Pilumnus hirtellus, (LINNAEUS, 1761)

Clark, Paul F., 2005, The evolutionary significance of heterochrony in the abbreviated zoeal development of pilumnine crabs (Crustacea: Brachyura: Xanthoidea), Zoological Journal of the Linnean Society 143 (3), pp. 417-446 : 418-430

publication ID	https://doi.org/ 10.1111/j.1096-3642.2005.00149.x
persistent identifier	https://treatment.plazi.org/id/0B568782-FFD7-8225-729C-E8CFB9E9F8B9
treatment provided by	Diego
scientific name	Pilumnus hirtellus
status

Treatment

PILUMNUS HIRTELLUS ( LINNAEUS, 1761) View in CoL

FIGURES 1–15 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 View Figure 10 View Figure 11 View Figure 12 View Figure 13 View Figure 14 View Figure 15

Pilumnus hyrtellus – Williamson, 1915: 490, figs 314– 316, 317 (ZI, Cr.).

Pilumnus sp. – Williamson, 1915: 492, figs 318, 319 (ZI, II).

Pilumnus hirtellus View in CoL – Boraschi, 1921: 9, Tav. 1, fig. 14 (ZI); Lebour, 1928: 532, fig. 1(18), pl. II, fig. 5, pl. X, figs 6–8, pl. XI, figs 1–4 (PZ, ZI-IV, Meg., Cr.I); Bourdillon-Casanova, 1960: 172, fig. 54 (PZ, ZI, Cr.I); Salman, 1982: 113, figs 1–7 (ZI-IV, Meg.); Ingle, 1983: 973, fig. 15a,b,f (Meg., Cr.I); Hong, 1988: 1019, fig. 12 (ZI-IV, Meg.); Ingle, 1991: 232–234, figs 1.13k, 1.18b, 1.23c, 1.25g, 1.29b, 1.32c, 1.34o, 1.36h, 1.38d, 2.30b–t (ZI-IV, Meg.); Ng & Clark, 2000: 228–231, figs 20–23 (Z1).

Zoea I

Carapace ( Fig. 1A View Figure 1 ): dorsal spine smooth, curved distally, approximately twice as long as rostral spine; rostral spine smooth, shorter than antennal protopod; lateral spines unarmed; 1 pair of posterodorsal setae; ventral margin without setae; eyes sessile.

Antennule ( Fig. 4A View Figure 4 ): uniramous, endopod absent; exopod unsegmented with 4 terminal aesthetascs (2 broad and long, 2 shorter and slender) plus 2 terminal setae of unequal length.

Antenna ( Fig. 5A View Figure 5 ): protopod distally spinulate, longer than rostral spine but slightly shorter than exopod; endopod absent; exopod longer than protopod, unsegmented, distally spinulate with 2 medial setae of unequal length.

Mandible: endopod palp absent.

Maxillule ( Fig. 6A View Figure 6 ): epipod seta absent, coxal endite with 7 setae; basial endite with 5 terminal setal processes and 2 small teeth; endopod 2-segmented, proximal segment with 1 seta; distal segment with 6 (2 subterminal, 4 terminal) setae; exopod seta absent.

Maxilla ( Fig. 7A View Figure 7 ): coxal endite bilobed with 5 + 4 setae; basial endite bilobed with 5 + 4 setae; endopod bilobed with 3 + 5 (2 subterminal, 3 terminal) setae; exopod (scaphognathite) margin with 4 setae and 1 long distal plumose stout process.

First maxilliped ( Fig. 8A View Figure 8 ): coxa without setae; basis with 10 setae arranged 2,2,3,3; endopod 5-segmented with 3,2,1,2,5 (1 subterminal, 4 terminal setae), respectively; exopod 2-segmented, distal segment with 4 long-terminal plumose natatory setae.

Second maxilliped ( Fig. 9A View Figure 9 ): coxa without setae; basis with 4 setae arranged 1,1,1,1; endopod 3-segmented, with 1,1,6 (3 subterminal, 3 terminal) setae,

respectively; exopod 2-segmented, distal segment with 4 long-terminal plumose natatory setae.

