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

Abstract

SUMMARYHermaphroditism and gonochorism are two contrasting forms of sexuality. Hermaphroditic species are generally seen as species adapted to conditions of low density, stabilized by poor mate search efficiency and high costs of searching. They can adjust allocation of reproductive resources to each sex function in response to current social conditions, making reproduction more efficient, at least in principle. By contrast, gonochorism (separate sexes) is advantageous when mates are frequent, making it ineffective to maintain two sex functions in a single individual. This, however, also rules out the need for a flexible response to mating opportunities as known for hermaphrodites. In the hermaphroditic marine polychaete worm Ophryotrocha diadema we showed earlier that group size is assessed through a chemical cue. In this study we verified the accuracy of the response to gradients of the chemical cue used to assess group size by O. diadema by checking reduction in egg production as the group of partners increases, as expected according to sex allocation theory. Furthermore we compared the effect of such a gradient with a similar gradient in a closely related gonochoric species (O. labronica). Here sex allocation adjustment is not predicted, thus an adaptive change in egg production in response to group-size cues is not expected. In fact, our results show that the group-size effect only occurs in O. diadema and not in O. labronica. Moreover, our study provides evidence of high perceptual abilities of chemical cues in O. diadema, suggesting that perceiving social cues and adjusting sex allocation appropriately are special properties of hermaphrodites.

Does the cost of a function affect its degree of plasticity? A test on plastic sex allocation in three closely related species of hermaphrodites

“…O. diadema worms strongly decreased the amount of resources they invested in the female function when the number of rivals increased from low to intermediate mating opportunities. These results confirm that these worms make accurate estimates of their social environment as they respond to changes from one to three adult conspecifics (Schleicherová et al, , 2010. Instead, O. adherens worms processed variations between low and intermediatemating opportunities, but did so less accurately than O. diadema worms did.…”

“…Indeed, these worms sense the number of potential mates through waterborne chemical cues and adjust their male and female allocation appropriately, whereas encounter probability has no effect on sex allocation . The effect of these chemical cues is so strong that we can manipulate the sex allocation ofworms by simply varying the amount of cues rather than varying the number of worms in the enclosures (Schleicherová et al, 2010). These worms have kept plasticity in sex allocation although cultured in dense laboratory populations for generations (Schleicherová et al, 2013).…”

“…Tan et al, 2004 in leeches;Baeza, 2007 in shrimps; Hoch and Levinton, 2012 in barnacles;Schärer and Ladurner, 2003 in flatworms). Conversely, other species show a plastic female allocation and an almost fixed male allocation (Lorenzi et al, 2005;Schleicherová et al, 2006Schleicherová et al, , 2010. In contrast to the above animals, which mate through behavioral interactions, sessile colonial invertebrates may withhold female investment pending sperm reception (Hughes et al, 2002 in bryozoans).…”

Demographic costs of sex allocation: hermaphrodites perform better in sparse populations

“…When more than two worms are present, they adjust their sex allocation by investing proportionally more resources into the male function (Lorenzi et al 2005, mate promiscuously (Sella & Lorenzi 2000) and can share the paternity of a single egg-cocoon with other hermaphrodites . Sex allocation adjustments are not costly in the short term ) and polychaetes sense the number of conspecifics and/or potential mates through waterborne chemical cues (Schleicherová et al , 2010Minetti et al 2013). …”

Crepidula Slipper Limpets Alter Sex Change in Response to Physical Contact with Conspecifics

“…Sex allocation in both simultaneous and sequential hermaphrodites is often influenced by the presence of conspecifics (Collin et al, 2005;Velando et al, 2008;Schleicherová et al, 2010). Sex allocation theory predicts that the optimal size to change sex in sequential hermaphrodites and the optimal allocation to male and female function in simultaneous hermaphrodites depend on the composition of an individual's mating group or local population (Charnov, 1982, and references therein).…”