Pilumnus vinaceus A. Milne-Edwards 1880
Creators: name: Alves, Douglas Fernandes Rodrigues
name: Pantaleão, João Alberto Farinelli
name: Costa, Rogério Caetano
name: Mantelatto, Fernando Luis
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Pilumnidae
Pilumnus
Pilumnus vinaceus

description: Pilumnus vinaceus A. Milne-Edwards, 1880
Zoea I (Figs. 1 A-G; 2A-D)
Dimensions: CL: 0.23 ± 0.01 mm; RDL: 0.83 ± 0.01 mm (n = 10).
Carapace (Figs. 1 A-C): dorsal spine longer than rostral spine, curved distally backward, without setae; rostral spine straight, shorter than antennal protopod; lateral spines well developed and smooth; 1 pair of posterodorsal simple setae; lateroventral margins with 8-10 denticles, without setae; eyes sessile.
Antennule (Figs. 1A, B, D): uniramous; primary flagellum unsegmented with 4 terminal aesthetascs (2 long and stout, 2 shorter and thinner) plus 2 small simple setae; accessory flagellum absent.
Antenna (Figs.1A, B, E): uniramous, protopod well developed and long, with two rows of lateral spines unequal in size on distal half; endopod absent; exopod slightly longer than protopod, with one row of lateral spines unequal in size distally, with 1 pair of unequal medial setae.
Mandible (not drawn): incisor and molar process developed; palp absent.
Maxillule (Fig.2A):uniramous;coxal endite with 7sparsely plumose setae; basial endite with 2 cuspidate and 3 plumodenticulate setae, plus 2 setal buds, microtrichia on proximal margin; endopod 2-segmented, proximal article with 1 sparsely plumose seta; distal article with 6 (2 subterminal sparsely plumose, 2 terminal sparsely plumodenticulate and 2 terminal sparsely plumose) setae. Exopod and epipod setae absent.
Maxilla (Fig. 2B): biramous; coxal endite bilobed, with 5 + 4 sparsely plumose setae; basial endite bilobed, with 5 + 4 sparsely plumose setae; endopod bilobed, with 3 + 5 sparsely plumose setae; exopod (scaphognathite) with 4 marginal plumose setae and a stout distal process; microtrichia on distal margin of endopod, proximal margin of basial endite and distal process of the scaphognathite.
First maxilliped (Fig. 2C): biramous; coxa without setae; basis with 10 sparsely plumose setae (2+2+3+3); endopod 5-segmented with 3 sparsely plumose, 2 sparsely plumose, 1 sparsely plumose, 2 sparsely plumose and 5 (1 subterminal simple and 4 terminal sparsely plumose) setae, respectively; exopod 2-segmented, distal article with 4 long terminal plumose natatory setae.
Second maxilliped (Fig. 2D): biramous; coxa without setae; basis with 4 sparsely plumose setae (1+1+1+1); endopod 3-segmented, with 1 sparsely plumose, 1 sparsely plumose and 6 (2 simple and 1 sparsely plumose subterminal, and 1 simple and 2 sparsely plumose terminal) setae,respectively;exopod 2-segmented,distal article with 4 long terminal plumose natatory setae.
Third maxilliped: absent.
Pereiopods: absent.
Pleon (Figs. 1B, F): 5 pleonites; pleonite 1 without setae, pleonites 2-5 with one pair of posterodorsal simple setae and small denticles on posterodorsal margin; pleonites 2 and 3 with one pair of dorsolateral processes; pleonites 3-5 with long and acute posterolateral processes.
Pleopods: absent.
Telson (Figs. 1B, F, G): bifurcated; furcae distally spinulated, with one pair of well-developed lateral spines, one pair of small lateral spines just below the long ones, and one pair of dorsal spines;inner margin with 3 pairs of serrate setae.
DISCUSSION
The first stage larval morphology of Pilumnus vinaceus conforms to the general Pilumnidae pattern, with characteristics such as: [1] antenna of type 2, exopod is well developed, generally similar in size or longer than protopod and with medial setae; [2] maxilliped 1 with 2+2+3+3 setae on the basis and with 3,2,1,2,5 setae in the endopod; [3] maxilliped 2 with 1+1+1+1 setae on the basis and 1,1,6 setae on the endopod; [4] telson of type 10, furcal rami armed with dorsal and lateral spines (see Clark & Cuesta, 2015). Additionally, some zoea I morphological characters are common among all species within the genus: [1] carapace with lateral spines, but without setae on the ventral margin;[2] antenna lacking endopod (except for P. kempi, P.sluiteri and P. vespertilio); [3] maxillule with seven setae on the coxal endite; [4] maxilla exopod (scaphognathite) margin with only four setae (see Tables 1 and 2, and references therein).
Despite this great morphological similarity of zoea I within Pilumnus, there are consistent differences of zoea I of P. vinaceus comparing with some of the other species from the Indo-West Pacific (Tables 1 and 2). Some species of the Indo-West Pacific show unusual morphological variations when compared to the western Atlantic species, mainly in the following characters: [1] setae on the carapace; [2] presence of antennule endopod; [3] variable number of setae in the maxilla, first maxilliped and pleon (see Tables 1 and 2, and references therein). The species with the most conspicuous morphological dissimilarities in the zoea I in the genus are those with a reduced number of zoeal stages: P.vespertilio (3 stages) (Lim &Tan,1981; Terada, 1990) and P. kempi and P. sluiteri (both 2 stages) (Siddiqui&Tirmizi,1992; Clark&Ng,2004).These three species are the only zoea of the genus that have the endopod of the antennule already developed in the first zoeal stage (see Tables 1 and 2, and references therein). Clark (2005) verified that the timing of appearance and rate of development of pilumnine characters occur at different stages and can be attributed to three heterochronic mechanisms: postdisplacement, predisplacement and acceleration.
