Brood Reduction, Not Poecilogony, in a Vermetid Gastropod with Two Developmental Outcomes within Egg Capsules.

A small vermetid gastropod broods capsules containing nurse eggs and embryos that develop into small veligers. A few of these veligers continue development and growth while nurse eggs and developmentally arrested sibling veligers disappear. Survivors hatch as crawling pediveligers and juveniles. None of the veligers, if removed from capsules, swim in a directed way or withdraw into their shells, indicating that even the developing veligers are unsuited for extracapsular life until they can crawl. The shells of arrested veligers decalcify while their siblings grow. Few of the developmentally arrested veligers that were isolated from siblings and fed algal cells resumed detectable growth. Nurse eggs rather than cannibalism provide most of the food, but full growth of developing veligers depends on limited sharing; arrest of some siblings is a necessary adjunct of the nurse-egg feeding. Here, two developmental outcomes for larvae produced by developmental arrest of some (often termed poecilogony) serves instead as a means of brood reduction. Brood reduction is often attributed to family conflicts resulting from genetic differences. Another hypothesis is that a mother who cannot accurately sort numbers of nurse eggs and developing eggs into capsules could rely on brood reduction to adjust food for her offspring. At the extreme, an entirely random packaging would produce a binomial distribution of embryos in capsules, a very uneven distribution of food per embryo, and some capsules with no embryos. Males have yet to be found in this species, but even if reproduction is asexual, selection could still favor brood reduction.