Sperm precedence in a novel context: mating in a sessile marine invertebrate with dispersing sperm

The compound ascidian Diplosoma listerianum releases aquatic sperm which ar e dispersed passively to potential mates as individual gametes prior to sto rage of sperm, internal fertilization and brooding of embryos. The storage of exogenous sperm enables D. listerianum to produce a lengthy series of pr ogeny following a brief period of mating. Molecular paternity analysis foll owing sequential mating of colonies in laboratory culture revealed a consis tent pattern with a clear initial bias in paternity towards the first of tw o acting males. The sites of sperm storage and fertilization and the morpho logy of the ovary in D. listerianum suggest that this bias reflects first-i n-first-out use of individual stored gametes. The proportion of second-male paternity subsequently increased with time within the progeny arrays. This may have reflected the ageing or passive loss of first-male sperm. It is a lso possible that the modular nature of the organism contributed to this te mporal trend any recently budded colony modules maturing in the interval be tween matings would have been available exclusively to second-male sperm as virgin zooids. Two sets of mating trials were run. In the first, the colle ction of progeny suffered an interruption of 13 days and each male gained a larger proportion of recorded paternity within the progeny analysed when m ating first rather than when mating second. In one mating combination, the first male obtained almost 100% of recorded paternity In the second set of trials, with different clonal combinations, the complete sequence of progen y was collected and the estimated overall proportion of second-male paterni ty (P-2) was consistently > 0.5. Taken as a whole, the results suggest that the overall P-2-value can vary widely within the population studied. Propo sed mechanisms of mating-order effects in species with copulatory mating in clude several which can have no counterpart in indirect aquatic mating sinc e they involve the active removal, sealing off, volumetric displacement or incapacitation of first-male ejaculates. It is nevertheless clear that mati ng-order effects can be pronounced during the type of non-copulatory mating examined here, which is widespread in marine invertebrates.