CHANGES IN ABUNDANCE OF THE COMMENSAL CRABS OF CHAETOPTERUS

Abstract

Marked changes in relative abundance of Polyonyx macro-cheles and Pinnixa chaetopterana, commensal crabs common in tubes of the annelid Chaetopterus variopedatus, have occurred in the last half century at Woods Hole, Massachusetts and at Beaufort, North Carolina. At Woods Hole Polyonyx, rare in 1903, exceeded Pinnixa in abundance in 1959 and constituted 66% of the crabs in Chaetopterus tubes. The Polyonyx-Pinnixa ratio at Beaufort has shifted from 83:17 (1905) to 39:61 (1959). In this region Pinnixa greatly exceeds Polyonyx in abundance in tubes from mud flats, but is less abundant in tubes from sand substrates. The Polyonyx-Pinnixa ratio at Clearwater, Florida, in 1960 was found to be similar to that at Beaufort in 1905, 82:18. Observations on the life history and ecology of the two crabs in the Beaufort region are recorded.