Athanas dimorphus, Ortmann, 1894

Morphological description of Athanas dimorphus View in CoL

Zoea I

Carapace length: 0.34 ± 0.02 mm (n = 10 larvae)

Cephalothorax ( Fig. 1A View FIGURE 1 ): rostrum absent; pterygostomian spine present; eyes sessile.

Antennule ( Fig. 1B View FIGURE 1 ): peduncle 2-segmented with 2, 4 plumodenticulate setae, respectively; endopod as long plumose seta; exopod with 5 terminal aesthetascs (2 thinner, 3 more robust).

Antenna ( Fig. 1C View FIGURE 1 ): peduncle unsegmented with 1 medial distal spiniform projection near endopod; endopod with 1 spiniform projection; exopod (scaphocerite) 3-segmented with 11 plumose setae (6, 1, 4), distal segment with 1 simple seta.

Mandible ( Fig. 1D View FIGURE 1 ): palp absent.

Maxillule ( Fig. 1E View FIGURE 1 ): coxa with 2 (1 medial simple, 1 terminal simple) setae; basis with 2 spiniform projections; endopod with 2 terminal simple setae; exopod absent.

Maxilla ( Fig. 1F View FIGURE 1 ): coxa with 2 medial simple setae; basis bilobed, proximal lobe with 2 medial simple setae and distal lobe with 3 medial simple setae (distal one shorter); endopod with 1 medial proximal simple seta and 2 terminal (1 simple and 1 plumose) setae, without microtrichia; exopod (scaphognathite) with 5 plumose setae, without microtrichia.

First maxilliped ( Fig. 1G View FIGURE 1 ): protopodite with 1 medial simple seta; endopod unsegmented with 2 terminal simple seta; exopod unsegmented with 4 terminal natatory plumose setae plus 1 lateral distal simple seta.

Second maxilliped ( Fig. 1H View FIGURE 1 ): coxa without setae; basis with 1 medial simple seta; endopod 3-segmented with 0, 0, 4 (3 distal simple (1 medial and 2 lateral), 1 terminal serrulate) setae; exopod unsegmented with 4 terminal natatory plumose setae.

Third maxilliped ( Fig. 1I View FIGURE 1 ): coxa and basis without setae; endopod 4-segmented with 0, 0, 2 (1 medial and 1 lateral) distal simple setae, 3 (2 medial distal simple, 1 terminal serrulate) setae; exopod 3-segmented with 0, 1 medial distal simple seta, 4 terminal natatory plumose setae.

Pereiopods ( Fig. 1J View FIGURE 1 ): first as biramous bud; fifth pereiopod as uniramous bud.

Pleon ( Fig. 1A View FIGURE 1 ): with 6 pleonites (6th pleonite fused to telson); lateral spines absent; pleopods absent; anal spine absent.

Telson ( Fig. 1K View FIGURE 1 ): triangular, with 8+8 (6 inner plumose, 2 outer laterally plumose) setae, the 2 inner pair of setae shorter than others.

Discussion

Larval data

Although Athanas have 43 described species, the early larval stages are known for only five species (references in Table 1 View TABLE 1 ): A. dimorphus , A. djiboutensis Coutière, 1897 , A. japonicus Kubo, 1936 , A. nitescens and A. parvus De Man, 1910 . Besides the studies where the larvae were described from laboratory reared material, there are also other larval descriptions of this genus based on planktonic material ( Sars 1906; Williamson 1915; Webb 1921; Kurian 1956; Bourdillon-Casanova 1960; Williamson 1967), but these descriptions of plankton samples were not considered in the comparisons as there is no certainty about the identification of the species to which the larvae were assigned.

Setae—lps: laterally plumose seta; nps: natatory plumose seta; pds: plumodenticulate seta; ps: plumose seta; s: seta; sp: spine/ spiniform seta/spine-like; srs: serrulate seta; ss: simple seta; NR: not reported. *According to the figure shown in the description. **Setae are separated by commas indicate that the coxa and/or bases are bilobed; ***Microtrichia in A. nitescens described as minute spines sparsely and A. parvus described as fine marginal hairs. **** Athanas dimorphus ( India), Athanas japonicus and Athanas parvus also have minute spinules. Underline indicates characters in common with zoea I of Athanas dimorphus from Brazil.

The characters of zoea I of A. dimorphus described in this study were compared with the larvae described in eight studies, from four Athanas species, as well as with the three Indian Ocean specimens identified as A. dimorphus ( Table 1 View TABLE 1 ). Some larval descriptions, for example, of A. dimorphus ( Gurney 1927; Bhuti et al. 1977), A. nitescens ( Lebour 1932) and A. djiboutensis ( Gurney 1938) show very poor details in both the description and illustrations of the structures, or even lack illustrations, which makes comparisons of characters between species difficult. As an example, to describe the zoea I, authors can use terms such as “pointed” or “ending in a point” to describe the apex of the endopod of the antenna, leading to confusion when such a distinct character is not depicted in the illustrations.

The zoea I of A. dimorphus described in this study shares some features typical of most zoea I of Athanas (references in Table 1 View TABLE 1 ): rostrum absent (except in A. dimorphus from Pakistan and A. nitescens ); pterigostomial spine present or not reported; sessile eyes (except in A. nitescens , which has stalked eyes); endopod of the antennule as a plumose seta; antenna with unsegmented peduncle with a terminal spine near the endopod (absent in A. djiboutensis ) and 11 plumose setae on the exopod (12 in A. djiboutensis ); mandible without palp (not reported in A. dimorphus from Egypt and rudimentary in A. djiboutensis and A. parvus ); maxillule without exopod; first pereiopod as a biramous bud and fifth as an uniramous bud (rudimentary in A. dimorphus from Pakistan); 6 pleonites and without spines, the 6th pleonite not differentiated; anal spine absent or not reported.