Third maxilliped: absent.

Pereiopods: absent.

Abdomen ( Figs 12A View Figure 12 , 14A View Figure 14 ): 5 somites; somite 2 with pair of dorsolateral processes directed anteriorly; somite 3 with pair of dorsolateral processes directed ventrally; somites 1–2 with rounded posterolateral processes and 3–5 with spinous posterolateral processes; somite 1 without setae; somites 2–5 with 1 pair of posterodorsal setae; posterior margin of somites 2– 5 spinulate; pleopod buds absent.

Telson ( Figs 12A View Figure 12 , 14A View Figure 14 , 15A View Figure 15 ): each telson fork long, spinulate and gradually curved distally; 1 long spinulate and 1 small lateral spine; dorsal medial spinulate spine present; posterior margin with 3 pairs of stout spinulate setae.

Zoea II

Carapace ( Figs 1B View Figure 1 , 3A View Figure 3 ): now 2 pairs of anterodorsal setae; ventral margin with 1 anterior seta plus 2 posterior seta; eyes stalked, otherwise unchanged.

Antennule ( Fig. 4B View Figure 4 ): now with 2 stout and 3 slender aesthetascs plus 2 setae, otherwise unchanged.

Antenna ( Fig. 5B View Figure 5 ): endopod bud present, c. 43% of protopod; otherwise unchanged.

Mandible: unchanged.

Maxillule ( Fig. 6B View Figure 6 ): epipod seta present; basial endite with 8 setal processes, teeth no longer apparent; exopod seta present; otherwise unchanged.

Maxilla ( Fig. 7B View Figure 7 ): basial endite now with 5 + 5 setae; exopod margin with 11 setae and stout process no longer prominent; otherwise unchanged.

First maxilliped ( Fig. 8B View Figure 8 ): exopod distal segment now with 6 long-terminal plumose natatory setae; otherwise unchanged.

Second maxilliped ( Fig. 9B View Figure 9 ): exopod distal segment now with 6 long-terminal plumose natatory setae; otherwise unchanged.

Third maxilliped ( Fig. 10B View Figure 10 ): present, rudimentary, biramous, endopod longer than exopod.

Pereiopods ( Fig. 11A View Figure 11 ): present, rudimentary and chela bilobed.

Abdomen ( Figs 12B View Figure 12 , 14B View Figure 14 ): somite 1 with 1 medial seta.

Telson ( Figs 12B View Figure 12 , 14B View Figure 14 , 15B View Figure 15 ): unchanged.

Zoea III

Carapace ( Figs 2A View Figure 2 , 3B View Figure 3 ): dorsal spine with 1 pair of setae; now with 5 pairs of anterodorsal setae; ventral margin with 1 anterior seta plus 4 posterior setae; otherwise unchanged.

Antennule ( Fig. 4C View Figure 4 ): biramous; endopod bud swollen and developing; exopod now with 1 subterminal seta plus 2 broad and long, 3 shorter and slender aesthetascs plus 3 setae, all terminal.

Antenna ( Fig. 5C View Figure 5 ): endopod more developed c. 58% of protopod; otherwise unchanged.

Mandible: unchanged. Maxillule ( Fig. 6C View Figure 6 ): basial endite now with 9 setal processes; otherwise unchanged.

Maxilla ( Fig. 7C View Figure 7 ): basial endites with 6 + 6 setae; exopod margin with 18 setae of equal length; otherwise unchanged.

First maxilliped ( Fig. 8C View Figure 8 ): endopod distal segment now with 6 (2 subterminal, 4 terminal) setae; exopod distal segment now with 8 long-terminal plumose natatory setae; otherwise unchanged.

Second maxilliped ( Fig. 9C View Figure 9 ): exopod distal segment now with 8 long-terminal plumose natatory setae; otherwise unchanged.

Third maxilliped ( Fig. 10C View Figure 10 ): developing, epipod bud present.

