Abstract
Sibling cannibalism is commonly observed in marine species. For instance, intrabrood cannibalism has been documented in marine gastropods with direct development, suggesting a relationship between embryo behavior and the evolution of life history strategies. However, there has been little effort to document the factors driving sibling cannibalism in marine species. The kin selection theory suggests that the level of relatedness plays an important role in cannibalism patterns. We examined Crepidula coquimbensis, a marine gastropod that broods and encloses its brooded offspring in capsules. Encapsulated embryos show sibling cannibalism and high levels of intracapsular multiple paternity. Given these features, cannibalistic behavior may be driven by kin-relatedness. To test this hypothesis, we constructed artificial aggregations of embryos to mimic three levels of relatedness: high, medium and low. For each category of aggregation, the cannibalism rate and benefits (i.e. size at hatching of surviving offspring) were estimated. In addition, at the end of embryo development, we performed parentage analyses to determine if cannibalism was associated with the relatedness between cannibal and victim embryos. Our results show that the intensity of sibling cannibalism increased in aggregations characterized by the lowest level of relatedness. There were important benefits of cannibalism in terms of hatching cannibal size. In addition, cannibalism between embryos was not random: the variation in reproductive success between males increased over the course of the experiment and the effective number of fathers decreased. Altogether, these results suggest that polyandry may play an important role in the evolution of sibling cannibalism in C. coquimbensis and that kin selection may operate during early embryonic stages in this species.
Highlights
Sibling cannibalism is a widespread phenomenon observed in nature [1,2,3,4]: cannibal individuals prey on sibs for food and to escape from limiting resources [5,6,7]
Different cannibalistic behavior Three different cannibalistic behaviors were observed between siblings: (1) cannibalism by engulfing [45]: one embryo cannibalizes a smaller embryo by engulfing it (Fig. 1B); (2) hypercannibalism by engulfing: one cannibal engulfs another embryo which had previously cannibalized an embryo (Fig. 1C); and (3) hypercannibalism by sucking: one cannibal preys on another cannibal embryo, sucking its contents and killing it (Fig. 1D)
Our results showed that in C. coquimbensis: (1) three cannibalistic behavioral strategies were observed among embryos; (2) the intensity of sibling cannibalism was greater at the lower levels of relatedness, with important benefits in terms of size at hatching and (3) cannibalism between embryos was associated with an increased variance in male reproductive success
Summary
Sibling cannibalism is a widespread phenomenon observed in nature [1,2,3,4]: cannibal individuals prey on sibs for food and to escape from limiting resources [5,6,7]. Experimental data show that food limitation acts as an important trigger (e.g. spadefoot toad tadpoles [7], tiger salamander larvae [17], parasitic wasps [18,19] and a terrestrial gastropod [20]). Higher cannibalism rates have been observed in spadefoot toad tadpoles for which there are few heterospecific prey [7]. Oxygen may become a limiting factor for embryos in species that aggregate their offspring [21,22,23]. In the marine gastropod Acanthina monodon, which encapsulates its eggs during their development, the cannibalism rate has been shown to increase as levels of oxygen availability decrease inside capsules [24]
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