Also based on the morphological comparison of already described zoeae I of Pilumnus species, we noted that such morphology does not fit with the general zoeal pattern reported to the genus and can be used as additional support to check the taxonomic position of P. indicus, at least with the universe of larval description available. This suggestion is supported by some zoea I characteristics of P. indicus, which are distinct in relation to the other species of Pilumnus, such as: [1] absence of lateral spines on carapace; [2] rostral spine apparently shorter than in other species (smaller than antennule); [3] dorsolateral processes only in the second pleonite (see Table 1, and references therein). Therefore, the position of this species within the Pilumnidae should be reviewed since the unclear taxonomic retrospect described as Parapilumnus indicus by Deb (1987) but in the later brachyuran revision (Ng et al., 2008) considered as Pilumnus indicus with no further details to adopt this status (Tatiana Magalhães, pers. obs.).
a = aesthetascs;end.= endopod;nd = not described;RDL = rostrodorsal length;s = setae;so = somite;** = data from figures observation.
References: P.kempi (Siddiqui & Tirmizi, 1992; Clark & Ng,2004 d); P.longicornis (Clark & Paula, 2003); P.minutus (Terada, 1984; Ko, 1994); P.scabriusculus (Terada, 1990); P.sluiteri (Clark & Ng, 2004); P.verpertilio (Lim & Tan,1981; Terada,1990 b; Clark & Paula,2003 c); P.indicus (Terada,1980); P.limosus (García-Guerrero et al., 2005).
In the present study, it was not possible to obtain other zoeal stages of P. vinaceus during cultivation in the laboratory, due to the small size of zoeae I (see Table 2), which made it impossible to feed them with newly hatched Artemia nauplii, generally used as food for larvae of other species of Pilumnus (Bookhout & Costlow-Jr., 1979; Terada, 1990; Clark & Ng, 2004; Guerao et al., 2005). Here we conjecture that P. vinaceus probably have four zoeal stages. This assumption is supported by the considerable morphological similarity of its zoea I in relation to all other species that have four zoeal stages in the genus (see Tables 1 and 2, and references therein). However, we suggest that future efforts should be devoted to completing the knowledge about the remaining larval stages of P. vinaceus. For this, we recommend that the cultivation of other organisms (e.g., microalgae and/ or rotifers) with smaller sizes when compared to Artemia nauplii may be fundamental to the successful obtention of P.vinaceus larval stages. Microalgae and rotifers had already been used as live food alternatives in several studies aiming the cultivation of decapod larvae (Zhang et al., 1997; Simões et al., 2002).
Considering only Pilumnus species that occur in the Atlantic, there is a relevant morphological similarity of zoea I (see Table 2, and references therein).This similarity is even greater if we consider only the species that occur in the western Atlantic, and the few differences among species are: (1) two terminal setae in the antennule (P. vinaceus), instead of one (P. dasypodus and P. reticulatus); (2) pleon with dorsolateral processes from the second to the fifth pleonites (P. reticulatus), instead of second to the third (P. dasypodus and P. vinaceus) (Table 2). Thus, considering only the morphology of zoea I, the similarity between P. vinaceus and P. dasypodus is now observed (see Table 2). Importantly, such species are also remarkably similar in the postlarval stages and they were considered synonymous until recently (see Magalhães et al., 2021). All this nebulous scenario with some inconsistencies detected among the larvae of these species were also the subject of taxonomic doubts and proposed synonymy between some of these pairs of close related species given the enormous morphological similarity detected among adults (Magalhães et al., 2021). Only after the study from Magalhães et al. (2021) using the molecular tool in combination with a thorough morphological analysis of a large number of specimens and the type series it was possible to elucidate these doubts.
After the morphological description of zoea I of P.vinaceus it is necessary to update the previous key for the identification of the known zoeal stages of brachyuran crabs from tropical and subtropical Brazil, southwestern Atlantic, proposed by Koettker et al. (2012).In this key, the morphological characters that lead to P. dasypodus are: rostral spine visibly shorter than antenna;no mediolateral spines on abdominal somites 4-5. However, such characters are identical to those of P. vinaceus. So, we suggest that this part of the key could be used as follows: a) pleonites 2-5 with a pair of mediolateral processes (Pilumnus reticulatus); b) pleonites 2-3 with a pair of mediolateral processes; antennule with four aesthetascs and two simple setae (Pilumnus vinaceus); c) pleonites 2-3 with pair of mediolateral processes; antennule with four aesthetascs and one simple setae (Pilumnus dasypodus) (see Table 2).
It is important to consider that there is still a great deficit in the morphological knowledge of Pilumnus larval forms. Of the 19 species belonging to the genus that occur in the western Atlantic (Magalhães et al., 2021), only three (15.8%) have the morphology of the first zoeal stage known, considering the present description. Thus, there is still an amount of zoeal descriptive work to be undertaken for the knowledge of the real diversity of larval morphology within Pilumnus. Such work would also lead to a better understanding of pilumnid evolution and phylogeny, which represents one of the most diverse genus of brachyuran crabs.
description: Published as part of Alves, Douglas Fernandes Rodrigues, Pantaleão, João Alberto Farinelli, Costa, Rogério Caetano & Mantelatto, Fernando Luis, 2022, A description of the first zoeal stage of Pilumnus vinaceus A. Milne-Edwards, 1880 (Decapoda, Pilumnidae), with a revision of the first zoea morphology of Pilumnus Leach, 1815, pp. 1-9 in Papéis Avulsos de Zoologia 62 on pages 3-7, DOI: 10.11606/1807-0205/2022.62.037, http://zenodo.org/record/7176920