Pilumnus reticulatus Stimpson, 1860 (Decapoda: Brachyura: Pilumnidae): a reappraisal of larval characters from laboratory reared material*

Pilumnid crabs inhabit coastal waters worldwide and belong to the family Pilumnidae of the superfamily Xanthoidea (Guinot, 1977). Within Pilumnus Leach, 1815, complete zoeae development has been published for the following species: P. dasypodus Kingsley, 1879 by Sandifer, 1974 and Bookhout and Costlow, 1979; P. sayi Rathbun, 1897 by Bookhout and Costlow, 1979; P. vespertilio (Fabricius, 1793) by Lim and Tan, 1981; P. hirtellus (Linnaeus, 1761) by Salman, 1982 and Ingle, 1983, 1992; P. reticulatus Stimpson, 1860 by Montú et al., 1989; P. kempi Deb, 1987 by Siddiqui and Tirmizi, 1992; P. scabriusculus Adams and White, 1849 by Terada, 1990; and P. minutus De Haan, 1835 by Terada, 1984 and Ko, 1994, 1997. Hale (1931) and Wear (1967) described abbreviated or direct development in P. vestitus Haswell, 1882, P. novaezealandiae Filhol, 1886 and P. lumpinus Bennet, 1964. Detailed descriptions and illustrations of some stages of different species of Pilumnus, obtained from northeastern Atlantic and Mediterranean plankton samples, have also been published (see Ingle, 1992, for a review). Seven species of Pilumnus have been recorded from the coasts of the southwestern Atlantic (Melo, 1996). Pilumnus reticulatus is known from the Caribbean to the north of Argentina, and along the Pacific coast of Panama. It inhabits mainly the intertidal or shallow waters, but may reach 75 m in SCI. MAR., 66 (1): 5-19 SCIENTIA MARINA 2002

Seven species of Pilumnus have been recorded from the coasts of the southwestern Atlantic (Melo, 1996).Pilumnus reticulatus is known from the Caribbean to the north of Argentina, and along the Pacific coast of Panama.It inhabits mainly the intertidal or shallow waters, but may reach 75 m in depth.It is the only species in the genus that crosses the Rio de la Plata biogeographical barrier (Rathbun, 1930;Boschi, 1964;Melo, 1996).Three other species have been synonymised under P. reticulatus, viz. P. tessellatus A. Milne Edwards, 1880, P. meridionalis Nobili, 1901, and P. fragosus A. Milne Edwards, 1880(see Rathbun, 1930).Rathbun (1930), however, notes that there is considerable variation in P. reticulatus, and for convenience recognises two forms , "forma fragosa" and "forma tessellata".The present South American specimens belong to Rathbun´s "forma tessellata" (Boschi, 1964).
Larvae of Pilumnus reticulatus from the Rio de la Plata differ in several characters from the previous description by Montú et al. (1989).However, this description was cited neither in later studies on other Pilumnus species (Terada, 1990;Siddiqui and Tirmizi, 1992;Ko, 1994Ko, , 1997) ) nor in regional treatises (Melo, 1996;Pohle et al., 1999Pohle et al., : 1335Pohle et al., -1336)).Therefore, the aim of this study is to describe and reappraise the morphology of Pilumnus reticulatus zoeas and megalopa by comparing the characters with those of Montú et al. (1989) and with those of the following Pilumnus species: P. dasypodus, P. sayi, P. vespertilio, P. hirtellus, P. kempi, P. scabriusculus and P. minutus.

