Description of the fi rst juvenile stage of Dilocarcinus septemdentatus ( Herbst , 1783 ) ( Crustacea , Decapoda , Trichodactylidae )

A total of 38 freshwater brachyuran species occur in Brazil, of which 79% are distributed in the northern region. However, for only two species of freshwater crabs are descriptions available for their juveniles. Th e importance of these studies lies in understanding of life-cycle aspects as well the potential elucidation of phylogenetic relationships within the group because the characters are usually solely based on adult specimens. Th e morphology of the fi rst juvenile stage of the Dilocarcinus septemdentatus (Herbst, 1783) is described and illustrated for the fi rst time based on specimens hatched by an ovigerous female collected in the northeastern region of the Brazilian Amazon. Juveniles of Dilocarcinus pagei Stimpson, 1861 and D. septemdentatus have one fl agellar segment on the exopod of the fi rst maxilliped while the only described juvenile of an unidentifi ed species of Trichodactylus Latreille, 1828 has two. Dilocarcinus septemdentatus diff ers from other congeneric species mainly in the number of setae on the antennae and number of segments of the mandibular palp. Variations in the number of segments and orIgInal artIcle This article is part of the tribute offered by the Brazilian Crustacean Society in memoriam of Michael Türkay for his outstanding contribution to Carcinology CORRESPONDING AUTHOR Cleverson Rannieri M. dos Santos crsantos@museu-goeldi.br SUBMITTED 23 February 2017 ACCEPTED 09 September 2017 PUBLISHED 27 November 2017 Guest Editor Célio Magalhães DOI 10.1590/2358-2936e2017031 orcid.org/0000-0003-0368-5300 www.scielo.br/nau www.crustacea.org.br


IntroductIon
Freshwater crabs are Brachyura Heterotremata that reproduce exclusively by direct development with the larval stages occurring inside the egg and newly hatched juveniles exhibiting characteristics similar to those of the adults (sensu Williamson, 1969;Cumberlidge and Ng, 2009).This group includes more than 1,280 species in 220 genera in the Neotropical, Afrotropical, Palearctic, Oriental and Australasian regions (Cumberlidge and Ng, 2009).A total of 43% of all Brazilian freshwater decapod fauna occur in the Amazon basin (Magalhães, 2003).Sternberg et al. (1999) separated the freshwater crabs into two monophyletic groups, the first one with five families and the other one consisting of the family Trichodactylidae H. Milne-Edwards, 1853, but this classification is controversial.Bott (1970), Cumberlidge (1999) and Martin and Davis (2001) included the Trichodactylidae in the superfamily Portunoidea Rafinesque, 1815Rafinesque, , but Števčić (2005) ) transferred it to the superfamily Trichodactyloidea H. Milne-Edwards, 1853, which was followed by Ng et al. (2008), Cumberlidge andNg (2009), andDe Grave et al. (2009).The family has two subfamilies, Dilocarcininae Preztmann, 1978, consisting of 12 genera and 33 species, and Trichodactylinae H. Milne Edwards, 1853, encompassing 3 genera and 16 species (De Grave et al., 2009).In Brazil, Trichodactylidae is represented by 10 genera and 29 species that occur in all of the river basins in the country, excluding the temporary rivers of the semiarid northeastern region, but are primarily distributed in the Amazon basin (Magalhães, 2003).These animals are found in streams, lakes, wetlands, backwaters and swamps, where they occupy various microhabitats, including submerged leaf litter, branches and logs, as well as aquatic vegetation, marginal pits and cracks under and between riverbed stones (Magalhães, 2000;2003).
Studies of the juvenile stage of the Decapoda are important because all the characters used to identify species are exclusively based on adult morphology, which complicates their identification when they are juvenile.According to Martin et al. (1984), juvenile morphological studies may also help to clarify the phylogenetic relationships within Brachyura and facilitate their taxonomic identification.Considering the large number of brachyuran species in Brazil, there is little knowledge about the juvenile development of crabs.Juvenile development has only been described for three freshwater crabs: the trichodactylids Trichodactylus sp. by Müller (1892) and D. pagei by Vieira et al. (2013), and the pseudothelphusid Kingsleya ytupora Magalhães, 1986by Wehrtmann et al. (2010).However, the latter authors described only the carapace of the first and second juvenile stages.
In this study we aim to provide the full description of the first juvenile stage of D. septemdentatus as well as to highlight the main differences in comparison with other Brazilian species from different habitats already described.

MaterIal and Methods
One ovigerous female of D. septemdentatus was collected using a 3-mm mesh sieve on the banks of the Rio Preto creek (00º59'59.2"S47º06'53.7"W) in Primavera, state of Pará, Brazil.The juveniles began hatching while still in the field and were preserved in 70% alcohol.

