Biological Journal of the Linnean Society, 2007, 90, 55–60. With 4 figures © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 55–60 55 Blackwell Publishing LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066© 2006 The Linnean Society of London? 2006 90? 5560 Original Article RESTING EGG SHAPE VARIATION IN A FRESHWATER FAIRY SHRIMP A. THIÉRY ET AL . *Corresponding author. E-mail: alain.thiery@up.univ-mrs.fr Modelling intraspecific resting egg shape variation in a freshwater fairy shrimp Tanymastix stagnalis (L., 1758) (Crustacea, Branchiopoda) ALAIN THIÉRY 1 *, NICOLAS RABET 2 and GABRIEL NÈVE 3 1 ER Biodiversité & Environnement, Laboratoire de Biologie Animale, Case 18, Université de Provence, 3 Place Victor Hugo, 13331 Marseille Cedex 3, France 2 Laboratoire de Biologie du Développement, Université Paris 7, 7 Quai St Bernard, 75005, Paris, France 3 EA Evolution Génome Environnement, Case 36, Université de Provence, 3 Place Victor Hugo, 13331 Marseille Cedex 3, France Received 2 March 2005; accepted for publication 1 March 2006 The shape of the resting eggs of a large branchiopod crustacean, the Anostraca Tanymastix stagnalis, is represented very accurately by analytical expressions. The occurrence of atypical shape of some T. stagnalis eggs may be viewed as a simple change of the analytical expression describing the usual egg shape. Their unusual shape may be explained by a higher embryo volume within an envelope of a given size. Biological implications are briefly discussed and hypothesized in an evolutionary point of view. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 55–60. ADDITIONAL KEYWORDS: Anostraca – cyst volume – shape model. INTRODUCTION Fairy shrimps (Branchiopoda, Anostraca) and partic- ularly Tanymastix stagnalis are known to live in tem- porary freshwater bodies, such as ponds, ditches, and rock pools (Young, 1976; Thiéry, 1987; Grainger, 1991). One of the main adaptations for survival in this kind of harsh habitat is the production of a unique type of egg, the cyst or resting egg, which is able to enter dia- pause and survive in the sediments during durable severe droughts common in arid biomes (Thiéry, 1996). Morphologically, in Anostraca, the shape of resting eggs has been recognized as a character of good taxonomic value (Mura & Thiéry, 1986; Thiéry & Gasc, 1991) with, at maturity, little intraspecific vari- ations in shape, such as small variations in diameter and in shape pattern over a large distribution area (Thiéry, Brtek & Gasc, 1995). The genus Tanymastix is distributed over Western Europe and North Africa (Brtek & Thiéry, 1995) and is represented by four species of which T. stagnalis (syn. Tanymastix lacunae) has the largest distribu- tion, from Mediterranean countries to Northern Germany. The three remaining species are endemic with restricted distributions: Tanymastix stellae in Corsica-Sardinia (Mura & Barsotti, 1985; Defaye, Rabet & Thiéry, 1998), Tanymastix motasi in Romania (Orghidan, 1945), and Tanymastix affinis in Western Morocco (Mura & Thiéry, 1986; Thiéry, 1987). The lens shape of resting eggs is typical of the genus (Freiner & Grüttner, 1984; Al-Tikrity & Graigner, 1990) and is unique among the Anostraca (Mura & Thiéry, 1986; Thiéry & Gasc, 1991; Thiéry et al., 1995). During a sampling of rock pools in the Fon- tainebleau forest, where abundant populations of T. stagnalis were known (Rabet, 1994), some cysts of an atypical shape were found in several clutches. Dur- ing a survey of populations in Southern France, some atypical eggs were found from Avignon area. Although polymorphism of antennae morphology has been described in Anostraca (Margalef, 1958; Nourisson, 1960), this is the first description of egg shape polymorphism in this order. The present study aims: (1) to describe the atypical cyst; (2) to quantify