Studies of the Southern Oyster Borer, Thais haemastoma
Original work was carried on at the U.S. Bureau of Fisheries Laboratory on Apalachicola Bay from August 1935 to April 1936. Since then observations have been made in Texas, Louisiana and Mississippi. Five papers on specific aspects of the biology of the animal have been written since on this and other predatory gastropods. Here all commentaries are drawn together and unpublished matter is presented.
The name Thais haemastoma is used because separations based upon the rugosity of the shells do not hold up. Perfectly smooth and very rugose specimens are found in the same bays, with various shell characteristics being related to various oyster reefs on which they grow.
Radular movement is by the band-over-pulley method suggested by Husley (1853), Herrick (1906) Gunter (1936) and Carriker (1943). Evidence is presented showing that Thais can kill oysters without mechanical injury, presumably by some paralytic material. About one-third of the oysters are opened by large Thais without any boring whatsoever. Smaller Thais are more prone to bore complete holes into the shell cavity of the prey. In Apalachicola Bay large Thais may eat one oyster about every 8 days and it was calculated that on St. Vincent’s Bar 24 million adult oysters could be killed in a year.
The resting gonads consist of a thin layer of tissue on the body over the liver and they are lavender-grey in the males and yellowish-orange in the females. They begin to thicken in January and the color intensifies. Egg laying takes place from April to July on the Gulf coast. No young or small Thais were seen in Apalachicola Bay probably because of heavy freshwater drainage in the springs of 1934 and 1935. Several hundred Thais were measured and each month the length frequency mode was at 80.0 mm. The largest known specimen of Thais, a Louisiana specimen, was 103 mm long.
A heavy kill of Thais took place in the spring of 1935 and no adults survived in Apalachicola Bay except on Hiles’ Bar near Indian Pass, which is close to the ocean. The Thais seemed to perish when salinity dropped to 9‰ and stayed that way for several weeks. Both oysters and mussels survived at salinities lower than Thais could withstand.
Thais shells are extremely hard and are difficult to break with a hammer. Nevertheless, they are cracked by stone crabs. They are also invaded by commensals such as the boring clam Diplothyra smithii, the annelid worm Polydora websteri, and the boring sponge Cliona.
In Louisiana a so-called conch line was established by St. Amant (1938) when it was found that adult conchs did not get much beyond the area n Barataria Bay where the salinity fell to around 20‰. This was confirmed later by J. G. Mackin and Gunter at about 18‰, but has not been published. It has also been found that baby conchs are found landward of this line. It was found by experiments that conchs were generally killed by water registering 9‰ salinity and, additionally, that snails taken from low-salinity water survived transfer to still lower salinities or lived longer even in lethally low salinities than those coming from higher salinities.
Attempts to trap conchs on oyster beds were unsuccessful because no baits more attractive than the surrounding oysters and mussels could be found. The conchs’ activity stopped at temperatures of 10°C and below.
Studies of the Southern Oyster Borer, Thais haemastoma.
Gulf Research Reports
Retrieved from http://aquila.usm.edu/gcr/vol6/iss3/5