What is Pilumnus palmeri Garth, 1986 (Crustacea: Brachyura)?

Abstract

The poorly known xanthiform crab Pilumnus palmeri Garth, 1986, is known only from two poorly preserved specimens collected from Ecuador. Garth had only provisionally placed the species in Pilumnus Leach, 1816, and it was subsequently referred to a new genus, Garthopilumnus Števčić, 2011 and a new family Garthopilumnidae Števčić, 2011. Garthopilumnus currently sits as a valid genus within Pilumnidae Samouelle, 1819. A re-examination of the holotype male, now in poor condition, shows that it is a juvenile, and key aspects of the original figure of the species given in the account of the type are inaccurate. Given the incompleteness and immaturity of available material, we could not confidently assign the species to an existing family, but we are also not convinced a new family for the species is warranted. The morphological features of Garthopilumnus palmeri suggest it is a member of the Portunoidea, where we provisionally place it.

Keywords:
Classification; juvenile; Portunoidea; systematics; unknown family; xanthiform crab

INTRODUCTION

Among the more unusual xanthiform crabs described by Garth (1986) is Pilumnus palmeri, a small spinose species collected off the coast of Ecuador by “Velero III” as part of the Alan Hancock Expeditions. The species was described on the basis of two incomplete specimens: the male holotype, comprising a more-or-less complete body, chelipeds and one ambulatory leg (according to the type account and figures; Garth, 1986: fig. 4); and a second specimen comprising only of a partial carapace. The taxonomic position of the crab, however, was immediately unclear. Garth (1986) tentatively assigned the species to PilumnusLeach, 1816, noting similarities with Lophoplax sculptaStimpson, 1858, and PlanopilumnusBalss, 1933, both of which he regarded as pilumnids at that time. Without examining the specimens, Števčić (2011) made the species the type of a new genus, Garthopilumnus, for which he also proposed a new family Garthopilumnidae. No explanation was provided.

Garthopilumnus palmeri remains known only from the type material and all knowledge of the species to date has been limited to Garth’s original account. Here, we re-assess the taxonomic position of G. palmeri based on re-study of the type material. A key intent of the present study is to highlight the peculiar features of G. palmeri so that adults of the species may be discovered by other workers in the future and its affinities determined.

MATERIAL AND METHODS

The type specimen is deposited in the Los Angeles County Museum of Natural History (LACM), California, U.S.A. The terminology used essentially follows Davie et al. (2015). Measurements used, in millimetres, are for the maximum carapace width and length, respectively (when possible). Pereopods 2-5 are abbreviated P2-5, respectively; and gonopod 1 is abbreviated G1.

SYSTEMATICS

Superfamily Portunoidea Rafinesque, 1815

Family Incerta Sedis

Genus Garthopilumnus Števčić, 2011

Garthopilumnus Števčić, 2005: 133 (nomen nudum).

GarthopilumnusŠtevčić, 2011: 127. - Ng et al., 2008: 141, 144. - Poore and Ahyong, 2023: 668, 676, 678.

Type species. ?Pilumnus palmeriGarth, 1986, by original designation. Gender masculine.

Remarks. See Remarks for species below.

Garthopilumnus palmeri (Garth, 1986 )

(Figs. 1, 2)

?Pilumnus palmeriGarth, 1986: 7, figs. 4A-G. - Ng et al., 2008: 142.

Garthopilumnus palmeri - Števčić, 2005: 133. - Števčić, 2011: 127. - Poore and Ahyong, 2023: fig. 14.108c.

Figure 1.
Garthopilumnus palmeri (Garth, 1986), holotype male (9.3 × 6.5 mm) (LACM 1938.1181), Ecuador. A, Dorsal habitus; B, right chela (outer view); C, left chela (outer view); D, right third maxilliped (only proximal articles present); E, basis-ischium and coxa of ambulatory leg; F, pleon (distal segments only); G, G1s. Scales = 1 mm. After Garth (1986: figs. 4A-G).

