Relationship between Function and Mechanical Properties of the Pleopods of Isopod Crustaceans

In the isopod crustaceans Idotea wosnesenskii and I. resecata, the first three pairs of abdominal appendages (pleopods) are locomotory, and the remaining two pairs are respiratory. We investigated the mechanical and morphometric properties of the pleopods in order to study constraints that may influence this division of labor. Measurements of the flexural (bending) stiffness of the pleopods demonstrated that the podites of the first and second pairs of pleopods were generally an order of magnitude stiffer than the fourth and fifth pleopods; the third pleopods were variable in stiffness. Anterior pleopods tend to be slightly thicker and have thicker cuticles, but these morphological differences do not entirely account for differences in flexural stiffness among pleopods; there must also be differences in modulus of elasticity (E). Estimates of E based on these stiffness data yield values in the middle of the range typical of arthropod cuticles. The differences in cuticle thickness and material properties between the anterior and posterior pleopods may help explain why the swimming pleopods do not make a significant contribution to gas exchange. Additional key words: cuticle, gills, stiffness, swimming In this study, our goal has been to investigate the physical and mechanical constraints on the closelylinked swimming and respiratory systems in isopod crustaceans. Much recent research has shown that the mechanical properties of an organism or its parts can have a major influence on the organism's way of life. The mechanical properties encompass both the geometric (morphological) features and the material properties of the structure, and either or both can play a role. For example, Koehl (1977a,b) showed that species of sea anemones from high-current and low-current areas have body walls with essentially the same material properties, but their different geometries give them different mechanical properties. Telewski and Jaffe (1986), comparing the bending resistance of pine trees from sheltered and windy areas, also found differences in mechanical properties due mostly to geometric differences. In contrast, Charters et al. (1969) found differences in both material and geometric properties between specimens of Eisenia (an upright alga) from sheltered and high-current environments. Components of an organism commonly have mula Corresponding author: Dept. of Entomology, 2045 Haworth Hall, University of Kansas, Lawrence, KS 66045-2106, USA. tiple functions, both primary and secondary. In geckos, the outer layer of skin is loosely attached and has numerous zones of weakness where it can easily be torn (Bauer et al. 1989); when grasped by a predator, large areas of skin can be sloughed off, giving the lizard a b tter chance of escaping. Thus, a predator-escape function has been added to the skin's primary function as an integumentary barrier. Similarly, in isopod crustaceans, the abdominal appendages, or pleopods, are the primary site of gas exchange (McLaughlin 1980; Schram 1986) and ion exchange (Holliday 1988). Many aquatic isopod groups have independently evolved a secondary function for pleopods in which they are used as paddles for swimming (Schram 1986). Early morphological studies (reviewed by McLaughlin 1980) suggested that such swimming isopods, rather than using all pleopods for both functions, display a division of labor among the pleopods. Recent experiments have confirmed that the anterior pleopods are used for swimming and the posterior pleopods act as gills (Alexander 1988; Alexander & Chen 1989). In the present study, we investigated what mechanical requirements of paddles might prevent them from acting as gills, and what physical requirements of gills might prevent them from paddling. Our approach was to measure the flexural stiffness (resistance to lengthThis content downloaded from 207.46. 13.52 on Mon, 24 Oct 2016 04:13:10 UTC All use subject to http://about.jstor. org/terms Alexander, Blodig, & Hsieh