Pereiopods ( Fig. 11B View Figure 11 ): developing with some cheliped segmentation.

Abdomen ( Figs 13A View Figure 13 , 14C View Figure 14 ): now with 6 somites; somite 1 with 1 pair of dorsal medial setae plus 1 posterodorsal seta; somite 6 without setae; somites 2–6 with uniramous pleopod buds present without endopods; otherwise unchanged.

Telson ( Figs 13A View Figure 13 , 14C View Figure 14 , 15C View Figure 15 ): unchanged. Zoea IV

Carapace ( Figs 2B View Figure 2 , 3C View Figure 3 ): 6 pairs of anterodorsal setae; ventral margin with 1 anterior seta plus 7 posterior setae; otherwise unchanged.

Antennule ( Fig. 4D View Figure 4 ): protopod with 1 seta; endopod bud now developed; exopod now with 1 proximal subterminal aesthetasc, 5 distal subterminal aesthetascs;

3 stout and long, 2 shorter and slender aesthetascs plus 3 setae, all terminal.

Antenna ( Fig. 5D View Figure 5 ): endopod more developed c. 80% of protopod and with minute terminal setal buds; otherwise unchanged.

Mandible ( Fig. 10A View Figure 10 ): palp present. Maxillule ( Fig. 6D View Figure 6 ): coxal endite with 8 setae; basial endite with 10 setal processes; otherwise unchanged.

Maxilla ( Fig. 7d View Figure 7 ): exopod margin with 22 setae of equal length; otherwise unchanged.

First maxilliped ( Fig. 8D View Figure 8 ): coxal segment with epipodal bud; exopod distal segment with 10 long-terminal plumose natatory setae.

Second maxilliped ( Fig. 9D View Figure 9 ): coxal segment with epipodal bud; exopod distal segment with 10 longterminal plumose natatory setae.

Third maxilliped ( Fig. 10D View Figure 10 ): arthrobranch gill bud developing.

Pereiopods ( Fig. 11C View Figure 11 ): developing with some segmentation.

Abdomen ( Figs 13B View Figure 13 , 14D View Figure 14 ): somite 1 with 2 pairs of dorsal medial setae plus 1 posterodorsal seta; pleopods with endopod buds present (biramous); otherwise unchanged.

Telson ( Figs 13B View Figure 13 , 14D View Figure 14 , 15D View Figure 15 ): 1 pair of dorsal medial setae; otherwise unchanged.

Variability

The setation of the maxillule coxal endite, fourth zoeal stage, varied and of the six appendages examined they scored 1 ¥ 7; 3 ¥ 8; 1 ¥ 9; 1 ¥ 10 setae. In other descriptions of Pilumnus hirtellus, Salman (1982) recorded 7–8 setae and Ingle (1991) eight setae for this character.

Previous descriptions

Although nine larval references are listed in the synonymy for P. hirtellus only the study of Salman (1982) appears to be complete. However, a number of characters in his zoeal description are problematical because some setal counts remain conservative throughout development. For example, the number of anterodorsal setae on the carapace remains at two pairs of setae for ZII–ZIV and the setation of the ventral carapace margin is four in all zoeal stages. In contrast, the setae for these two characters in the present study are shown to increase in number with successive moults (see Figs 1–3 View Figure 1 View Figure 2 View Figure 3 ). Furthermore, a number of characters are not fully described such as the appearance of the mandibular palp and the development of the third maxilliped. Table 1 lists the differences between the zoeal description of Pilumnus hirtellus by Salman (1982) and the present study.

Comparative zoeal development

The timing of expression of ontogenetic characters was analysed from the zoeal development of Pilumnus hirtellus (the present study), Actumnus setifer (by Clark & Ng, 2004a) and P. sluiteri (by Clark & Ng, 2004b). Characters relating to number of zoeal stages; appearance of sixth somite division; appearance of appendages; appearance of endopod buds; appearance of the epipod on the first maxilliped and appearance of setae were compared and are summarized in Tables 2–24.