Juvenile development of Dilocarcinus septemdentatus
Nauplius, 25: e2017031 The juveniles were counted and dissected in the laboratory.The appendages were fixed in a 96% alcohol and glycerin mixture (ratio of 1:1) and drawn in dorsal view using a Zeiss microscope equipped with a camera lucida.The dorsal and ventral views of the juveniles were photographed using a Leica M205A stereomicroscope equipped with a DFC420 camera, and the setae were counted for ten individuals.Measurements were made using an ocular micrometer.Descriptions are reported from the proximal to the distal segments, following Clark et al. (1998), and setae classified as simple or plumose.
Female and juveniles are deposited in the Museu Paraense Emilio Goeldi (MPEG) under catalog number MPEG 1174.

results
A total of 101 juveniles hatched from the eggs.The female had a carapace width of 35 mm.The juveniles had a mean carapace width of 1.62 mm (range: 1.43 -1.75 mm) and a mean carapace length of 1.57 mm (range: 1.23-1.71mm).
Similar to the adults, the carapace of the first juvenile stage of Dilocarcinus septemdentatus are wider than long, the abdomen extends beyond the first transverse suture of the thoracic sternum (fourth sternite), and the chelipeds have spines in the merus and carpus.

Dilocarcinus septemdentatus (Herbst, 1783)
Description of the first juvenile stage.Carapace (Fig. 1A) slightly wider than long, subquadrangular, posterior margin wider than anterior one, convex in anteroposterior direction; front bilobed, slightly deflected.Dorsal region convex with simple and plumose setae sparsely; early differentiation of the gastric, cardiac, intestinal and branchial regions.Anterolateral margin with minute spine.Small red and brown spots over the entire dorsal region of carapace.Sternum (Fig. 1B) without setae.
Abdomen (Fig. 1C) attached to sternum, with six somites wider than long, extending beyond transverse suture of fourth sternite.Ventrally with four pairs of pleopodal buds from second to fifth somite.
Telson subtriangular, with small simple and plumose setae scattered on dorsal region.Antennule (Fig. 2) with the basal segment welldeveloped with 8-11 simple and 12-15 plumose setae.Peduncle two-segmented, first segment smooth, second segment with 4 simple setae.Endopod (ventral flagellum) two-segmented, proximal segment with 1 subterminal simple seta, distal segment with 4 simple setae (2 subterminal and 2 terminal).Exopod (dorsal flagellum) 3-segmented; first segment smooth; second segment with 1 simple seta and 4 aesthetascs ending in a globose tip with a sharp point; third segment with 2 simple setae and 4 aesthetascs ending in a globose tip with a sharp point.
Chelipeds (Fig. 1A, B) symmetrical, covered with simple and plumose setae.Presence of small red and      brown dorsal spots on all chelipeds segments.Ischium with conspicuous dark pigmentation in ring format.Merus with prominent spine on anterolateral region.Carpus with distal spine on dorsolateral inner portion, near articulation with propodus.
Pereopods 2-5 (Fig. 1A, B) similar, with simple and plumose setae.Presence of small red and brown dorsal spots along pereopods.Ischium of all pereopods with conspicuous dark pigmentation in ring format.

