STRATHMANN, Richard R.; STRATHMANN, Megumi F.; HADFIELD, Michael G.; University of Washington; University of Washington; University of Hawaii: Nurse Egg Feeding Enhanced by Developmental Arrest of Sibling Veligers

A complex form of adelphophagy, with nurse egg feeding by some siblings and arrested growth at the veliger stage for others, occurs in the brooded capsules of a small vermetid gastropod. Capsules initially contain more than 150 eggs, each of 0.1mm diameter. Most eggs do not develop and are consumed as nurse eggs. About ten to twenty develop to well formed small veligers with foot, operculum, velar lobes, prototrochal cilia, eyes, lenses, cephalic tentacles, statocysts, and calcified shell. Only 2 to 5 of these veligers grow to crawling juveniles while all of the nurse eggs and usually all of the small sibling veligers disappear. Growth of the small sibling veligers is arrested. Their digestive gland becomes pale and shrinks while their shells decalcify. The few non-growing veligers that survive to hatching are unsuited for extracapsular larval life, lying on the bottom or tumbling as they swim and not withdrawing into the remnant shell. In some capsules, early growth is very uneven. The number of sibling veligers consumed is small compared to the number of nurse eggs, but full growth of the large hatchlings presumably results from limited sharing of nurse eggs. The absence of growth of their more numerous small sibling veligers is therefore an adjunct of nurse-egg feeding. The veligers resemble those of another nurse-egg feeding vermetid in their large mouths but differ in retention of functional opposed-ciliary bands. Thus barriers to evolutionary reversion to a pelagic feeding veliger may be weak for this species.