Different effects of mating group size as male and as female on sex allocation in a simultaneous hermaphrodite

Abstract

Sex allocation theory predicts that the optimal sexual resource allocation of simultaneous hermaphrodites is affected by mating group size (MGS). Although the original concept assumes that the MGS does not differ between male and female functions, the MGS in the male function (MGSm; i.e., the number of sperm recipients the focal individual can deliver its sperm to plus one) and that in the female function (MGSf; the number of sperm donors plus one) do not always coincide and may differently affect the optimal sex allocation. Moreover, reproductive costs can be split into “variable” (e.g., sperm and eggs) and “fixed” (e.g., genitalia) costs, but these have been seldom distinguished in empirical studies. We examined the effects of MGSm and MGSf on the fixed and variable reproductive investments in the sessilian barnacle Balanus rostratus. The results showed that MGSm had a positive effect on sex allocation, whereas MGSf had a nearly significant negative effect. Moreover, the “fixed” cost varied with body size and both aspects of MGS. We argue that the two aspects of MGS should be distinguished for organisms with unilateral mating.

Mating behavior and reproductive morphology predict macroevolution of sex allocation in hermaphroditic flatworms

“…Finally, outcrossing frequency is a strong predictor of sex allocation, with more female-biased allocation under high selfing rates [ 20 ]. Similar patterns are found in animals with generally a more male-biased allocation with higher density and/or mating group size (both of which likely correlate with sperm competition intensity) (e.g., [ 22 – 25 ]). Furthermore, there is some evidence of a more female-biased allocation with increasing body size (reviewed in [ 12 ]) and with increased selfing [ 26 , 27 ].…”

The effects of parasitism on sex allocation of a hermaphroditic acorn barnacle

“…The MGS as males (i.e., the number of female partners within reach of the penis; Yamaguchi et al 2013; Tamechika et al 2020) is expected to be reduced by infestation, as the penises of infested barnacles are shorter than those of uninfested ones. In sessile animals such as barnacles that mate using a penis, MGS is limited by the area within reach of the penis (Neufeld and Palmer 2008; Hoch 2016).…”

“…Then, we dissected out the operculum (consisting of two plates, the scutum and tergum), soma, penis, and eggs (if present) from the samples (see Appendix Table 1). The operculum was dried in an oven at 60 °C for 12 h and weighed to provide an index of body size (Kado et al 2009; Tamechika et al 2020). We counted the number of eggs for brooded individuals, and then randomly selected three egg capsules for each individual and measured their major and minor axes under a stereoscopic microscope.…”

“…Sex allocation theory was originally developed for barnacles (Charnov 1980(Charnov , 1982(Charnov , 1987, and empirical studies have shown the adaptive plasticity of sex allocation in hermaphroditic barnacles (Raimondi and Martin 1991;Yusa 2018). Barnacles are sessile and mate using a long penis (Anderson 1993; except for the unique mode of "sperm casting": Barazandeh et al 2013Barazandeh et al , 2014Barazandeh and Palmer 2015), and these characteristics facilitate the determination of MGS by researchers (Hoch 2008;Neufeld and Palmer 2008;Tamechika et al 2020). Some barnacles are infested by parasites such as isopods and rhizocephalans (Crisp 1960;Høeg et al 1990;Høeg 1995;Fong 2016;Fong et al 2019a).…”