Figure 2.
Garthopilumnus palmeri (Garth, 1986), holotype male (9.3 × 6.5 mm) (LACM 1938.1181), Ecuador. A, Dorsal view of carapace; B, ventral view of damaged cephalothorax; C, presumed left P2 merus; D, presumed left P3 merus (dorsal margin damaged); E, presumed right P3 merus; F, presumed right P5 merus; G, presumed left P2 carpus and propodus; H, I, left ambulatory carpus; J, presumed right P2 or P3 propodus (ventral margin damaged); K, presumed left P5 propodus; L, basis-ischium and coxa of ambulatory leg; M, presumed left P2 dactylus; N, right ambulatory dactylus; O, outer view of left chela. Scale bar = 2mm.

Material examined. Holotype male (9.3 × 6.5 mm) (LACM 1938.1181), off Cape San Francisco, Ecuador, 0°37'10"N 80°00"30"W, 15 fathoms, station AHF A5486, mud-rock, coll. R/V “VELERO III”, 23 February 1938.

Remarks. Števčić (2005: 133) treated Garthopilumnus very briefly: “Type specimen very damaged. Etymology: according to John S. Garth, American carcinologist. Type species: Pilumnus palmeriGarth 1986. Gender: masculine. Probably new family (Garthopilumnidae)”. He did not examine the specimens and based his prognosis entirely on Garth’s (1986) description and figures. As noted by Ng et al. (2008: 144), both the new genus and family names are not available from Števčić (2005). Števčić (2011: 127, 135) subsequently rectified the nomenclatural problem when he formally diagnosed the family and genus. To date, Garthopilumnidae of Števčić (2005) (unavailable) and Števčić (2011) (available) have been treated as synonyms of Pilumnidae, and Garthopilumnus as a valid genus, tentatively placed in the Pilumnidae (Ng et al., 2008; Poore and Ahyong, 2011).

Ng et al. (2008: 144) had doubts about the genus and family, commenting that decisions about their validity should only be made after actual specimens are examined. We recently re-examined the type specimen of Pilumnus palmeriGarth, 1986. The species was described on the basis of one supposedly complete but damaged holotype male measuring 9.3 × 6.5 mm and a partial carapace from relatively shallow water in Cape San Francisco, Ecuador; it has not been reported since. Garth (1986) noted that the holotype was not in good condition and figured one intact leg (the left P2) with the remaining legs schematically indicated (as conjectures based on the fragments); he also figured the chelae, base of the legs, maxilliped 3, damaged pleon, and gonopods (Fig. 1).

The present condition of the specimen is very poor, being fragile and fragmented, with the carapace partly damaged on the right side, detached from the sternum, with the dorsal and lateral parts of the sternum crushed. The chelipeds are intact, but all the ambulatory legs are disarticulated and badly damaged (Fig. 2 C -O). The posterior part of the carapace and sternum are badly damaged with parts missing. Only fragments of the third maxillipeds remain.

From re-examination of the remnants of the holotype, it is clear that the figure provided by Garth (1986) (present Fig. 1) is inaccurate in several aspects. Firstly, in the carapace figured by Garth (1986) shape and proportions are incorrectly rendered with the anterolateral margin much shorter than the posterolateral margin, so much so that the carapace appears very “front-heavy”, with the posterolateral margin strongly concave (Fig. 1 A ). In fact, the carapace is evenly hexagonal in shape (Fig. 2 A ), with the antero- and posterolateral margins subequal in length, and the posterolateral margin is almost straight rather than prominently concave. The chelipeds are accurately figured (Fig. 2 O ). The pleon as figured by Garth (1986) (Fig. 1 F ) is relatively intact and the figure is accurate, with somites 3-5 completely mobile. The remnants of the G1 are as figured by Garth (1986) (Fig. 1 G ), but they are soft, clearly immature and not developed, without any spinules or setae; and there is no trace of the G2, which suggests the specimen is a juvenile. The condition of the penis could not be ascertained as the posterior part of the sternum is missing.

