Shenius anomalus (Shen, 1935)

Ng, Peter K. L., Clark, Paul F. & Cuesta, Jose A., 2010, Establishment of a new subfamily for Shenius anomalus (Shen, 1935) (Crustacea: Decapoda: Brachyura: Dotillidae), Journal of Natural History 44 (25 - 26), pp. 1531-1553 : 1541-1552

Shenius anomalus zoea I

( Figures 7–10 )

Material examined

Shenius anomalus : 1 female ( ZRC 1988.792 ), Mandai , Singapore, coll. P. K. L. Ng, 21 February 1987, Z1 hatched 23 February 1987; Baruna trigranulum ( Dai and Song, 1986) : 1 female ( ZRC 1988.791 ), Pandan mangroves, Singapore, coll. S. Harminto, 3 October 1986, Z1 hatched 7 October 1986; Dotilla myctiroides (H. Milne Edwards,

1852): 1 female ( ZRC 1988.789 ), Punggol Point, Singapore, coll. S. Harminto, 31 October 1986 , Z1 hatched 1 November 1986.

Description

Carapace ( Figure 7A,B ). Dorsal spine present; rostral spine present and much longer than antennal protopod and with distal spinulation; lateral spines absent; anterodorsal setae absent; one pair of posterodorsal setae; ventral margin without setae; eyes sessile.

Antennule ( Figure 7C ). Uniramous, endopod absent; exopod unsegmented with three (one broad, two slender) terminal aesthetascs of unequal length and one terminal seta.

Antenna ( Figure 7D ). Long protopod with spinulation, shorter than rostral spine; endopod absent; exopod minute with one terminal seta.

Mandible. Palp absent.

Maxillule ( Figure 8A ). Epipod seta absent; coxal endite with four setae; basal endite with five setal processes and two small setal buds; endopod two-segmented, proximal segment without setae; distal segment with four terminal (subterminal setae absent); exopod seta absent.

Maxilla ( Figure 8B ). Coxal endite not bilobed with six setae; basal endite bilobed with 5 + 4 setae; endopod bilobed, with 2 + 3 terminal (subterminal setae absent) setae; exopod (scaphognathite) margin with four setae and one long distal stout process.

First maxilliped ( Figure 9A ). Coxa with one seta; basis with 10 setae arranged 2,2,3,3; endopod five-segmented with 2,2,1,2,5 (one subterminal, four terminal) setae, respectively; exopod two-segmented, distal segment with four long terminal plumose natatory setae.

Second maxilliped ( Figure 9B ). Coxa without setae; basis with three setae arranged 1,1,1; endopod three-segmented, with 1,1,6 (three subterminal, three terminal) setae, respectively; exopod two-segmented, distal segment with four long terminal plumose natatory setae.

Third maxilliped. Absent.

Pereiopods. Absent.

Abdomen ( Figure 10A,B ). Five somites; somite 2 with one pair of dorsolateral processes directed anteriorly; somite 3 with one pair of dorsolateral processes directed ventrally; somites 1 and 2 each with rounded posterolateral processes and 3–5 each with short posterolateral spinous processes; somite 1 without setae; somites 2–5 each with one pair of posterodorsal setae and small spinules on posterior margin of somites 3–5; pleopod buds absent.

Telson ( Figure 10A–C ). Each fork long, gradually curved distally, not spinulate with one lateral spine and two smaller dorsomedial spines; posterior margin with three pairs of stout spinulate setae.

Remarks

All the dorsal spines of the first-stage zoeae examined for the present study were damaged because the original material is relatively old and not sufficiently well preserved. However, the authors had access to an old figure by Sundowo Harminto which illustrated the first-stage zoea of S. anomalus . Although his larval illustration lacks many details, it does clearly figure the carapace spines. By using the scale bar provided by Harminto, the rostral spine measured ca. 0.52 mm and the dorsal spine measured ca. 0.25 mm. Therefore, it appears that the rostral spine length is approximately twice that of the dorsal.

Brachyuran first-stage zoeae of congeneric species appear to have identical setotaxy ( Christiansen 1973; Clark 1983, 1984; Ng and Clark 2000). This similarity provides a degree of predictability within a taxon. Conversely, setal differences (incongruence) within a group suggest incorrect assignment of taxa and lack of systematic compatibility. Consequently the first-stage zoeal morphology described in the present study may provide additional information regarding the classification of S. anomalus .

