Monday, May 21, 2007

I received an email from Terri Kirby Hathaway from the NC Sea Grant website and she writes...
Your shells are the eastern mudsnail (/Ilyanassa obsoleta/), very commonin NC estuaries. This animal is also called eastern mud nassa, mud dogwhelk, and the common mudsnail. We find lots of them up here on DareCounty beaches.

"Ilyanassa obsoleta (Say, 1822)

Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Prosobranchia
Order: Neogastropoda
Family: Nassariidae

Ilyanassa obsoleta has a plain, dark black or brown shell, 1.5-3 cm high, with 5-6 whorls (a whorl is one complete turn of the spiral shell) and a blunt, conical spire. The shell's surface is decorated with a network of weak, beaded lines. The aperture (the opening through which the snail emerges) is roughly oval and about half the height of the shell, with a notch at the bottom. The outer rim of the aperture is thin and sharp, and the inner side of the aperture has a spiral fold near its base. The inside surface of the shell is a deep, purplish black. The shell is often covered with mud and algae (and sometimes with the Atlantic bryozoan Alcyonidium polyoum), and in most older snails the tip of the shell is eroded.

Ilyanassa obsoleta is the most abundant intertidal snail on San Francisco Bay mudflats and in the lower reaches of marsh channels, where it is often found in large herds. As this snail crawls over the mud surface it leaves a grooved trail behind, and when the tide goes out it usually burrows under the surface to avoid drying out. It has been collected in the bay in salinities of 10-32 ppt and water temperatures of 13-22° C. It feeds on diatoms and algal detritus that it gleans from the surface layers of the mud, captures and consumes minute worms (in the family Spionidae) that live in the mud, and scavenges on dead fish, crabs and other animal remains. It is fed on in turn by ducks and the larger shorebirds.

Ilyanassa obsoleta deposits its eggs in small, bristly, faceted capsules that are attached to hard surfaces. Each capsule is about 3 mm high and contains several eggs. The capsules are often laid in rows on eelgrass blades, or in rows or clusters on shells, stones or debris on the mudflats. The young snails emerge as free-swimming larvae. The larvae drift in the plankton for 20-30 days, feeding on phytoplankton, before settling to the bottom and metamorphosing into tiny snails. Ilyanassa obsoleta can live for up to 5 years.

The San Francisco Bay mudflats where Ilyanassa obsoleta is now found were once occupied by the native hornsnail Cerithidea californica. Race (1982) describes how each spring the two populations of these snails collide in the lower marsh channels. By mid-summer Ilyanassa obsoleta has restricted the native hornsail to the upper channels and shallow pools in the upper marsh through a combination of egg predation and direct competitive interference. The high marsh pools (also called salt pannes) are too salty for Ilyanassa obsoleta, and thus provide a habitat refuge for the native snail."
from the website:

They also mention that these are not a native species on the west coast and probably came in with a shipment of atlantic clams in the early part of the 20th century.