So far, it has not been possible to propose a set of unique and diagnostic larval characters to define Athanas , due to high interspecific variation within the genus. Observed morphologic diversity may be explained by: (1) comparisons being biased due to lack of detail in the description/illustrations of some species coupled with relatively few species in the genus with known larval morphology, and (2) Athanas is perhaps not a monophyletic group, as already proposed by Anker & Jeng (2007). Considering that this genus has a high morphological complexity and absence of phylogenetic studies ( Anker & Jeng 2007), the addition of more larval data, presented in this study, will lend to future taxonomic discussions on Athanas .

Comparing the first larval stage of A. dimorphus from Brazil with previous described larvae of Athanas ( Table 1 View TABLE 1 ), we found morphological similarities: (1) segments in the peduncle of the antennule; (2) spiniform projection on the endopod of the antenna; (3) spines on the basis of the maxillule; (4) setae on the coxa of the maxilla; (5) segments on the endopod and exopod of the first maxilliped; (6) segments and setae on the endopod of the second and third maxillipeds. And morphological differences: (1) setae on the peduncle of the antennule; (2) aesthetascs on the exopod of the antennule; (3) segments on the exopod of the antenna; (4) simple seta on the exopod of the antenna; (5) setae on the endopod of the maxilla; (6) setae on the basis of the first and second maxillipeds; (7) setae on the endopod of the three maxillipeds; (8) simple seta on the exopod of the first and third maxillipeds; (9) natatory plumose setae on the exopod of the second maxilliped; (10) segments in the exopod of the third maxilliped; (11) plumose setae on the telson.

Specifically for A. dimorphus , some morphological differences between the zoea descriptions from distinct localities ( Brazil, Egypt, India and Pakistan) were observed ( Table 1 View TABLE 1 ), the most striking are the segments and setae on the peduncle of the antennule; aesthetascs on the exopod of the antennule; setae on the basis of the maxilla; segments on the endopod of the second maxilliped; and setae on the telson ( Fig. 2 View FIGURE 2 ).

Some features are unique to the zoea I of A. dimorphus from Brazil and can be used to separate the zoea I of this species from other described larvae of Athanas , including the Indian Ocean larvae of A. dimorphus ( Table 1 View TABLE 1 ), such as: segments on the peduncle of the antennule with 2 and 4 plumodenticulate setae, respectively; maxilla with 2 and 3 simple setae on the proximal and distal lobe of the basis; basis and exopod of the first maxilliped with 1 simple seta; basis of the second maxilliped with 1 simple seta.

The larvae of the species examined here, as well as larvae from previous studies of A. dimorphus ( Gurney 1927; Bhuti et al. 1977; Ghory & Kazmi 2009), are characterized by presenting some general characters that indicate extended larval development, such as: sessile eyes, segmentation on the exopod of the antenna and unsegmented endopod, swimming maxillipeds, pereiopods absent or present as a bud, pleopods and uropods absent and telson fused to the sixth pleonite ( Guerao & Cuesta 2014). Extended development is characterized by 6 to 13 zoea stages, and 1 to 5 decapodite stages ( Anger 2001; Bauer 2004; Guerao & Cuesta 2014). Species with extended larval development are consistent with the life history of an alien species, as it provides the species with the ability to reach distant localities, and in some cases may explain wide distributions ( Crandall et al. 2010; Fujita et al. 2016; Oliveira et al. 2019). The data obtained will serve as a background for future larval studies in Athanas , besides expanding the knowledge of the larval morphology of Alpheidae .

Taxonomic implications

Reviews on larval morphology in genera and families of caridean shrimps such as Hippolytidae Spence Bate, 1888 ( Terossi et al. 2010); Processidae Ortmann, 1896 ( Terossi & Mantelatto 2014); Alpheus Fabricius, 1798 ( Pantaleão et al. 2020); Synalpheus Spence Bate, 1888 ( Santos et al. 2020); Ogyrides Stebbing, 1914 ( Pantaleão et al. 2021), demonstrated that the morphology of zoea I can be used to separate species. Within these taxa, variation (mainly using setal and segmentation data) is consistent with species separation and the intraspecific variation, when it exists, is very subtle. In our analyses of A. djiboutensis , A. japonicus , A. nitescens and A. parvus ( Table 1 View TABLE 1 ) we confirm that variation in larval morphology can be used to define certain Athanas species, and therefore have a potential to possibly define other species within the genus.

Based on larval morphology, our results demonstrate that A. dimorphus can be divided into three morphotypes, which are correlated to geographical regions ( Brazil, India and Pakistan) ( Table 1 View TABLE 1 , Fig. 2 View FIGURE 2 ). Although there are clear similarities between Brazil and Pakistan larvae ( Fig. 2 View FIGURE 2 ), other characters are found to represent distinct defining characters separating these morphotypes ( Table 1 View TABLE 1 ). Unfortunately, several characters are missing to sufficiently define the larvae from Egypt, leaving a clear definition of these populations wanting. In effect, the present description and review supports the idea that A. dimorphus is a species complex, as suggested by Anker (2001).

In conclusion, A. dimorphus is still considered to be an alien species in Brazilian waters. We do, however, suspect that we might be dealing with new taxa, but in order to reveal these taxa, a taxonomic revision using morphology (adults and larvae) and genetics on the suggested A. dimorphus species complex is needed. To start, one must include specimens from the specie’s type locality ( Tanzania), and/or coloration. After this revision, we will be able to attribute the specimens that occur in Brazil to A. dimorphus sensu stricto or to a new species.