Garth (1986: 9-10) commented that “Despite its tentative assignment to the genus Pilumnus, the affinities of this delicately sculptured and richly ornamented crab remain obscure. The recent rediscovery by Takeda (1977, p. 120, text-fig. 1) of Lophoplax sculpta (Stimpson, 1858), a similarly tomentose species with raised, naked areolets, resulted in a search among goneplacid genera for a crab related to the proposed new species. Although the critical first abdominal somite is not present, so that it is impossible to determine whether or not it occupies the entire space between the coxae of the last pair of legs, the other characters of the crab, including the shape of the carapace and the form of the chelipeds, are strongly xanthid. Although it might be placed in PlanopilumnusBalss (1933) [regarded by Garth as a pilumnid], a genus containing such sculptured species as P. vermiculatus (A. Milne Edwards, 1873), but none apparently with spinulous margins, it seems best to refer the new species to Pilumnus, sensu lato …..”. Lophoplax was recently reviewed by Ng and Rahayu (2023) and any similarities with P. palmeri are superficial at best. Lophoplax does have smooth raised regions (areolets) on the carapace but it is not spiniform, with the chela normal and the ambulatory meri cristate. Planopilumnus and the Planopilumnidae Serène, 1984, was revised by Ng (2010) and shown not to be pilumnoids, and none of the genera recognised are spiniform or have the strong carapace regions of P. palmeri. The pilumnoid genus Vellumnus Ng, 2010 does have some regions raised with distinct setal patterns, but the carapace is not spiniform. In any case, all these taxa have normal truncate bilobed frontal margins, without any triangular median lobes.

Although placed in Pilumnidae until now on account of its Pilumnus-like spininess, G. palmeri appears to be neither a pilumnid nor even a pilumnoid given the straight and relatively stout G1 remnants, which are usually sinuous and slender even in juveniles.

The figures of the male pleon and gonopods have resemblances to members of the Eriphiidae MacLeay, 1838. The carapace shape of G. palmeri is superficially similar to that of EriphiaLatreille, 1817, and EriphidesRathbun, 1897, in that it is almost trapezoidal, the front is relatively broad, and the posterolateral margins strongly converging towards a short posterior carapace margin. However, there are no species from the eastern half of the Pacific that have the sharp spines and teeth along the anterolateral margin and surfaces of the chelae like G. palmeri. All eastern Pacific species of Eriphia have flattened, rounded tubercles on the outer surface of the chelae (see Koh and Ng, 2007; Davie and Ng, 2000); the only species that has more spiniform chelae being Eriphia scabriculaDana, 1852, although this species has a very different carapace (Koh and Ng, 2007). Garthopilumnus palmeri superficially resembles Eriphides hispida (Stimpson, 1860) but the latter species has a more granular carapace and is nowhere as spiniform (see Rathbun, 1930). Most members of the Eriphiidae are relatively large crabs, reaching maturity only at sizes around 20 mm in carapace width or more (Koh and Ng, 2007), and the fact that the holotype male of G. palmeri already measures 9.3 mm in carapace width but is still immature is perhaps indicative. None have a frontal margin even approaching the sinuous form of Garthopilumnus with the median lobe triangular. In addition, species of Eriphia and Eriphides have a proportionately higher and more thick-set carapace, appearing stocky (cf. Rathbun, 1930; Koh and Ng, 2007; Davie and Ng, 2000), which this contrasts with the flatter carapace of Garthopilumnus (Figs. 1A, 2A). The broad front of G. palmeri is reminiscent of DomeciaEydoux and Souleyet, 1842 (Domeciidae Ortmann, 1893), a genus also found in the eastern Pacific, but in the latter genus, the posterior margins of the carapace are not as strongly converging, the chela is not as spiniform, the armature of the front is not the same and the eye form is different. Significantly, thoracic sternites 2 and 3 are completely fused in Domeciidae, without trace of the transverse groove clearly evident in Garthopilumnus (see Ng et al., 2023).

The relatively broad front and strongly converging posterolateral margins of the carapace also resembles members of the genus DacryopilumnusNobili, 1906 (Dacryopilumnidae Serène, 1984) (see Serène, 1984; Li et al., 2010) but all known species of this genus have smoother anterolateral margins and disproportionately larger eyes. Both Domecia and Dacryopilumnus also have relatively simple and stout G1s, although adults of both genera are small, reaching maturity at 6 to 8 mm in carapace width. The type specimen of G. palmeri at 9.3 mm in carapace width is still immature.

