Genus Dacrydium Torell 1859 



Dacrydium vitreum (Holboll in Mdller 1842). Glassy teardrop. 

 Figure 41. 



This species is somewhat shrouded in controversy re- 

 flected in the variability of its distribution as claimed by 

 different authors. Ockelmann (1958) voiced his concern in 

 believing that this species "has a panarctic distribution, but is 

 absent from truly boreal regions." He postulates that there are 

 "4 different forms from the N. Atlantic, most probably dis- 

 tinct species." We, however, have found no clear differentia- 

 tion among the various sources examined and. therefore, are 

 maintaining, at this time, the integrity of this species as it 

 presently exists. Our distributional records, however, con- 

 tain an intriguing inshore-offshore disparity which should be 

 investigated further (see below). 



Johnson (1934) and La Rocque (1953) reported the glassy 

 teardrop as occurring from the Arctic Ocean to Florida: Mor- 

 ris (1951) included the preceding range and added that it also 

 occurs in the English Channel, at the Azores, and in the 

 Mediterranean. Ockelmann (1958), who reported it widely 

 distributed throughout Arctic regions, stated that it also prob- 

 ably occurs in the Gulf of St. Lawrence and in Nova Scotia, 

 and extends from Newfoundland to Cape Cod. Clarke (1962), 

 in addition to Arctic and subarctic distributional data, re- 

 ported it occurs in Norway, Northern Eurasia, West Europe, 

 the Canaries, and on the Middle Atlantic Ridge in the region of 

 the Azores. Abbott's (1974) distributional range is from 

 Greenland to the Gulf of Mexico, he also included Norway. 



Dacrydium vitreum is common; our collection contains 522 

 specimens from 95 samples (Table 5). 



The NEFC sampling data shows two separate groupings of 

 samples, one of which occurs in the Gulf of Maine Basin area, 

 including the Scotian Shelf and Browns Bank, ranging onto 

 the southwestern part of Georges Bank; the second grouping 

 occurs in deep offshore waters beyond the shelf break on the 

 continental slope, ranging from the Northeast Peak of 

 Georges Bank to slightly south of Delaware Bay with one 

 other sample occurring south of Cape Hatteras at the shelf 

 break (Fig. 41 ; Theroux and Wigley footnote 4, table 67). This 

 distributional disparity bears further investigation as this pat- 

 tern may indicate the existence of two separate populations 

 or, indeed, separate species occupying different bathymetric 

 ranges and habitats. 



In addition to Ockelmann's (1958) reporting of this species 

 as panarctic in the North Atlantic only, Gosner (1971) re- 

 ported it occupying the Boreal and Virginian zoogeographic 

 provinces as well. 



The glassy teardrop enjoys a wide bathymetric range 

 occupying water depths from 6 to 4,454 m (Ockelmann 1958; 

 Clarke 1962). 



Our samples are also widely distributed by depth, ranging 

 from 38 to 3,055 m with a mean depth of 305 m. The majority 

 of samples and specimens are in the 100-199 m grouping, 

 which contains 52% of the samples and 71% of the specimens, 

 and the 200-499 m grouping containing 38% of the former and 

 26% of the latter. Significantly smaller amounts occur in the 

 other depth range groupings: the 25-49, 500-999, 1,000-1,999 

 and the 2,000-3,999 m groupings (Table 92). There is no depth 

 information concerning one sample which contains three 

 specimens. 



No samples obtained from till, shell, or sand-shell sedi- 

 ments contained specimens of the glassy teardrop . The major- 

 ity of both samples and specimens occurred in clay where 

 54% of the former and 74% of the latter were obtained. The 

 next most plentiful amounts of samples and specimens oc- 

 curred in silty sand where 19 and 14%, respectively, occur- 

 red. Smaller amounts occurred in gravel, sand-gravel, sand, 

 and silt substrates (Table 93). There are 3 samples containing 

 1 1 specimens which are unclassified with regard to sediment 

 type. 



Genus Geukensia Poel 1959 



Geukensia demissa (Dillwyn 1817). Atlantic ribbed mussel. Fig- 

 ure 46. 



This is a common and locally abundant species which has 

 been introduced into California at San Francisco Bay (Abbott 

 1974). 



The range of this species extends from the Gulf of St. 

 Lawrence to South America as well as having been intro- 

 duced in California (Johnson 1934; Morris 1951, 1973: La 

 Rocque 1953; Ockelmann 1958: Abbott 1968, 1974; Emerson 

 et al. 1976). 



The NEFC collection contains 38 specimens from 1 1 sam- 

 ples (Table 5). 



Our samples are from the shores of Cape Cod. Mass.. with 

 one sampling site on the Connecticut shore (Fig. 46: Theroux 

 and Wigley footnote 4, table 75). 



This species occurs in the Boreal, Virginian, and Carolinian 

 provinces (Coomans 1962); Gosner(1971) listed it as occupy- 

 ing the Boreal and Virginian provinces, while Dance (1974) 

 placed it in the Boreal, Transatlantic, and Californian pro- 

 vinces. 



This species is found in the intertidal region from low tide to 

 approximately 6 m (Abbott 1968, 1974; Porter 1974). 



All of our samples, with the exception of one containing 

 two specimens which had no depth information, were col- 

 lected at a depth of 1 m which places them in the 0-24 m depth 

 range grouping (Table 94). 



This species is an inhabitant of salt marshes, mud-sand 

 flats, and also occurs on muddy or peaty bottoms (Morris 

 1951. 1973; Abbott 1968, 1974; Emerson et al. 1976). 



Seventy-five percent of our samples and 94% of our speci- 

 mens occurred in silty sand substrates with 25% of the sam- 

 ples and 6% of the specimens occurring in sand (Table 95). 

 There are 7 samples containing 20 specimens which are un- 

 classified with regard to sediment type. 



Genus Modiolus Lamarck 1799 



Modiolus modiolus (Linne 1758). Northern horse mussel. Fig- 

 ure 62. 



The northern horse mussel is widely distributed throughout 

 northern seas ranging in the Atlantic from Arctic seas to Cape 

 Hatteras, N.C., and in the Pacific from Arctic seas south to 

 San Pedro, Calif.; it is also widely distributed throughout 

 Arctic regions and in northern Europe (Johnson 1934: Morris 

 1951, 1973; La Rocque 1953: Ockelmann 1958: Tebble 1966: 

 Abbott 1968, 1974). 



Modiolus modiolus is the largest and commonest mussel of 

 New England (Abbott 1968): it is represented in the NEFC 



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