Spermcasting of spermatozeugmata by the bivalves Nutricola confusa and N. tantilla

The dynamics and consequences of the varied reproductive modes of marine invertebrates is a rich and vibrant field of inquiry for ecological and evolutionary studies. One mode of reproduction that is not as well‐studied as others is “spermcasting” or “spermcast mating,” when males broadcast sperm and females retain eggs and brood developing embryonic stages. This type of reproduction occurs in two small (maximum adult shell length ~5–6 mm) venerid bivalves, Nutricola confusa and N. tantilla, that live in protected bays of the temperate eastern Pacific. Females of these species brood developing embryos in chambers formed by the inner demibranchs, and release fully formed juveniles. We discovered that upon exposing clams to fluvoxamine, a selective serotonin reuptake inhibitor, males release spermatozeugmata, clusters of sperm cells attached by their heads to a central core. Spermatozoa of Nutricola have unusually long, needle‐shaped heads that are approximately one quarter of the total length of the cell. These heads are curled and “packaged” into the hemispherical‐shaped cores of spermatozeugmata. The cores are about one‐third as long as the heads, and the tails protrude out of the opposite side of the cap of the core. The spermatozeugmata display two different swimming patterns, one where the tails beat in synchrony, and the other where they do not. The size of the cores is not significantly different in the two species, but spermatozeugmata of N. tantilla have significantly longer and wider tails than those of N. confusa. Advantages to spermcasting spermatozeugmata instead of individual spermatozoa may include enhanced dispersal and increased probabilities of fertilization. One consequence of spermatozeugmata (rather than individual spermatozoa) entering female brood chambers might be lowering of the effective population size. For species like these, which lack pelagic larvae, spermatozeugmata could increase dispersal and gene flow.