Many thousands of starfish are transported in 

 this way from place to place and settle on new 

 grounds when tidal currents slacken. 



The starfish leaves no identifying marks on its 

 victim, and only empty shells remain as evidence 

 of a destructive attack. Tlie recent death of 

 oysters is indicated by the cleanliness of the valves, 

 which contain no foreign growth and are still 

 attached to each other. The method by which 

 the starfish succeeds in forcing oysters or clams to 

 relax their muscles and open the valves has 

 puzzled biologists for a long time. It seemed 

 doubtful that the starfish could exhaust its 

 victim and open it by main force, and suggestions 

 were made, not well corroborated by observations, 

 that the prey was killed by suffocation or that a 

 substance secreted by the stomach of the starfisli 

 produced relaxation of the adductor muscle of the 

 oyster. Sawano and Mitsugi (1932) reported 

 that an extract of starfish stomachs poured over 

 the heart of living molluscs produced tetanus and 

 often inhibited the heart beat; this seemed to 

 give some support to the "anesthetic" hypothesis. 

 Critical experiments made in Woods Hole by 

 Lavoie (1956) show, however, that the effects of 

 extracts prepared from digestive organs of starfish 

 and introduced into the adductor muscle or 

 poured over the heart of Mytilus were generally 

 identical with those produced by plain water. 

 On the other hand, the force exerted by the tube 

 feet of starfish in opening shellfish was measured 

 manometrically and was found to exceed 3,000 g. 

 The measurement was made using mussels in 

 which the adductor muscles were severed and 

 replaced by steel springs or plastic cylinders. 



Lavoie noticed that a tiny opening of about 0.1 

 mm. between the valves of the mollusks was 

 sufficient to permit tlie insertion of starfish 

 stomach. The pulling of valves apart is probably 

 repeated at intervals while the stomacii remains 

 partially compressed. The observations of Feder 

 (1955) on Pisaster ochraceus show that this starfisli 

 can open its prey by force alone. Another 

 Pacific Coast species, Evasferias troschelii, was 

 found to exert a force in excess of 5,000 g. during 

 an attack on artificial clams baited with Mytilus 

 meat (Christensen, 1957). The fact that star- 

 fishes are able to open mollusks by force alone 

 does not eliminate tlie possibility of an addi- 

 tional narcotizing' eflfect produced by starfisli secretion. 

 The problem of how the starfish opens its prey 



has not yet been finally solved, although present 

 evidence favors the mechanical hypothesis. 



Not all starfishes feed by everting their stomachs 

 and digesting the body of tlie victim without 

 ingesting it. Many of them are scavengers feeding 

 on dead animals found on bottoms while others 

 are capable of catching and consuming live fishes. 

 Many interesting cases of starfish attacks on 

 various marine animals including fishes are 

 described by Gudger (1933). 



Starfish are usually found in water of high 

 salinity and do not invade the oyster grounds in 

 brackish waters. The salinity level between 

 16%o and 18°/oo below which A. jorhesi cannot 

 exist is a natural barrier to the distribution of 

 this species. This conclusion is based on field 

 observations along the Atlantic coast and on 

 experiments at the Bureau of Commercial Fish- 

 eries Biological Laboratory, Milford, Conn. (Loos- 

 anoft", 1945). In New England waters, starfish 

 are controlled by mopping or dredging to remove 

 them, and by dispersing calcium oxide and other 

 chemicals to kill tliem or to make a protective 

 chemical barrier around an oyster bed. 



Flatworms 



Turbellarians of the genus Stylochus and Pseu- 

 dostylochiis, commonly known as oyster leeches, 

 are predators which attack adult and j'oimg mol- 

 lusks and frequently inflict serious damage to 

 oyster populations. In 1916 and 1917 attacks of 

 Stylochus on oysters in Cedar Keys on the west 

 coast of Florida killed about 30 percent and in 

 one or two localities 90 percent of the adult 

 oysters. The mortality of oysters in Apalachicola 

 Bay, Fla., allegedly caused by the "leech," was 

 investigated for the LT.S. Bureau of Fisheries by 

 Pearse and Wharton (1938), who could not state 

 definitely that the distruction was due to S. ini- 

 inicus Palombi" and suggested that the oysters 

 were first weakened by some unknown cause and 

 that Stylochus invaded those which were unable 

 to protect themselves. S./rotitalis tolerates water 

 of low salinity (6 7oo), but according to Pearse 

 and Wharton does not lay eggs in salinities less 

 than 157oo. 



S. ellipticus (Girard), found in Atlantic coastal 

 waters and also reported from the Gulf (Hyman, 

 1939, 1954), lives among oysters, shells, barnacles, 

 and rocks. The turbellarian was reported to 

 destroy young oysters on the flats at Milford, 



'0 The identification was corrected by Hyman fl9y9) who found that the 

 Florida leech belongs to the species .S. fro-ntalis Verrill. 



438 



FISH AND ViaLDLIFE SERVICE 



