Cannarsa E and Meconcelli S (2017), Increased population density reduces body growt...

ECB-ART-46157

Curr Zool 2017 Apr 01;632:151-157. doi: 10.1093/cz/zow030.

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Increased population density reduces body growth and female investment in a simultaneous hermaphrodite.

Cannarsa E , Meconcelli S .

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Sex allocation theory applied to hermaphrodites assumes that there is a trade off between the allocation of resources to male and female functions, within a fixed reproductive resource budget. Charnov''s classic resource allocation model predicts a more female-biased sex allocation when competition among different sperm donors is low due to diminishing fitness returns for male investment. By manipulating the social group size, one automatically changes the population density at which individuals live. Increasing population density may affect reproductive allocation, leading to resource competition and/or to increased concentration of harmful metabolites. This could lead to an over- or underestimation of the individual adjustment of sex allocation responses to mating opportunities. In this article, we tested the effects of density and social group size separately on female investment and body growth (considered as proxy of the overall energy budget) in the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema. We manipulated social group size (i.e., monogamous and promiscuous regimes) and density (i.e., 4 levels) using a full-factorial design, to identify the underlying factor affecting female allocation (in terms of egg production) and body growth. In contrast to findings of previous experiments, we found that an increase in population density reduced body growth and egg production of hermaphrodites irrespective of social group size. We advance the hypothesis that the increase of catabolites and oxygen consumption in high-density conditions reduces the overall resource budget and this could obscure group size effects on female fecundity.

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References [+] :

Brauer, Phenotypically flexible sex allocation in a simultaneous hermaphrodite. 2007, Pubmed

	Figure 1. Mean egg number per capita (±SE) laid by hermaphrodites reared in monogamy and promiscuity, according to 4 density levels (very high: 0.5 ml of water per worm; high: 2.5 ml per worm; intermediate: 7.5 ml per worm; and low 25 ml per worm). Continuous line = monogamy. Dashed line = promiscuity.
	Figure 2. Cocoon number (± SE) per capita laid by hermaphrodites reared in monogamy and promiscuity, according to 4 density levels (very high: 0.5 ml of water per worm; high: 2.5 ml per worm; intermediate: 7.5 ml per worm; and low 25 ml per worm). Continuous line = monogamy. Dashed line = promiscuity.
	Figure 3. Mean values (± SE) of body growth reached by hermaphrodites reared in monogamy and promiscuity, according to 4 density levels (very high: 0.5 ml of water per worm; high: 2.5 ml per worm; intermediate: 7.5 ml per worm; and low 25 ml per worm). Continuous line = monogamy. Dashed line = promiscuity.