The carapace of Garthopilumnus also resembles some taxa of Cancridae (see Schweitzer and Feldmann, 2010; Schram and Ng, 2012), particularly that of the Miocene Tasadia carniolica (Bittner, 1844) (Jansen and Müller, 1984: pl. 5) with spiny, well demarcated carapace regions, and the Late Eocene, Anisospinos berglundiSchweitzer and Feldmann, 2000, but the carapace is less spiny and the gastric regions are poorly demarcated (cf. Schweitzer and Feldmann, 2010: figs. 2, 3; Schram and Ng, 2012: fig. 1H [incorrectly labelled as Notocarcinus sulcatusSchweitzer and Feldmann, 2000]). Nevertheless, the proportionally narrow male thoracic sternum of cancroids excludes Garthopilumnus, which has a wide xanthiform-portuniform thoracic sternum.

One of the most distinctive features of Garthopilumnus is the form of the frontal margin. The frontal margin (excluding the inner orbital angles) superficially appears to have three rounded lobes, but is actually divided into four lobes, the lateral lobes being broadly convex with the two medians acutely triangular and closely adjacent medially, separated only by a narrow notch (Fig. 2 A ). This form of frontal arrangement is present in many portunoids (and some cancroids). In addition, the fragments of the ambulatory legs (P2-P5) in the form of the merus, with the distal part distinctly tapering (Fig. 2 C -F) represents a form commonly observed in (although not exclusive to) portunoids, as is a very short P5 merus (Fig. 2 F ). A disarticulated article we interpret to be the left P5 propodus (Fig. 2 K ) is very short, typical of many portunoids, especially those with P5 adapted for swimming. Unfortunately, the diagnostic foliaceous P5 dactylus is missing so we cannot be certain; that being said, many portunoids (notably those in the Geryonidae, Carcinidae and some in the Portunidae) have a falciform instead of foliaceous P5 dactylus.

Among known portunoids, some, such as ParathranitesMiers, 1886 (Polybiidae) and EchinolatusDavie and Crosnier, 2006 (Geryonidae), have a carapace that superficially resembles Garthopilumnus, sharing a subhexagonal shape and more pronounced spination than most other swimming crabs. Notably though, Garthopilumnus has prominent, convex raised regions on the epigastric, protogastric, and metagastric regions (Fig. 2 A ), more so than Parathranites or Echinolatus (cf. Dell et al., 1970; Moosa, 1996; Crosnier, 2002; Davie and Crosnier, 2006). Such strongly raised regions are more often seen in xanthiform crabs, such as eriphioids, xanthoids, and pilumnoids, than in portunoids. Noteworthy also is that each protogastric region is longitudinally divided into two halves in Garthopilumnus, with the lateral one asymmetrical in form (Fig. 2 A ).

At present, the incompleteness and immaturity of known specimens means our knowledge of the morphology of G. palmeri is very limited. Given these limitations, our current best estimate of the systematic position of Garthopilumnus is among the Portunoidea, rather than Pilumnoidea. Although the nearest candidates appear to be among the polybiines and geryonids of the Portunoidea, we cannot yet determine which family can best accommodate Garthopilumnus. Whether Garthopilumnus may warrant its own family, or even a superfamily, remains to be seen.

ACKNOWLEDGEMENTS

We are most grateful to Adam Wall and Jody Martin (LACM) for arranging the loan of the specimens; and to Lily Berniker (American Museum of Natural History, New York) for helping to arrange the loan from LACM and allowing the first author to examine the specimen at the museum. We also thank Danièle Guinot, Peter Davie, Ernesto Campos, and Michel Hendrickx for feedback and ideas on what Pilumnus palmeri might be.

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ADDITIONAL INFORMATION AND DECLARATIONS

• Consent for publication

  All authors declare that they have reviewed the content of the manuscript and gave their consent to submit the document.

• Data availability

  All study data are included in the article.

• Funding and grant disclosures

  Not applicable.

• Study association

  Not applicable.

• Study permits

  Not applicable.

Edited by

Associate Editor:

Marcos S. Tavares

Data availability

All study data are included in the article.

Publication Dates

• Publication in this collection
  23 Sept 2024
• Date of issue
  2024

History

• Received
  30 Nov 2023
• Accepted
  18 Jan 2024