2006 Volume Special2006 Issue 6 Pages 23-31
Each January for 7 years (1999-2005), tubes of the polychaete Chaetopterus sp. were excavated at low tide from Dog Island, Florida, in the northern Gulf of Mexico, and examined for the presence of two common crab commensals, the anomuran Polyonyx gibbesi and the brachyuran Pinnixa chaetopterana. Of 114 tubes examined, 4 (3.5%) had no crabs, 77 (67.5%) had Pinnixa-only, 23 (20%) had Polyonyx-only, and 10 (9%) contained both species. For Pinnixa-only tubes, 90% contained a mixed-sex pair; for Polyonyx-only, 57% contained a mixed-sex pair and the remainder a single crab. All tubes containing both species had either one or two adult Pinnixa and a small Polyonyx (CW < 3.5 mm, 9 tubes) or a single adult Polyonyx and a small Pinnixa (CW < 3 mm, 1 tube). No tubes contained adults of both species. Among adult females, 24% of the Pinnixa and 44% of the Polyonyx were ovigerous. The Polyonyx-Pinnixa ratio of 20:80 showed a greater relative abundance of Pinnixa compared to other reports of this association from elsewhere along the Atlantic coast of the United States or the Gulf of Mexico. Experiments introducing single adult intruder crabs of either species into in situ tubes showed that adult crabs of either species evict same-sex conspecifics or heterospecifics; with time most tubes have a mixed-sex pair of a single species.
