DWARFS OR GIANTS? SEXUAL SIZE DIMORPHISM IN (COPEPODA, )

BY

P. ØSTERGAARD1,2,4),G.A.BOXSHALL1) andD.L.J.QUICKE2,3) 1) Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K. 2) Department of Biological Sciences, Imperial College London, Silwood Park Campus, Ascot SL5 7PY, U.K. 3) Department of Entomology, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.

ABSTRACT

Sexual size dimorphism in the Chondracanthidae is very marked: we investigate whether it is a consequence of dwarfing of males or females becoming giants. Plotting male size against female size indicates that the relationship is allometric, i.e., showing a trend towards increasing sexual size dimorphism. This is supported by the results of an analysis of sexual size dimorphism against morphological distance, although not significant when phylogeny is controlled for. It appears that both sexes are driving the sexual size dimorphism, and chondracanthid females are probably selected for high fecundity leading to large body size, whereas males are probably selected for small size.

RÉSUMÉ

Le dimorphisme sexuel concernant la taille chez les Chondracanthidae est très marqué : nous avons recherché si ce dimorphisme s’expliquait par des mâles devenant nains ou des femelles devenant géantes. La comparaison de la taille du mâle par rapport à celle de la femelle indique que leur relation est allométrique, montrant une tendance vers l’augmentation du dimorphisme sexuel de la taille. Ceci est soutenu par les résultats de l’analyse du dimorphisme sexuel de la taille par rapport à la distance morphologique, bien que non significatifs quand on considère la phylogénie. Il apparaît que les deux sexes sont impliqués dans l’acquisition du dimorphisme sexuel relatif à la taille ; la sélection chez les Chondracanthidae femelles en vue d’une fécondité élevée a abouti à une grande taille du corps, tandis que chez les mâles, la sélection a probablement conduit vers une petite taille.

4) Author for correspondence; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2005 Crustaceana 78 (4): 397-408 Also available online: www.brill.nl 398 P. ØSTERGAARD, G. A. BOXSHALL & D. L. J. QUICKE

INTRODUCTION

Sexual dimorphism is defined as morphological differences between sexually mature males and females (Fairbairn, 1997) and sexual size dimorphism (SSD) is a common phenomenon within sexually reproducing species, though its expression varies with males typically being larger than females in mammals and birds, while the opposite is the case for most other taxa. Among the parasitic a variety of different SSD strategies is found. Some species do not show any SSD in body length (which does not mean they cannot show SSD in other morphological traits, e.g., maxilliped and size). Examples include the minimus Otto, 1821 (Caligidae) and the harpacticoid Tegastes cnidicus Humes, 1981 (Tegastidae) (Kabata, 1979; Humes, 1981). However, most species show some degree of body length SSD. Occasion- ally, males are larger than their female counterpart, but in such cases they are only slightly larger (to a maximum of 1.25 times the body length of the female). Ex- amples include both symbionts of , such as Monocheres cagarren- sis Johnsson & Bustamente, 1997 (Asterocheridae) on sponges (Johnsson & Bus- tamante, 1997), and fish parasites such as Müller, 1785 (Caligi- dae) (Kabata, 1979). In the great majority of cases, SSD is female-biased and can be extreme. In fish parasites such as Caligus zei Norman & Scott, 1906 (Caligi- dae) (Kabata, 1979), and in symbionts of hosts such as Scottomyzon gibberum (T. Scott & A. Scott, 1894) (Asterocheridae) on (Gotto, 1993) and Lichomolgus digitatus Humes & Ho, 1968 () on corals (Humes & Ho, 1968) the females are about 1.25 times the body length of the males. However, more extreme cases of SSD are found in the fish parasite families Lernaeopodidae (e.g., Clavellisa scombri (Kurz, 1877)) and Sphyriidae (e.g., Tri- paphylus musteli (Van Beneden, 1851)), where the postmetamorphic adult females are approximately 25 times the body length of the males (Kabata, 1979, 1992). Fe- males of the fish parasite Linnaeus, 1767 (Pennellidae) are about 45 times the body length of the males (Kabata, 1992). The most extreme case of sexual size dimorphism involves Gonophysema gullmarensis Bresciani & Lützen, 1960 in the ascidian, Ascidiella aspersa (Müller, 1776), which is crypto- gonochoristic, with the mature male reduced to little more than a testis, which is housed within a special receptacle in the female genital apparatus (Bresciani & Lützen, 1961). The proportional relationship between the mean values of the trait of two sexes is the most intuitive quantitative measure of SSD and is often expressed as the ratio of male : female body size. However, the use of ratios has been questioned and various other measures have been proposed. For example, the use of residuals from the regression of male against female values has been suggested (Ranta