The Urosalpinx could have been feeding on commensals on the crab carapace. The Orlando hypothesis may also have some merit-riding crab back to the next oyster patch is quicker than crawling, and probably more fun as well.
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
[log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!