14 



The Animal Ecology of the Cold Spking Sand Spit 



seem destined to become smaller and smaller nntil entirely washed away. But from 

 this fate they are preserved by an interesting association. The current that rushes 

 through the channels carries with it an abundant supply of microscopic food, such 

 as lamellibranchs can make use of. This food is taken advantage of by the mussels which 

 come to line the muddy banks on the channels, and form so close a wall that erosion 

 is almost completely stopped (Fig. 6). Thus the mussels assist the Spartina in their 

 constructive work. The mussels that line the banks are Modiola plicatula, Modiola 



Fig. 6.— Photograph taken at the inside of the hook of the sand spit, 

 looking north, at low tide, through one of the passages scoured out by the tidal 

 currents. At the base of the Spartina patches, on the right, are seen some of the 

 beds of mussels which protect the roots from exposure. 



modiolus, and Mytilus edulis, the first-named being the most abundant. The channels 

 have irregular bottoms in which shallow pools of water stand when the tide is out. 

 Here the mud snails, Nassa obsoleta, aggregate, scarcely submerged. High up on the 

 stems of the Spartinas, exposed to the air during perhaps half the day, are found cling- 

 ing the Littorinas, whose lack of a siphon makes it necessary for them to keep out of 

 the mud. Littorina rudis and Littorina palliata were still the prevailing species in 

 1898 (see Balch, 1899), but in 1901 Littorina littorea, which is rapidly advancing up 

 the harbor, had gained a marked predominance. * The independence of the sea water 

 that is exhibited at the tip of the sand spit is also illustrated in the marsh at the head 

 of the harbor through which Cold Spring creek runs. On this marsh Littorina occurs 



* This habit of clinging to rushes is even more exagger- 

 ated in the southern Littorina irrorata. For, in a visit to 

 the Lagoon on Ship Island, Mississippi Sound, in March, 

 1902, 1 found nearly all of them living on the stems of the 



short marsh grass twenty to thirty centimeters above the 

 water level and exposed to the sunlight. Cooke (1895, pp. 

 20, 93, 151, note) cites other cases of Littorina living out of 

 water. 



168 



