BACKGROUND In simultaneous hermaphrodites with copulation and internal fertilization it is often unclear whether reciprocal sperm exchange results from the unconditional willingness of both partners to donate and receive sperm, or whether it follows from a more controlled process such as conditional reciprocal sperm exchange, i.e. sperm trading. While in some sea slugs mating is assumed to be based on sperm trading, it seems to be unconditional in others. Here, we describe the unusual mating behaviour of Elysia timida, a small sacoglossan, focussing on indications for conditional reciprocity. RESULTS E. timida shows an as yet unique combination of a long series of hypodermic transfers followed by a short phase with standard insemination into a female genital aperture. Hypodermic transfer takes place in the form of repeated small injections into the dorsal surface of the partner, interrupted by synchronised circling movements. In the final mating phase sperm is transferred into the female genital aperture in a short period. In both phases the two mating individuals show a high degree of transfer symmetry and synchrony. While total duration and number of transfers were balanced within pairs, they varied significantly between pairs. Furthermore, looking at individual hypodermic transfers within pairs, reciprocal transfers lasted longer than unilateral transfers. Final sperm transfers were always reciprocal except for two cases which also diverted from the usual pattern in ways that were suggestive of a conflict over reciprocity. CONCLUSION Our results suggest that individual mating decisions in E. timida depend on what the partner does, indicating conditional reciprocity. If hypodermic transfers also involve the transfer of sperm (which remains to be confirmed), this system represents an up to now unique transition stage between hypodermic and standard insemination, both of which are widespread in this group of sea slugs, but never have been observed to co-occur within the same species.

中文翻译：

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海蛞蝓 Elysia timida (Opisthobranchia, Sacoglossa) 的交配行为：皮下注射、精子转移和平衡互惠。


背景 在同时进行交配和体内受精的雌雄同体中，通常不清楚互惠精子交换是否源于双方无条件捐献和接受精子的意愿，或者是否源于更受控制的过程，例如有条件互惠精子交换，即精子交易。虽然在一些海蛞蝓中，交配被认为是基于精子交易，但在其他海蛞蝓中，交配似乎是无条件的。在这里，我们描述了 Elysia timida（一种小型 sacoglossan）不寻常的交配行为，重点关注条件互惠的迹象。结果 E. timida 显示出迄今为止独特的组合，包括一系列皮下移植，然后是短阶段的标准授精到女性生殖器孔中。皮下注射以重复小量注射的形式发生在伴侣的背部表面，并被同步的盘旋运动打断。在交配的最后阶段，精子在短时间内转移到雌性生殖孔中。在这两个阶段，两个交配个体都表现出高度的转移对称性和同步性。虽然总持续时间和传输次数在配对内是平衡的，但它们在配对之间差异很大。此外，观察配对内的个体皮下转移，相互转移比单侧转移持续时间更长。最终的精子转移总是互惠的，除了两个案例，这两个案例也偏离了通常的模式，暗示了互惠方面的冲突。结论 我们的结果表明，E. timida 的个体交配决定取决于伴侣的行为，这表明有条件互惠。 如果皮下移植还涉及精子的转移（仍有待证实），那么该系统代表了迄今为止皮下注射和标准授精之间的独特过渡阶段，这两种技术在海蛞蝓中广泛存在，但从未被观察到在同一物种内同时发生。