Copulation in the neritic chaetognath Sagitta crassa

The number of spermatophores found attached to Sagitta crassa,the dominant chateognath in Tokyo Bay, ranged from one to four.Individuals with one spermatophore were most abundant, accountingfor 70% of chaetognaths with spermatophores (A). Using the meanminimum body length of chaetognaths with spermatophores (A),those without spermatophores were subdivided into two categories:those >7 mm (B) were considered to be mature animals, andthose <7 mm (C) to be immature. The ratio of chaetognathswith spermatopbores (A) to all chaetognaths (A+B+C) varied withtime, with high values from 0040 through 0120 h as in a previousstudy. Variations in the ratio of chaetognaths with spermatophores(A) to all adults (A+B) showed a pronounced peak at 0100 h.Thus, it was confirmed that copulation of S. crassa is a periodicactivity with a peak soon after midnight. Most of the chaetognathpairs exchange one spermatophore. The frequency of copulationis considered to be two times per night at most because thechaetognaths have a single pair of seminal vesicles. However,the presence of chaetognaths with 3–4 spermatophores suggeststhat occasionally a reciprocal exchange of spermatophores doesnot occur. This could be called false copulation. It is suggestedthat copulation in S. crassa including false copulation occursat most four times per night.