RATES OF DISARTICULATION IN CRABS AND URCHINS ON THE CONTINENTAL SHELF AND SLOPE: HOW SPINELESS AND CRABBY WOULD YOU BE AFTER EIGHT YEARS ON THE SEA FLOOR?

Decapod crustaceans and echinoids are two types of animal that are rarely found preserved intact. When they are preserved articulated, the deposit is considered an example of rapid burial, low oxygen, or other mode of exclusion of scavengers and microbes. To test these assumptions, frozen blue crabs (Callinectes sapidus) and echinoids (Heterocentrotus mammillatus and Echinometra lucunter) were deployed in mesh bags attached to weighted PVC poles and placed in a wide variety of depths and environments of deposition in both the Bahamas and Gulf of Mexico.

Urchin tests were found to be surprisingly durable. All specimens were rapidly denuded of spines. The corona, however, was recovered intact more often than not after 5 - 8 years on the sea floor. Urchins deployed at15m were recovered whole in 4 out of 9 specimens. At 33m, 75% of urchin tests (n=12) remained intact after seven years. At depths from 70m to 100m, 66% of tests were intact after 5+ years (n=31) despite some being deployed on hardgrounds. Sites below the photic zone had 67% whole tests 5+ years after deployment (n=42). Disturbance of the mesh bags was minimal, but did occur at some shallow Bahamas sites where test breakage was no higher than at undisturbed sites.

Crab claws (the dactyl and fixed finger of the chela) showed remarkable longevity on the sea floor. Crabs at all depths at all sites in both the Bahamas and Caribbean had some remains left after 7-8 years. Crabs deployed in the Bahamas for 7 years at 15m had only a few disarticulated chelae left. At 33m, claws remained in the bag, along with the mandibles and a few lateral spines from the carapace. From 73m – 267m, crab deployments typically retained many disarticulated parts after 7 years. Even pieces of the delicate carapace were recovered. In the Gulf of Mexico, articulated claws were recovered at two different sites after 8 years at 60m. Exceptional preservation was observed in a sulfurous brine pool (>200ppt salinity) in which crab remains were recovered “pickled,” the dominant affect of the brine being decalcification of hard parts. Deeper Gulf of Mexico sites yielded a predominance of disarticulated claws. Petroleum seep sites at 550m-570m resulted in the least amount of crab remains after 8 years, while sites with carbonate sand and hard ground bottoms at 60 to 90 m yielded many more disarticulated remains.