430 U. S. BUREAU OF FISHERIES 



of the bars supplying the market are badly overfished, the supply 

 of cultch is inadequate, and unless additional bars can be brought 

 into production, the present rate of fishing on producing bars will 

 result in a steady decline in the size and quality of the shell stock. 



Life history of the leech. — A study of the two species of polyclad 

 worms or so-called leeches, Stylochus inimicus and Eustylochus, as- 

 sociated with the oysters of this region, and at times so abundant 

 as to do damage, was carried out by Dr. A. S. Pearse and G. Wharton. 

 Stylochus has been found at Bull's Bay in Charlotte County, Tampa, 

 Crystal River, Port Inglis, Cedar Key, Apalachicola Bay, Port St. 

 Joe, and Crooked Island Sound. It is more common in summer 

 than in winter. Eustylochus is being described as a new species, hav- 

 ing a wide range along the Atlantic and Gulf coasts, and found 

 in all of the localities just mentioned in Florida. This species is 

 apparently more common in winter than in summei- along the Florida 

 coast. 



The anatomy of both species of oyster "leeches" has been studied 

 in some detail. The largest specimens of Stylochus collected meas- 

 used 48 by 28 mm, while those of Eustylochus measured 22 by 9 mm. 



Stylochus is remarkably fecund. A single individual kept in sea 

 water in a finger bowl in the laboratory laid 22,370 eggs. The leeches 

 found in oyster shells in Apalachicola Bay were found to be brood- 

 ing clutches of eggs numbering as many as 18,028. For develop- 

 ment, a Stylochus egg requires from 9 to 15 days at summer tem- 

 peratures, and hatches into a ciliated larva similar to those 

 characteristic of leptoplanid polyclads. After a time it tends to 

 creep about on solid objects, and when adult entirely ceases to swim. 

 Eggs of this form were still being found on oyster bars as late as 

 January 11. 



Neither Stylochus nor Eustylochus have been observed to eat any 

 food other than the bodies of oysters, though the latter is believed 

 also to eat barnacles. Observations show that they bite holes in the 

 oyster's body, usually attacking the gills first. An oyster invaded 

 by a leech usually secretes a partition along its edge to shut out the 

 intruder and sometimes succeeds in covering it over so as to incorp- 

 porate it in the shell. An oyster occupied by a worm opens and 

 closes its shell more often than under normal conditions. 



Studies of the distribution of polyclads on the oyster beds in 

 Apalacliicola Bay at times showed more than 200 polyclads in a 

 bushel of oysters. In general, Stylochus was most abundant where 

 oyster bars were nearest the open sea; while Eustylochus was most 

 abundant where mussels and barnacles are found ; viz, close to shore 

 or in inclosed lagoons. Polyclads such as these show considerable 

 toleration to variations in environment, perhaps owing in part to 

 mucus secreted abundantly when unfavorably stimulated. Low 

 salinities undoubtedly cause their death. Eggs will not develop, and 

 adults commonly do not lay eggs in salinities below 15 parts per 

 mille and larvae have been killed in salinities as low as 13.9. When 

 subjected gradually to progressively lower dilutions, the adults will 

 live in a salinity as low as 6 parts per mille, Eustylochus enduring 

 lower salinities than Stylochus. Stylochus endures considerable 

 desiccation, reviving when immersed in sea water after a loss of 54 

 percent of its body weight. 



