ZOOLOGIA 28 (4): 465–478, August, 2011 doi: 10.1590/S1984-46702011000400008 Redescription of megalopa and juvenile development of Pachygrapsus gracilis (Decapoda: Grapsidae) from the Amazon region, reared in the laboratory Danielle C. B. Arruda1 & Fernando A. Abrunhosa1, 2 1 Laboratório de Carcinologia, Instituto de Estudos Costeiros, Universidade Federal do Pará. Campus Universitário de Bragança, Alameda Leandro Ribeiro, Aldeia, 68600-000 Bragança, PA, Brazil, E-mail: [email protected]; [email protected] 2 Corresponding author. ABSTRACT. This present study re-describes the megalopa stage and provides detailed morphological descriptions and main growth changes observed in stages I through VII of the juvenile instars of the dark shore crab Pachygrapsus gracilis (Saussure, 1858), from the Amazon region. The specimens in this study were reared in the laboratory and the megalopae were collected at Ajuruteua beach in northeastern Pará, Brazil. Previous studies had described the megalopa of P. gracilis from Mexican waters, as well as those of Pachygrapsus transversus (Gibbes, 1850) and Pachygrapsus marmoratus (Fabricius, 1787). A comparison between the Mexican and Amazonian populations of P. gracilis revealed significant morphological differences. The main difference is the presence of 3 elongated setae on the 5th pereiopods of individuals of the Amazonian population. The setal number and their arrangement in the appendages also differed. In P. gracilis, the male and female genital openings are observed from the juvenile instar III, whereas differentiation in male pleopods is observed only in juvenile instar V. In females, the pleopods undergo rapid differentiation during juvenile instar VI. These morphological comparisons and other observations on development are briefly compared and discussed with reports for other species. KEY WORDS. Description; growth; morphology; transitory stage. The dark shore crab Pachygrapsus gracilis (Saussure, 1858) Studies on crab growth are important because some mor- has a wide distribution, occurring in the western Atlantic from phological characteristics, including secondary sexual charac- the Caribbean to Texas, French Guiana, Brazil (Trinidad and ters, which arise at the juvenile phase, are useful in the from Ceará to Rio Grande do Sul) and Argentina, In the east- identification of species, as well as age and sex determinations. ern Atlantic the species occurs from Senegal to Angola (MELO The morphological characteristics found in crustaceans 1996, POUPIN et al. 2005). have contributed significantly to the understanding of the taxo- Pachygrapsus Randall, 1839 megalopae was described in nomic, systematic and phylogenetic nature of crustaceans. They the past from individuals collected in the natural environment: have also generated information which has helped the under- P. marmoratus (Fabricius, 1787) by GUERAO et al. (1997), P. trans- standing of their biology and assisted population management versus (Gibbes, 1850) by FLORES et al. (1998) and P. gracilis by (BARUTOT et al. 2001). However, morphological descriptions of CHÁRAZO-OLVERA & ROCHA-RAMÍREZ (2007). These studies used crustaceans must be accurate, and describe the setal arrange- specimens collected from the environment, probably due to ment, the number of the juvenile appendages and other struc- the difficulty of obtaining the stage of megalopae under labo- tures correctly. ratory conditions. Only two studies on the juvenile growth and In some cases, intraspecific variation has been found in morphology of this group are available in the scientific litera- individuals of different populations. Nelice M. Batistelli ture: GUERAO et al. (1997) for P. marmoratus and FLORES et al. (unpubl. data), for example, found discrepancies in setal num- (1998) for P. transversus. ber and setation arrangements in re-descriptions of various fid- GUERAO et al. (1997) described the morphology of the cara- dler crab larvae. More studies on the morphological stages and pace, pereiopods and third maxillipeds of the juvenile instar I new descriptions of larval and juvenile stages of crustaceans of P. marmoratus. Their data provide information on the size of are necessary in order to clarify these differences. the carapace up to the juvenile IV. FLORES et al. (1998) described Due to the scarcity of studies detailing the morphologi- in detail the megalopa, the first crab instar, and the juvenile cal development of crustaceans and the growth of these ani- development instars of P. transversus giving emphasis on the mals, it is necessary to conduct further studies on the growth process of pleopod differentiation up until juvenile instar VII. and morphology of juvenile instars. The present study re-de- © 2011 Sociedade Brasileira de Zoologia | www.sbzoologia.org.br | All rights reserved. 