dIscussIon
Direct development in Decapoda results from secondary embryogenesis of the ancestral larval stages (zoea and megalopa), with these phases occurring inside the egg (Felder et al., 1985).The main morphological differences between adults and juveniles of D. septemdentatus are: adults have 6-7 acute teeth on the anterolateral margin of the carapace whereas this margin is smooth in juveniles; adults have abdominal somites III-VI fused whereas they are free in juveniles; and the presence of four pairs of rudimentary pleopods on abdominal somites II-V in the juveniles (adult males have only the first and second pairs modified as gonopods, and adult females have all the four pairs of pleopods).In none of the juvenile specimens analyzed, it was possible to define the sex because there is no conspicuous modification in the pleopods.In D. pagei the sex differentiation begins at the second juvenile stage (Vieira et al., 2013), likely the same in D. septemdentatus.
The juveniles of D. septemdentatus (present study) and D. pagei (Vieira et al., 2013)  morphologically and can only be differentiated through a combination of characters.D. septemdentatus has an antenna consisting of 8 segments and 3 setae on the sixth segment; the mandibular palp is 3-segmented; the basal endite of the maxillule has 20-24 setae; the basal endite of the first maxilliped has 28-34 setae; the exopod flagellum of the second maxilliped has 1 segment; and the protopodite of the third maxilliped has 28-40 setae.For the same characteristics, D. pagei has 8 segments on the antenna and the sixth segment has 4 setae; the mandibular palp is 2-segmented; the basal endite of the maxillule has 25-28 setae; the basal endite of the first maxilliped has 33-37 setae; the exopodal flagellum of the second maxilliped has 2 segments; and the protopodite of the third maxilliped has 41 setae (Vieira et al., 2013).In general, D. septemdentatus has fewer setae than D. pagei.
Juveniles of Trichodactylus sp.(Trichodactylinae) described by Müller (1892) differ from D. septemdentatus and D. pagei (Dilocarcininae) mainly because the exopod of the antennule consists of 2 segments instead 3; the antenna consists of 9 segments instead of 8; and the exopodal flagella of the first and third maxillipeds have 2 segments instead of 1.Other variations among the Trichodactylidae species that had their first juvenile stage described are outlined in Tab. 1.
As Rodríguez (1992), Sternberg and Cumberlidge (2003) and Daniels et al. (2006) suggested that Trichodactyloidea might have a marine origin and affinities with the Portunoidea, we performed a comparison with the first juvenile stage of Callinectes sapidus Rathbun, 1896 (see Barutot et al., 2001), Callinectes ornatus Ordway, 1863 (see Bolla Junior et al., 2008), D. pagei, D. septemdentatus, andTrichodactylus sp. (Tab. 2).Only the number of segments in the endopodite of the antennule is the same across the five species.While the species of Callinectes Stimpson, 1860 have 7 segments in the exopod of the antennule and the antenna have 10 or 11 segments, Dilocarcinus and Trichodactylus species have, at most, 3 segments in the exopod of the antennule, and the antenna has 8 or 9 segments.There are also variations in the number, size and shape of the aesthetascs and setae of the cephalic and thoracic appendages, with Dilocarcinus and Trichodactylus having a lower number.These variations might be adaptations to the freshwater environment in order to reduce the area of contact with the external environment, which is a key factor for maintaining osmotic balance.According to Lockwood (1962), marine species tend to be characterized by a high surface permeability to ions and water to sustain the blood concentration and, in general, freshwater species have a less permeable area than estuarine species.With the reduction of body surface, freshwater can promote further restriction of the permeability favoring the process of osmotic regulation (Lockwood, 1962).
Table 1.Morphological characters that allow the differentiation and identification of the first juvenile stages of Dilocarcinus septemdentatus (Herbst, 1848) In D. septemdentatus, D. pagei and some Grapsoidea species from estuarine and semiterrestrial habitats the antenna has one simple and extremely long seta located on the last and penultimate segments (see Diaz and Ewald, 1968;Fransozo, 1986/87;Rieger and Nakagawa, 1995;Flores et al., 1998;Rieger and Beltrão, 2000), this simple seta is presumably sensorial ( Bauer and Caskey, 2006).Others types of setae have mechanical functions in Decapoda.Plumose, serrate and plumodenticulate setae are adapted for particle retention and the promotion of water flow, scrape and brush other setae and surfaces or to release food from substrates (Thomas, 1970;Farmer, 1974;Felgenhauer and Abele, 1983;Alexander and Hindley, 1985;Martin and Felgenhauer, 1986;Watling, 1989;Garm, 2004).
Compared to marine species, the number of aesthetascs is lower in juveniles of Trichodactylidae (6 to 8) and larger than in juveniles of semiterrestrial species of Sesarmidae Dana, 1851, with only up to 4 aesthetascs (see Müller, 1892;Diaz and Ewald, 1968;Fransozo 1986/87;González-Gordillo et al., 2010;Vieira et al., 2013).According to Shenoy et al. (1993), the aesthetascs of marine species present thin cuticles that are pointed or uniformly rounded and, in the estuarine species, the aesthetacs are short and robust.The morphological changes in the aesthetascs from marine to semiterrestrial species occur as reduction of number and size a well ass thickening trends.These modifications likely facilitate the conservation of water and sodium ions and have the function to prevent against the wear to which aesthetascs of freshwater and semiterrestrial species are subjected (Edmondson, 1929;Shenoy et al., 1993).

acKnowledgeMents
The authors thank Dr. Inocêncio Gorayeb, of the Entomology Laboratory of the Museu Paraense Emílio Goeldi (MPEG), for allowing the use of the microscope, and Dr. Alexandre Bragio Bonaldo and Dra.Yulie Shimano of the MPEG Arachnology Laboratory, for their permission and assistance in the use of the stereomicroscope.Rony R. R. Vieira was a recipient of a scientific grant from Museu Paraense Emílio Goeldi (Programa Capacitação Institucional -CNPq 170093/2016-6).

Table 2 .
Diagnostic characters that allow differentiation of the first juvenile stages of Brachyura species that occur in Brazilian waters for which the juvenile development is known.