseudojanira (see Poore and Just, 1990: fig. 4B). In Vermectias, this opening appears to be directly opposite the cuticular organ. The tissue connecting the spermatheca to the body wall could be part of the ovary/oviduct wall.

 

In ovigerous females the spermathecae and cuticular organs are absent (cast off in the previous molt), but there is a small pit with thickened cuticle in place of the cuticular organ. A similar pit is present in the groove of pereionites 5-7 in mancas, young females, and males. The pit of pereionite 5 in females is invariably a little larger than those of pereionites 6 and 7. In larger males the pits of pereionite 7 were not always found.

 

We believe the pattern of cuticular pits and the sequence of transformations from juvenile to brooding female (pit to cuticular organ and spermatheca to pit) indicate that these two structures evolved from a simple cuticular invagination separate from the oopore, and based on a structure (the pit) present in both sexes and not confined to pereionite 5. This feature is a unique asellotan attribute.

 

Conclusions on Female Structures.-Wilson (1986b) analyzed the evolutionary significance of the various forms of spermathecae and cuticular organs and the diverse positions of the opening of the cuticular organ in Asellota. His conclusions led him to ask: "How did the female reproductive sys-tem with a single orifice [see his fig. 6] evolve into a two-orifice system, separating the functions of insemination and egg release?" The basis for this question is the configuration found by Wilson (1986b) in Asellus and Stenetrium (both considered more primitive than the Janiroidea) and in the janiroid genera Munna and Santia. In these groups the opening of the cuticular organ is attached to the rim of the oopore field. Wilson considered this the primitive "single orifice" system from which he proposed a simple stepwise migration of the opening of the cuticular organ to an anterior and more dorsal position.

 

Evidence from Vermectias suggests the reverse, that separate but closely set openings for the cuticular organ and the oviduct in the lateral groove, the former in front of the latter, represent the primitive asellotan condition.

 

Two independent lines of evolution from this arrangement can be seen in the Asellota.

 

In one, separation of the pores in the groove led to an anterior position for the opening of the cuticular organ. This hypothesis offers an explanation of even the extreme anterodorsal position of the opening of the cuticular organ in the Ischnomesidae (personal observations). In some ischnomesid species, the anterior part of the lateral groove on pereionite 5 curves dorsad and widens under the posterior edge of pereionite 4 to enclose the opening of the cuticular organ. In more advanced species, without a lateral groove, the opening of the cuticular organ is in a similar position, but the tergal margin of the groove is reduced to a cuticular ridge.

 

Lincoln and Boxshall (1983) showed that Dendromunna (Dendrotiidae) has the openings of the cuticular organs on the dorsal surface. We have not studied that genus, but we suspect that the position of the openings evolved in a similar way.

 

In the second line, the distance between the two openings is reduced until the opening of the cuticular organ becomes attached to the cuticular edge of the oopore (e.g., Wilson, 1986b: figs. 2-4: Asellus, Stenetrium, Munna). In taxa with a cuticular spermatheca outside the oviduct (Asellus, Stenetrium), the cuticular organ is external to the oviduct. In Munna and Santia (see Wilson, 1986b: 300), where a spermatheca without a cuticle lining is enclosed in the oviduct, we believe the cuticular organ is also external to the oviduct until it breaks through its wall and enters the spermatheca (as in Jaera, Veuille, 1978: fig. 2B, and Notasellus, Wilson, 1986b: fig. 5C, both of which have well-separated oopores and openings of the cuticular organ).

 

Summary of Primitive Attributes. -On the basis of our analyses of characters in Vermectias, we consider the following to be plesiomorphic attributes of Asellota. None is likely to have been derived from known asellote structures and each could be ancestral to the states seen in other living asellotes: (1) Pleon with three free pleonites in front of the pleotelson. (2) Penes arising from the base of the protopodite of pereiopods 7. (3) Stylet of male pleopods 2 pointed, with a shallow lateral groove; entrance to the sperm groove situated ventrally at joint between articles 1 and 2 of the endopod. (4) Oopore and opening of the cuticular organ separate in a longitudinal ventrolateral pleural groove. (5) Cuticular pits present in the pleural groove of pereionites 5-7 in both sexes; the pits in pereionite 5 in females giving rise, through cuticular invagination, to cuticular organs and spermatheca.

 

Viewing the cuticular pits in Vermectias as autapomorphic would imply evolution of an internal spermatheca with a cuticular organ independent from all other Asellota. This seems improbable, since cuticular organs are not found in other isopod suborders. Their presence is likely to be autapomorphic for Asellota.

 

The Case of Angeliera

 

The elongate subcylindrical body of Vermectias led us to a brief survey of asellotes with a similar body form. Aselloidea, Gnathostenetroididae, Protojaniridae, and Janiroidea all include species of this shape, but, in most, it is considered a convergent adaptation to a similar interstitial life-style.

 

One genus, Angeliera Chappuis and Delamare Deboutteville, 1954, presently in the janiroid family Microparasellidae, attracted our attention, because it shares plesiomorphies with Vermectias: elongate penes arising near the base of pereiopods 7, and similar male pleopods 1 and 2 (A. phreaticola, personal observation). Besides, Angeliera has pereiopods similar to Vermectias, and females have strongly reduced pleopods 2. On the other hand, mouthparts in Angeliera (mandibles, maxillae 2, and maxillipeds in particular) are more specialized than those in Vermectias, and Angeliera has only one free pleonite in front of the pleotelson. Our material of Angeliera has not enabled examination of the oopore and opening of the cuticular organ.

 

Angeliera clearly does not belong in the Janiroidea, but, because of lack of information on female reproductive structures, and because its similarities with Vermectias are either plesiomorphic (penes, male pleopods), of undetermined polarity (e.g., pereiopods) or possibly convergent (general body shape) we cannot place it in the Vermectiadidae.

 

Other microparasellid genera (see Coineau, 1986) have medial penes, elongate male pleopods 1 (in some illustrations of pleopods 1 the rami appear to form an open gutter rather than being closely appressed in the ventral midline) and one free pleonite in front of the pleotelson. However, Micro-parasellidae have not been described or illustrated well enough to allow comparison with Vermectias and Angeliera.

 

Classification of Vermectiadidae

 

The morphology of the male pleopods 1 and 2 does not permit Vermectias to be placed in the Janiroidea. If these and other characters in Vermectias are plesiomorphic, as we hypothesize, the phylogeny of Asellota needs to be reappraised. In recent cladistic analyses ofasellotan relationships, five (Wilson, 1987) or six (Wiigele, 1989) super families were recognized. Both authors placed Aselloidea as the sister group of all other asellotes and Pseudojaniroidea as the sister group of the Janiroidea. The two phylogenies differ in the position, relative to these two, of Gnathostenetroidoidea, Pro-tojaniroidea (as Protojaniridae in Gnathostenetroidoidea in Wilson) and Stenetrioidea.

 

The Aselloidea as currently understood (Stenasellidae, Asellidae, Microcerberidae, and Atlantasellidae) is a heterogenous assemblage for which male and female reproductive structures are diverse and often poorly documented. Likewise, detailed descriptions of many characters described here for Vermectias, and critical to the higher classification of Asellota, are needed for gnathostenetroidids, protojanirids, and several supposed janiroid taxa. For these reasons, classification of Vermectiadidae within the currently used superfamilies would, in our opinion, be premature.”

 

(Just & Poore, 1992: 126-143)

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© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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