466 D. C. B. Arruda & F. A. Abrunhosa scribes the megalopa stage of P. gracilis inhabiting the Amazon RESULTS region, and describes important morphological changes ob- served during the growth from juvenile instars I to VII, of. MEGALOPA A total of 89 megalopae molted into the juvenile instar I. MATERIAL AND METHODS The remaining individuals died during the experiment. These were preserved. The first molting was recorded on the second In February 2008, 116 megalopa were obtained at day, and the last on the 18th day of culture after collections in Ajuruteua (00º49’20.2”S, 46º36’02.2”W) Beach, municipality the field (Fig. 1). Large frequencies of metamorphosed of Bragança (northeastern Pará state, Brazil). The specimens megalopae were recorded on the third and fourth days of rear- were collected off of mangrove leaves, pieces of trunks and roots ing. The mean intermolt period was 5.72 ± 4.45 days. brought by the tide. They were identified in the laboratory when JUVENILE GROWTH they reached the juvenile instar. Specimen identification fol- During the culture, the mortality of juveniles was quite lowed RODRIGUEZ (1980), MELO (1996) and POUPIN et al. (2005). low: only three specimens died in the juvenile instars I, II and In the laboratory, specimens were placed individually in III, respectively. The mean intermolt period gradually increased plastic recipients with sea water (salinity 30) and a small rock during the juvenile development and after consecutive molts shelter. The temperature was maintained at 26.5 ± 1.14ºC, pho- (Fig. 2). In juvenile instar I, the intermolt period mean was 6 ± 3 toperiod regime (12:12 h/light:dark cycle) and pH 8.1. Every days whereas in the juvenile instar VII it averaged 20 ± 12 days. two days the water of the recipients was changed. The indi- The carapace length of the juvenile instar I (1.96 ± 0.07 viduals were fed live Artemia nauplii daily and were monitored mm) was similar to that found for megalopa (1.97 ± 0.04 mm). twice a day. The exuviae and dead animals were preserved in The growth (carapace width) of each subsequent juvenile in- glycerol+ethylic alcohol 70% (1:1) solution. star showed an increase during the days of culture and molts At least 10 specimens of each stage were dissected with (Fig. 3). In the juvenile instar I, the mean carapace width was fine needles (BD Ultra-Fine®, 12.7 X 0.33 mm) under an opti- 2.21 mm and in the juvenile instar VII the mean carapace width cal Zeiss (Axioskop 40) microscope equipped with an ocular was 7.42 mm. The carapace width of male and female crabs micrometric. The appendages were extracted and transferred was similar during juvenile growth until the VII instar (Fig. 4). to another slide where they were stained with a solution of The percentage of molt increment showed a reduction methylene blue 1% (sometimes was used). The terminology at each molt event (Fig. 5). From the juvenile instar I to juve- used in the descriptions follows POHLE & TELFORD (1981), GUERAO nile instar II, the mean rate of increment was 31.13 ± 8.75%, et al. (1997), FLORES et al. (1998), GARM (2004), and CHÁRAZO- whereas from the juvenile instar VI to VII it was 18.51 ± 4.16%. OLVERA & ROCHA RAMÍREZ (2007). The studied specimens were The absolute molt increment increased at each molt event; from deposited at the crustacean collection of Museu Paraense Emílio juvenile instar I to II the mean was 0.68 mm and from the Goeldi, under the catalog number MPEG-817. juvenile instar VI to VII the mean was 1.17 mm (Fig. 5). The individuals were reared until juvenile instar VII. The width of the abdomen in juvenile forms increased During rearing, data on survival and intermolt periods were during juvenile growth (Fig. 6). However, the width of the abdo- obtained for individuals in each juvenile instar. The carapace men of males and females presented differences only in the ju- width (CW) was considered as an independent variable or as a venile instars VI and VII (Fig. 6). In the juvenile instar I, the reference (in axis x) because of its greater amplitude and be- mean abdomen width in males and females was similar, 0.70 ± cause it is the best representative dimension of organism size 0.04 mm and 0.60 ± 0.04 mm, respectively. In juvenile instars (HARTNOLL 1982). This variable was used for calculating the per- VI and VII, these values were very different for both sexes; fe- centage of molt increment and absolute increment after each males had 2.10 ± 0.4 mm and 3.20 ± 0.76 mm of abdomen width, ecdise. The abdomen width of male and female specimens was while males presented 1.70 ± 0.19 mm and 2.10 ± 0.15 mm, re- obtained by measuring the 4th abdominal somite. spectively. The percentage of molt increment (%MI) was estimated MORPHOLOGICAL DESCRIPTION by the following formula: %MI = [(CW2 – CW1) x 100]/CW1; where CW1 and CW2 are pre-molt and post-molt sizes, respec- Megalopa tively. Carapace (Fig. 7): length: 1.97 ± 0.04mm; width: 1.39 ± The absolute increment (AI) after each ecdise was ob- 0.04 mm; subretangular; surface showing four distinct lobes tained using the following equation: AI = CW2 – CW1; where on the anterior portion and few small setae; middle and poste- CW1 is the carapace width in pre-molt and CW2 the carapace rior portions fringed with numerous small marginal setae; an- width in post-molt.