shell invasion. This type of relation in which 

 one of the associates is inhibited while the 

 other is not affected is best described by the 

 term amensal as defined by Odum (1953). 



BRIEF REVIEW OF THE LIFE HISTORY 

 OF HYDRACTINIA ECHINATA 



Hydmctinia echinata is far less exclusive in 

 its choice of habitat than earlier observers indi- 

 cated. It has been dredged "on every sort of 

 bottom" (Sumner, Osburn, and Cole, 1913), and 

 has been found on a wide variety of substrate 

 (Hargitt, 1908). Bunting (1894) mentioned 

 that the hydroid lives on the sea mussel, 

 Mytilns rdulis (Linnaeus), and Moore (1937) 

 recorded its occurrence on the shell of another 

 bivalve, Pectunculus {^Glycymeris) , but I have 

 seen no other reports of the association of this 

 hydroid with a specific bivalve. 



Hydractinia echinata is the well-known hy- 

 droid which served formerly as the classic 

 example of symbiosis with the hermit crab, 

 Pagurus bernhardus (Linnaeus). Many papers 

 have dwelt on the symbiosis of pagurids and 

 actinians (see Balss, 1924, and Dales, 1957 for 

 a summary of the literature). From experi- 

 mental studies, however, Schijfsma (1935) 

 concluded that the association of H. echinata 

 and P. bernhardus could not be defined as true 

 symbiosi.s — that the hydroid is merely an 

 epizoon. 



Schijfsma (1939) described the early stages 

 of growth of colonies of the hydroids, and 

 Fraser (1944) and others have reported in de- 

 tail the specialization of individuals that make 

 up a colony of Hydractinia echinata. This 

 hydroid is polymorphic. Several types of 

 zooids develop, including special generative 

 zooids that produce male and female sporosacs. 

 The ova are fertilized in situ, and after being 

 discharged sink to the bottom where they de- 

 velop into mobile planulae in 24 to 48 hours. 

 The planulae are never free-swimming; rather 

 they crawl or glide in the manner of turbella- 

 rians. The mobile phase lasts at least 24 hours, 

 ending when the planulae fix themselves to a 

 substrate and develop into typical tentacular 

 zooids. 



The surface of a sea scallop shell can act 

 as a base for settling planulae. Once attached. 



the zooids grow from a stoloniferous network 

 of anastomosing tubes to develop into a colony. 

 The coenosarcal expansion is covered with a 

 heavy, chitinous perisarc from which rise the 

 ridged and jagged spines characteristic of the 

 encrusting colony. Nutritive zooids are the 

 most numerous ; other types include defensive, 

 sensory, and generative zooids. The mature 

 colony appears as a reddish velvety covering, 

 but feels rough to the touch because of the 

 numerous spines. Batteries of nematocysts in 

 the zooids protect the colony. 



ASSOCIATION OF HYDRACTINIA ECHINATA 

 AND PLACOPECTEN MAGELLANICUS 



The size of a colony of Hydractinia echinata 

 on the sea scallop varies with the length of 

 time the colony has been established and with 

 the surface area available. Where there is 

 ample surface for expansion, the zooids in the 

 advancing front of the colony tend to be ar- 

 ranged in concentric rows corresponding to the 

 growth marks on the shell. This arrangement 

 was observed by Frederick M. Bayer (personal 

 communication) of the University of Miami, 

 Fla., to whom material was sent for identifica- 

 tion ; his finding agrees with remarks by 

 Schijfsma (1939) that the course of stolons 

 is influenced by the surface sculpture of the 

 substrate. The colony, by advancing more rapid- 

 ly than the scallop grows, may eventually 

 arrive at the periphery of the shell and con- 

 tinue around the shell edge. Long, slender 

 zooids, armed heavily with nematocysts, are 

 especially abundant at the advancing edge. 

 Ultimately, the forward elements of the colony 

 come in contact with the extended mantle of 

 the scallop. The mantle withdraws, presumably 

 because of the nematocysts discharged into it.' 

 The mantle retreats steadily as the hydroid 

 colony encroaches inward (fig. 1, a-d). Evi- 

 dence of this sequence is seen clearly in figure 

 1, a. To be noted are the numerous relatively 



• In section, the nematocysts were found to be about 10 microns 

 lone and 4 microns wide and the thread was about at the limit of 

 visibility with an ordinary littht microscope. Thus a thread in the 

 scallop mantle cannot be disliutjuished with the histological technique 

 used. The scallop mantle did, however, show evidence of an in- 

 flammatory response (personal communication. Clyde Dawe. Nation- 

 al Institutes of Health. Bethesda, Md.l. This response was indicated 

 by the greatly increased number of cells under the epithelial 

 margin. Dawe considered these to be wandering amoebocytcs 

 coming to the site of the insult. 



274 



HYDRACTINA ECHINATA AND SHELL DEFORMITIES 



