Polygynandry and sexual size dimorphism in the sea spiderAmmothea hilgendorfi(Pycnogonida: Ammotheidae), a marine arthropod with brood‐carrying males

Abstract

Species that exhibit uniparental postzygotic investment by males are potentially good systems for investigating the interplay of sexual selection, parental care and mating systems. In all species of sea spiders (Class Pycnogonida), males exclusively provide postzygotic care by carrying fertilized eggs until they hatch. However, the mating systems of sea spiders are poorly known. Here we describe the genetic mating system of the sea spider Ammothea hilgendorfi by assaying nearly 1400 embryos from a total of 13 egg-carrying males across four microsatellite markers. We also determine the extent of sexual dimorphism in trunk and leg size, and assess how reproductive success in males varies with these morphological traits. We detected instances of multiple mating by both sexes, indicating that this species has a polygynandrous mating system. Genotypic assays also showed that: males do not mix eggs from different females in the same clusters; eggs from the same female are often partitioned into several clusters along a male's oviger; and clusters are laid chronologically from proximal to distal along ovigers. Females were on average larger than males with respect to leg length and width and trunk length, whereas males had wider trunks. Among the genotyped egg-carrying males, neither the number of eggs carried nor the number of mates was correlated with body-size traits. Nevertheless, the high variance in mating success, genetically documented, suggests that males differ in their ability to acquire mates, so future studies are needed to determine what traits are the targets of sexual selection in this species. In addition to providing the first description of the mating system in a sea spider, our study illustrates the potential uses of this group for testing hypotheses from parental investment and sexual selection theories.

Quantitative measures of sexual selection reveal no evidence for sex-role reversal in a sea spider with prolonged paternal care

“…Pycnogonids have been considered as ideal candidates for studying the evolution of male parental care (Tallamy 2001;Bain & Govedich 2004a), but P. stearnsi is only the second species for which the genetic mating system has been elucidated (the first was Ammothea hilgendorfi; Barreto & Avise 2008). Behavioural observations of a morphologically similar congener ( Jarvis & King 1972) led us to hypothesize that male P. stearnsi, owing to the substantial egg masses they carry, would show limited capacity for multiple mating.…”

“…Genomic DNA extraction and PCR conditions (reagent concentrations and thermal profiles) were performed for adults and larvae as described for another pycnogonid (Barreto & Avise 2008). Three highly polymorphic loci were chosen for this study after showing no significant deviations from Hardy -Weinberg equilibrium, no linkage disequilibrium and high parentage exclusion probabilities (electronic supplementary material, table S1).…”

Relative sexual size dimorphism in Centrobolus digrammus (Pocock) compared to 18 congenerics

“…Sexual size dimorphism is prevalent in arthropods and females are usually larger than males e. g. beetles [1] , sea spiders [2] , orthopterans [15] . Behavioural patterns such as provisioning versus non-provisioning relate to SSD [3] .…”

Multiple mating and clutch size in invertebrate brooders versus pregnant vertebrates

“…In turn, these inherent fertility differences between the genders routinely translate into shallower sexual-selection gradients (39) for females than for males in many species, meaning that a male's reproductive success can increase greatly with higher mate counts, whereas a female's genetic fitness inherently is truncated by her limited fecundity, regardless of her particular mating behavior. Thus, conventional evolutionary wisdom holds that males often experience stronger selection pressures for multiple mating than do females, and this difference in sexual selection pressure is precisely why the relatively few species with male-pregnancy and/or sex-role reversal are of special evolutionary interest (36,40).…”