Spawn and development of the gastropod Americominella longisetosa (Castellanos and Fernandez, 1972) (Mollusca: Buccinidae) from the Southwestern Atlantic deep sea

The buccinid Americominella longisetosa (Castellanos and Fernández, 1972) is widely distributed along the Southwestern Atlantic shelf where it is found at 200–1,250 m depth. A total of 38 egg capsules of this species were collected by the Argentine ship R/V Puerto Deseado from the Burdwood Bank/MPA Namuncurá surroundings (~ 54°36'S, 61°30'W at 298 m depth and 53°56'S, 61°30′W at 185 m depth). Egg capsules occur individually, not clustered in egg masses, are globose in shape, translucid, with thin walls measuring about 20 µm in thickness and having a triangular stalk on the side by which the capsule is fixed to its substrate. The substrate to which the capsules were attached was the internal matrix of the poriferan Mycale magellanica (Ridley, 1881). In each capsule, only one embryo develops from an egg of about 150 µm in diameter. The embryo develops by ingesting about 9,000 nurse eggs. In distinct egg capsules we found embryos at different stages of development indicating several spawn events. Hatching takes place as a crawling juvenile measuring about 8 mm in shell length and with 4 ¼ whorls, indicative of a long embryonic development within the capsule.