I. 



3. 



2 



Centimeters 







Centimeter 



Figure 391. — T. haemastoma floridana Conrad from the shores of Pensacola Bay, Fla. 1 — apertural view; 2 — abapertural 



view; 3 — egg cases. 



contains interesting material regarding this and 

 other species of the genus. 



T. haemastoma is a common oyster predator in 

 the waters of the South Atlantic and Gulf states. 

 There are two subspecies, T. haemastoma floridana 

 Conrad, which occurs from North Carolina to 

 Florida and the Caribbean (fig. 391), and T. 

 haemastoma haysae Clench (fig. 392), common on 

 oyster grounds of northwest Florida, Louisiana, 

 and Texas (Clench, 1947). T. haemastoma flori- 

 dana is a medium-size gastropod with a relatively 

 smooth shell and a smgle row of low spmes. T. 

 haemastoma haysae is a large, rugged snail, some- 

 times measuring 4}^ inches in height. It can be 

 distinguished from the other subspecies by double 

 rows of prominent spines around the whorls and 

 spire. 



The behavior of the two varieties apparently is 

 similar. The conchs feed on oysters and other 

 mollusks, penetrating their shells from the edge 

 by using the ABO gland or by drilling holes in the 

 shell (Burkenroad, 1931; Carriker, 1961a). The 

 entrance at the edge of the valves is often in- 

 conspicuous and may be easdy overlooked. 



Conchs multiply very rapidly because of their 

 great fecundity and high survival rate of larvae. 

 Thais haemastoma lays eggs in groups of about 800 

 to 975 enclosed in each egg case, with each female 



depositing more than 100 cases. These figures 

 refer to my laboratory observations on conchs 

 kept in captivity. The eggs are deposited in 

 horny and transparent egg cases of a creamy 

 color, which becomes brownish and finally turns 

 reddish-purple. The breeding season in Louisiana 

 waters begins by the end of March and reaches its 

 peak in April and May. There is usually a rapid 

 decline of egg laying in June and a complete 

 cessation of reproduction in July. At the begin- 

 ning of the breeding period the conchs become very 

 active and develop a strong tendency to climb on 

 structures and rocks to attach their egg cases 

 above the bottom. Because of this behavior they 

 can be trapped during the breeding season on 

 stakes which the oyster growers erect on the 

 grounds. Gregariousness is very pronounced, and 

 many conchs can be trapped in this way in a rela- 

 tively short time. The number of egg cases 

 attached to a single stake may be enormous. One 

 stake which I obtained as a sample was covered 

 with a solid mass of egg capsules over a 5-foot 

 length; the estimated number of cases was about 

 8,000. The incubation period is not known 

 definitely, but judging from the growth of hydroids 

 and other fouling animals on the conch cases, 

 I believe it is not less than 2 weeks. 



FACTORS AFFECTING OYSTER POPULATIONS 



433 



