Life history and sex allocation in the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema: the role of sperm competition

Abstract

Sex allocation theory predicts that, in hermaphroditic organisms, individuals allocate a fixed amount of resources divided among male and female functions to reproduction and that the proportion devoted to each sex depends on the mating group size. As the mating group size increases, hermaphrodites are predicted to allocate proportionally more resources to the male and less resources to the female function (approaching equal allocation to both sexes) to face increased sperm competition. Up to now little experimental evidence has been provided to support the theory in hermaphroditic animals. Facultative shift between male and female allocation in response to variation in local group size does occur in several taxa but not always in the expected direction and not with similar patterns. In the protandric and then simultaneously hermaphroditic polychaete worm Ophryotrocha diadema reproductive resources are flexibly allocated in the protandrous and the hermaphroditic phase. The cost of male reproduction during adolescence is spread over the whole energy budget of the animal as shown by the shortening of lifespan and the lowering of growth rate in individuals with enhanced male expenditure during the protandrous phase. Moreover, in this species, short term sex allocation adjustments differ from those described in other taxa. Individuals regulate their reproductive output so that where reproductive competitors are present, the number of female gametes is strongly reduced but the number of male gametes (although it changes) is not significantly increased. Resources subtracted from the female function are not directly allocated to sperm production, but to expensive male behaviors that are likely to enhance male reproductive success. These results are discussed in the light of the relevance of sexual selection in large populations of hermaphrodites.

Sex allocation and investment into pre- and post-copulatory traits in simultaneous hermaphrodites: the role of polyandry and local sperm competition

“…Allocation to sperm production thus continues to pay off, predicting a more linear male fitness gain curve and a shift towards a more equal sex allocation (figure 1d). While evidence for the former prediction is still relatively limited [26][27][28], the latter prediction is increasingly well supported [11,[29][30][31][32]. The decisive factor for a saturating male fitness gain curve would therefore appear to be the level of LSC resulting from variation in the mating group size (i.e.…”

Gender expression and group size: a test in a hermaphroditic and a gonochoric congeneric species of Ophryotrocha (Polychaeta)

“…Male and female allocation may not compete for a common resource pool if there are temporal differences in the activity of both sex functions or when the nutrients required to build male and female tissues differ. Under such conditions, male or female investment may not trade-off with each other, but with other life-history traits (Schärer et al, 2005;Lorenzi et al, 2006). According to Locher and Baur (Locher and Baur, 2002) trade-offs in resource allocation may not occur or be less pronounced under favourable conditions, whereas under stressful conditions, such as limited food supply, high temperature or drought, the energy intake might be carefully shared among different functions, including reproduction (Reznick, 1992).…”

No effect of group size on sex allocation in a protandric-simultaneous hermaphroditic shrimp

“…First, males should alter the timing of maturation as hermaphrodites according to male mating opportunities (Baeza 2007). This idea has received empirical support from some studies (in Lysmata wurdemanni -Baeza, 2007;Baeza & Bauer, 2004) but not from others (in Ophrytrocha diadema- Lorenzi et al, 2006). Second, hermaphrodites should adjust the proportion of resources allocated to sperm vs ova according to group size (number of male competitors in the population) (Charnov 1982;Fischer, 1984;Schärer & Ladurner, 2003).…”