Shenius anomalus was previously assigned to Macrophthalmidae Dana, 1851 . However, when comparing its zoea I morphology with that of a typical macrophthalmid such as Macrophthalmus (Mareotis) depressus Rüppell, 1830 , as described by Rice (1975), several important differences can be observed: macrophthalmids possess a “spine” or “tooth” ( Figure 11A ) on the ventral margin of the carapace (vs absent in S. anomalus , Figure 7A ); the antennal exopod to propodus percentage ( Figure 11B ) is ca. 39% (vs ca. 1% in S. anomalus , Figure 7D ); the antennal exopod is a spine (vs exopod terminates as seta in S. anomalus , Figure 7D ); the maxillule endopod has a setal formula of 1,5 (one subterminal, four terminal) setae (vs 0,4 terminal setae in S. anomalus , Figure 8A ); the bilobed endopod of the maxilla has 2 + 2 setae (vs 2 + 3 setae in S. anomalus , Figure 8B ); nine setae on the basis of the first maxilliped arranged 2,2,3,2 (vs 10 arranged 2,2,3,3 in S. anomalus , Figure 9A ); four setae on the basis of the second maxilliped arranged 1,1,1,1 (vs three arranged 1,1,1,0 in S. anomalus , Figure 8B ); a telson without lateral spines (vs one present in S. anomalus , Figure 10C ) and the shape of the telson ( Figure 11C ) is straight (vs medially with a waist in S. anomalus , Figure 10A,C ). Moreover, these morphological incongruences between the first-stage zoea of M. (M.) depressus and S. anomalus indicate that assignment of the latter species to the Macrophthalmidae is not well supported (see Table 1).

Shenius anomalus has also been assigned to the Camptandriidae . This family can be represented by the first-stage zoeae of Baruna trigranulum ( Dai and Song, 1986) , figured for the present study from the unpublished material of the present authors as part of an eventual larger study of the Ocypodoidea. There are a number of incongruent characters when the first-stage zooeal morphology of the two species is compared: for example in B. trigranulum the dorsal carapace spine ( Figure 11D ) is absent (vs present in S. anomalus , Figure 7A,B ); a plumose seta is present of the ventral ( Figure 11D ) carapace margin (vs absent in S. anomalus , Figure 7A ); the antennal exopod to propodus percentage ( Figure 11B ) is ca. 42% (vs ca. 1% in S. anomalus , Figure 7D ), antennal exopod is distally spinulate (vs not spinulate in S. anomalus ); antennal exopod with one subterminal seta (vs one terminal seta in S. anomalus ); abdominal somite 4 ( Figure 11F ) with one pair of lateral spines (vs absent in S. anomalus , Figure 9A,B ); fifth abdominal somite is distinctly wider than other somites and telson (vs somite 5 is not distinctly wider in S. anomalus ); in the furcal arms are “short” (vs much longer in S. anomalus ) and the shape of the telson ( Figure 11F ) is straight (vs medially wasted in S. anomalus , Figure 10A,C ). Although B. trigranulum and S. anomalus share the same setal formula for the maxillule endopod, the bilobed endopod of the maxilla, and the basal setation of the first and second maxillipeds; the larval differences cited here, especially the morphology of the antenna and abdomen, suggest that S. anomalus is not a camptandriid ( Table 1).