MATERIAL AND METHODS
One male and four ovigerous females of Pilumnus reticulatus forma tessellata were collected in the Rio de la Plata Estuary (35º22'S, 55º36'W) during a fishing cruise of the "BIP Eduardo Holmberg" in November 1997 (Instituto Nacional de Investigación y Desarrollo Pesquero, Argentina, Station 669).The sample was taken with a bottom trawl, 1cm mesh size.The bottom (12 m deep) was muddy, with abundant colonies of the tubicolous polychaete Phyllochaetopterus socialis (Claparède, 1868).Pilumnus reticulatus was found mainly in crevices among the tubes.Water salinity and temperature at the bottom were 29.9 PSU and 17.3ºC respectively.
Two ovigerous females were transported to the laboratory of the Departamento de Biología, Unversidad de Mar del Plata, and maintained in an aquarium containing natural sea water until the eggs hatched (December 26, 1997).The larvae were transferred to beakers of 500 ml capacity for mass culture.Natural sea water was used at a temperature of 20°C and salinity of 35 PSU.Larvae were subjected to continual artificial light regime: 8/16 h (L/D).From zoea I to megalopa, Artemia sp.nauplii were offered as food.
Drawings and measurements were made using a Wild MZ6 and a Zeiss Axioscop compound microscope, both equipped with a camera lucida.All measurements were made with an ocular micrometer.Drawings were based on five larvae, and measurements made on 20 larvae, per stage.For zoeae, rostro-dorsal length (RDL) was measured from the tip of the rostral to the tip of the dorsal spines; carapace length (CL) from the base of the rostrum to the posterior margin; and carapace width (CW) as the distance between the tips of the lateral spines.In the megalopa stage, carapace length (CL) was measured from the base of the rostrum to the posterior margin; carapace width (CW) as the maximum width.The long natatory setae on the distal exopod segments of the first and second maxillipeds of zoeae are drawn truncated in Figure 5. Also, long setae of the scaphognathite of the maxilla of zoea IV and the megalopa, and of pleopod 2-4 of megalopa, are drawn truncated (Figs. 4 and 7 respectively).
Samples of larvae and the two adult females of Pilumnus reticulatus are deposited in the United States National Museum of Natural History, Washington, under the catalog numbers USNM 291173 (females) and USNM 291174 (zoeae 1 to 4 and megalopa).Descriptions and figures are arranged according to the standard proposed by Clark et al. (1998).
Antenna (Fig. 2F), protopod well developed exceeding tip of rostral spine, bearing 2 rows of spines.Exopod elongated, as long as protopod, with 1 long and 1 smaller medial setae; distal half acute, with 1 row of even smaller spines.
Third maxilliped present as small bud.
Telson (Fig. 6A,a) bifurcated with 3 pairs of serrulate setae and medial cleft on posterior margin; 3 spines on proximal part of each furcal arm: 1 large lateral, 1 small lateral and 1 small dorsomedial.Furca and large lateral spine spinulated.
Pereiopods present as rudimentary buds, the first bilobed.

Interspecific comparisons
Zoeae development in Pilumnus comprises four stages except in P. vespertilio (three stages; Lim and Tan, 1981) and P. kempi (two stages; Siddiqui and Tirmizi, 1992).The size and shape of zoeae varies among Pilumnus species.When the distance between the tips of dorsal and rostral carapace spines (RDL) in zoea I is considered, their sizes increase in the following sequence: Pilumnus minutus, P. scabriusculus, P. reticulatus, P. dasypodus, P. sayi, P. hirtellus, P. verspertilio and P. kempi (Table 3).However, the carapace length (CL) of P. reticulatus was similar to that of the larger P. hirtellus and its carapace width (CW) was the largest among all the species (Table 3).When the RDL/CL ratio was considered, all the species could be divided into two groups: P. scabriusculus, P. minutus and P. reticulatus had ratios of < 1.8 and P. dasypodus, P. sayi, P. hirtellus and P. verspertilio had ratios of >2.7.There was a statistically significant difference in the medi-LARVAL DEVELOPMENT OF PILUMNUS RETICULATUS 15 an values among species (Kruskal-Wallis one way ANOVA on Ranks, p = 0.00361), but the group or groups that differ from the others could not be isolated.However, when the length of both carapace spines was compared, the rostral spine was short in P. minutus, P. kempi, P. scabriusculus and P. vespertilio, intermediate in P. hirtellus, P. dasypodus and P. sayi and longest in P. reticulatus.

