Nassarius fossatus (Gould, 1850)
Common name(s): Channeled basket whelk, Channeled nassa, Channeled dog whelk, Basket shell, Giant western nassa
|Synonyms: Nassa fossatus, Alectrion fossatus|
|Nassarius fossatus shell (contains a hermit crab) found intertidally near Rosario. Note the deep groove at the anterior (right) end that separates the anterior end of the body whorl from the rest of the shell.|
|(Photo by: Dave Cowles July 2006)|
How to Distinguish from Similar Species: Ilyanassa obsoleta and Searlesia dira do not have the deep anterior groove. Nassarius fraterculus has spiral ridges only on the body whorl. Several other species have axial ribs which reach to the angerior groove. Of these, N. mendicus has much more pronounced axial ribs than spiral ridges, while N. rhinites and N. perpinguis have small axial ribs and spiral ridges which intersect to form distinct beads (may need magnification to see).
Geographical Range: Vancouver Island, B.C. to Laguna San Ignacio, Baja California. This is the most common carnivorous snail on mud flats along the US west coast. It is most common in the northern parts of its range.
Depth Range: Low intertidal to 18 m
Habitat: Intertidal and more commonly subtidal on sandy areas and mud flats.
Biology/Natural History: Feeds as a predator or primarily a scavenger. Can crawl well on rock, or on top of or just below the surface of sand or mud. They crawl with both sides of the large foot, leaving a distinctive track in the sand. Attracted from long distances by rotting meat, which it can smell with its long proboscis. It can crawl rapidly. When feeding, it wraps its foot completely around the food until it is consumed. May also drill in clams or snails. Predators include the seastar Pisaster brevispinus. When contacted by P. brevispinus, N. fossatus may writhe so violently with its foot that it twists into somersaults and even vaults into the water column. At other times it may simply turn and crawl swiftly away, rocking its shell back and forth. Sometimes the colonial hydroid Clytia bakeri, which is one of few hydroids that grow on exposed sandy shores, is found growing on shells of this species. Deposits its egg capsules on eelgrass or other firm objects in mud flats in late winter and spring. A typical string of eggs may be 6 cm long and contain 45 eggs. The individual capsules are about 3 mm long. They are laid overlapping one another to produce a "shingled" appearance.
The wicker-basketlike appearance of the intersecting radial ribs and
spiral ridges is the reason this species is called a basket whelk.
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Brusca and Brusca, 1978
McConnaughey and McConnaughey, 1985
Morris et al., 1980
Ricketts et al., 1985
The outer margin of the aperture has small teeth on the margin and is ridged inside. It has a short siphonal notch or canal at the anterior end (right above) and a small notch at the posterior end.
The innr margin and columella have alarge callus which is usually orange.
This shell has a hermit crab inside.
Another, smaller individual
This anterior view of the shell clearly shows the groove which sets off the anterior end from the rest of the shell.