Significantly, the general morphology of S. anomalus first-stage zoeae resemble those of the known larval stages of Dotillidae species including the setal formula of the maxillule and maxilla endopods, and the basal setation of the first and second maxillipeds. With respect to the zoeae of Dotilla Stimpson, 1858 , the morphology of two species is known, namely that Dotilla blanfordi Alcock, 1900 , by Rajabai, 1958 (which maybe considered a little dated) and unpublished data on Dotilla myctiroides (H. Milne Edwards, 1852) ( Figure 12A–F ). The antennal morphology of both these zoeae is more camptandriid-like in that the exopod is well developed (ca. 90% of protopod) with a subterminal seta; however, the exopod is not distally spinulate. With regard to the morphology of the abdomen, both first-stage zoeae are similar to S. anomalus in that somite 5 is not broad and the telson is medially wasted. Furthermore, the first-stage zoeal descriptions are available for other Dotillidae genera, Ilyoplax pingi Shen, 1932 , by Jang et al. (1991), Ilyoplax tansuensis Sakai, 1935 , by Ko and Kim (1991) and Scopimera crabricauda Alcock, 1900 , by Rice (1976). The rostral spine is relatively long in all four species including S. anomalus compared with the other first-stage zoeae examined for the present study. Moreover, the considered rostral spine/dorsal spine ratio of 2: 1 for S. anomalus compares well with that of I. pingi (see Figure 13A ). Carapace lateral spines are present in I. pingi and Scopimera crabricauda ( Figure 13A,G , respectively) but absent for I. tansuensis and S. anomalus ( Figures 13D , 7A,B , respectively). The antennal protopod morphology is similar for all four species including S. anomalus in being relatively long and spinulate for the distal three-quarters. While absent in Scopimera crabricauda ( Figure 13H ) the exopod is minute in the other three species, with one terminal seta in I. tansuensis and S. anomalus ( Figures 13E , 7D ) and two in I. pingi ( Figure 13B ) The telson appears to be elongated and wasted in all zoeae illustrated ( Figures 13C,F,I , 10A,B ). For the present study the first-stage zoeae of I. tansuensis and S. anomalus are remarkably similar, but can be distinguished from those of I. pingi and Scopimera crabricauda . The zoeae of the latter two species can be on the presence and absence of an antennal exopod respectively and the former processing two terminal exopod setae ( Table 1).

In summary, the first-stage zoea of S. anomalus appears to have more characters in common with the dotillids I. tansuensis , I. pingi and Scopimera crabricauda than those of D. blanfordi and D. myctiroides , but there is enough support from zoeal morphology to suggest that S. anomalus be classified in the Dotillidae with perhaps assignment to its own subfamily emphasizing the incongruences with the other dotillid genera.

Conclusion

While the male abdomen, G1 and first-stage zoeal characters of Shenius places this genus within the Dotillidae as currently defined ( Ng, Guinot and Davie 2008), the carapace and ambulatory leg characters of Shenius are so atypical that it cannot be suitably assigned with any of the known dotillid genera. Therefore a new subfamily, Sheniinae , is established to accommodate Shenius . Unlike dotillines, which have a more ovate and swollen carapace with the ambulatory legs positioned more vertically, sheniines have a flattened and more quadrate carapace, with the ambulatory legs positioned laterally.

Figures

Figure 7. Shenius anomalus (Shen, 1935), zoea I. (A) Lateral view of carapace; b. anterior view of carapace; c. antennule; d. antenna.

Figure 8. Shenius anomalus (Shen, 1935), zoea I. (A) Maxillule; (B) maxilla.

Figure 9. Shenius anomalus (Shen, 1935), zoea I. (A) First maxilliped; (B) second maxilliped.

Figure 10. Shenius anomalus (Shen, 1935), zoea I. (A) Dorsal view of abdomen; (B) lateral view of abdomen; (C) telson.

Figure 11. Macrophthalmus (Mareotis) depressus Rüppell, 1830; zoea I modified from Rice (1975). (A) Lateral view of carapace; (B) antenna; (C) dorsal view of abdomen. Baruna trigranulum (Dai and Song, 1986); zoea I. (D) Lateral view of carapace; (E) antenna; (F) dorsal view of abdomen.

Figure 12. Dotilla blanfordi Alcock, 1900 (modified from Rajabai, 1959), zoea I. (A) Lateral view of carapace; (B) antenna; (C) dorsal view of abdomen. Dotilla myctiroides (H. Milne Edwards, 1852), zoea I. (D) Lateral view of carapace; (E) antenna; (F) dorsal view of abdomen.

Figure 13. Zoea I, lateral view of carapace, antenna and dorsal view of abdomen for (A–C) Ilyoplax pingi Shen, 1932; (D–F) Ilyoplax tansuensis Sakai, 1935; (G–I) Scopimera crabricauda Alcock, 1900 (after Jang et al. 1991; Ko and Kim 1991; Rice 1976, respectively).