DISCUSSION
The characteristics of the larvae of family Pilumnidae were summarised by Ko (1997), who stated that the larval development of 13 species had been described up to that moment, based on laboratory rearing.The descriptions of the larval development of Pilumnus kempi (Siddiqui and Tirmizi, 1992), P. scabriusculus (Terada, 1990) and P. sayi (Bookhout and Costlow, 1979) were not mentioned by Ko (1997).In fact, P. sayi and P. dasypodus larval development were very similar, although Bookhout and Costlow (1979) showed few, but clear, differences.Including P. reticulatus larvae, the postembryonic development of eight Pilumnus species can be compared.Rice (1980) and Martin (1984) suggested classifications of xanthoid larvae including 4 and 6 groups respectively.In both classifications Pilumnus correspond to group II, characterised by "antennal exopod acutely tipped", about equal in length to a protopod, armed with small spines distally and with prominent outer setae about halfway along their length; antennal protopod longer than rostrum.Group II xanthoids have 4 zoeae stages.
Pilumnus is a uniform group within the Pilumnidae (Ko, 1997), but the zoeae characterisation made by this author needs some modifications.All Pilumnus zoeae are characterised by a hook-like or slightly curved dorsal spine and the presence of lateral spines, or dorso-lateral processes, on abdominal somites 2 and 3.The process of the second abdominal somite was often described as a knob (Lim and Tan, 1981;Salman, 1982;Ko, 1994) or as a projection that is "stout with horn-like tip directed anteriorly" (Bookhout and Costlow, 1979); that of the third somite is described as a spine (Bookhout and Costlow, 1979), a hook (Salman, 1982) or a knob (Lim and Tan, 1981;Ko, 1994), and is usually posteriorly curved.Sandifer (1974) stated that the armature of the abdomen (presence of mediolateral processes on somites 4 and 5, and posterolateral spines on somite 2) may be important as an aid in the identification of Pilumnus zoeae.Other characters that are present throughout the development are: RDL/CL ratio, carapace setae, a fringe of microtrichia on the maxillar endopod, the number of setae on the distal segment of the endopod of the first maxilliped, posteromarginal denticles on abdominal somites 2 to 6, and the number of spines on the furca.The following characters varied among the 8 species studied: P. reticulatus, P. sayi and P. minutus have mediolateral spines on abdominal somites 4 and 5; there are posterolateral spines on abdominal somites 3 to 5, except in P. vespertilio and P. kempi (2 to 5); P. reticulatus, P. kempi and P. hirtellus have denticles on the posterior margin of abdominal somites 2 to 5, and 6 when it was distinguished from the telson; the telson carries 3 pairs of spines on the distal border of all species except P. minutus, which had an additional internal pair from Z2; there were 3 spines in the base of each furcae (large and small lateral, small dorsal) except in P. vespertilio (2 spines).Interestingly, the basis of the first maxilliped has 10 setae (2, 2, 3, 3) in all species.
Several other differences among Pilumnus zoeae were observed regarding the setation formulae (Table 3): the number of setae of carapace, antennular exopod (including the aesthetascs), maxillular and maxillar coxal and basial endite, scaphognathite, distal segment of the endopod of first maxilliped, abdominal proximal somite and telson, and the presence of fringes of microtrichia in the maxilla.Additional differences concerning the timing of appearance of some structures in the larval sequence are summarised in Table 3: the bud of antennular and antennal endopod, the epipod of maxilla, the mandibular palp, the buds of third maxilliped, pereiopods and pleopods and the sixth abdominal somite.The abbreviated development can account for the observed early appearance of these structures in P. vespertilio and P. kempi (Lim andTan, 1981, Siddiqui andTirmizi, 1992).The zoeae examined during this study were larger (see Table 2) than those previously described by Montú et al. (1989), and also differed in setation.
On the other hand, megalopae of different Pilumnus species are similar in general morphological features and there is considerable variation in the setation formula among them (Table 4).Lim and Tan (1981) stated that "the megalops of P. vespertilio and P. dasypodus appear almost identical in general and gross features".Setation formula varies among species but also within them, and there are remarkable differences even between the two descriptions of P. dasypodus megalopae (Sandifer, 1974;Bookhout and Costlow, 1979).Consequently, it is not possible to use setation for detailed interspecific comparisons.Regarding size and shape, the megalopa of P. reticulatus, P. vespertilio and P. dasypodus were shorter than those of P. sayi, P. minutus and P. kempi but the former had a wider carapace relative to its length.

TABLE 1 .
-Time to the first appearance of each larval stage and measurements of Pilumnus reticulatus.

TABLE 3 .
-Comparison of morphological features and setation formulae of the zoeae development of Pilumnus species (P.dasypodus,