CONJOINED TWINS IN THE BURROWING CRAYFISH, VIRILASTACUS RUCAPIHUELENSIS RUDOLPH & CRANDALL, 2005 (DECAPODA, PARASTACIDAE) BY ERICH H. RUDOLPH1) and ANDREA W. MARTÍNEZ Departamento de Ciencias Básicas, Universidad de Los Lagos, Casilla 933, Osorno, Chile ABSTRACT The occurrence of two pairs of newly hatched conjoined twins is reported in an oviposition of a female-phase, intersex specimen of the burrowing parastacid Virilastacus rucapihuelensis, caught in the peatlands of Rucapihuel, southern Chile. These conjoined twins were found among 49 normal, stage 1 juveniles and constitute the third reported case of Siamese juveniles in freshwater astacids. Their external morphological traits are described, as well as the type and extent of their fusion; a morphological comparison is made between them and previous cases of conjoined twins in Astacoidea. The stage of embryonic development when they would have formed, and the probable causes of their origin, are also analysed. RESUMEN Se reporta el hallazgo de dos parejas de gemelos siameses, recientemente eclosionados, en una ovipostura de un espécimen intersexo en fase hembra del parastácido excavador. Virilastacus rucapihuelensis, capturado en los humedales de la localidad de Rucapihuel en el sur de Chile. Estos siameses se encontraron con otros 49 juveniles en estado 1 normales y constituyen el tercer caso reportado de juveniles siameses en astácidos de agua dulce. En ellos se describen sus rasgos morfológicos externos, como así mismo el tipo y extensión de su fusión, se establece una comparación morfológica entre ellos y con aquellos casos previos de siameses en Astacoidea. También se analiza el momento del desarrollo embrionario en que ellos se habrían formado y las probables causas de su origen. INTRODUCTION There are numerous reports of teratisms and other morphological abnormalities in crustaceans. Many of these are compiled and systematized in Hart & Clark (1987) and Jara & Palacios (2001). However, records of conjoined twins are 1) Fax: +56.64333455; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2008 Crustaceana 81 (11): 1347-1355 Also available online: www.brill.nl/cr DOI:10.1163/156854008X369528 1348 ERICH H. RUDOLPH & ANDREA W. MARTÍNEZ scarce. Ryder (1886) reports the occurrence of conjoined twins in larvae of the American lobster, Homarus americanus H. Milne Edwards, 1837, fused together along the lateral or dorsal surface of their cephalothoraxes, but all with separate and divergent abdomens. Herrick (1895) describes the occurrence of Siamese embryos during H. americanus development. Zipf (1956) reports finding a pair of stage 1 juveniles in the crayfish, Cambarus longulus longulus Girard, 1852, fused along the dorsal cephalothoracic surface. Williams (1988) describes an adult shrimp of Trachypenaeus similis (S. I. Smith, 1885) as a conjoined twin, indicating that it is composed of two cephalothoraxes joined to only one abdomen. Harzsch et al. (2000) report the discovery of a pair of Siamese embryos in Homarus americanus, fused together along the cephalic region and analyse the structure of their nervous system. Jara & Palacios (2001) report the discovery of a pair of Siamese juveniles fused along the dorsal cephalothoracic region in the freshwater anomuran, Aegla abtao Schmitt, 1942. Harlioglu˘ (2002) reports a pair of stage 1 juveniles of the signal crayfish, Pacifastacus leniusculus (Dana, 1852), fused along the dorsal cephalothoracic region. Alwes & Scholtz (2006) study the embryonic development of the parthenogenetic marbled crayfish [“Marmorkrebs”: as yet without a scientific name or clear affilation], and describe the occurrence of a pair of twin embryos fused along the cephalic region, and sharing an optic lobe and the primordium of an antennule. In this article, we report the first finding of two pairs of recently hatched conjoined twins, detected in an oviposition of a female-phase intersex specimen of the burrowing parastacid, Virilastacus rucapihuelensis Rudolph & Crandall, 2005, and describe their external morphology. MATERIAL AND METHODS At the end of April, 2007, one intersex, female-phase type 2 specimen of the burrowing crayfish, Virilastacus rucapihuelensis was caught in peatland located in Rucapihuel (40◦35S73◦34W), southern Chile (for characteristics of this sexual morph, cf. Rudolph et al., 2007), measuring 27.9 mm cephalothoracic length (CTL), and with developing eggs. The eggs were observed in the laboratory under a Wild stereoscopic microscope, and the presence of developing embryos within the eggs was confirmed. Subsequently, the intersex specimen was placed in a darkened, acrylic 35 × 31 × 12 cm aquarium and maintained for a period of three months in conditions similar to those described by Rudolph & Rojas (2003). During the hatching process, two pairs of conjoined twins were obtained and, immediately after their death, these were photographed with Wild equipment. The photographs were manipulated in Adobe Photoshop 6.0. The length of the cephalothorax and of.