Table l. — Station locations of Nekton gama dives to observe depth distribution and density 

 of the siphonophore Nanomia cara, 15-28 June 1976. 



'Dives conducted by authors. 



(Figure 1, Station 3) to 17°C off Cape Ann (Station 

 15). Bottom temperatures at stations deeper than 

 100 m ranged from 5.5° to 7.5°C (Table 1). In gen- 

 eral, the thermocline shoaled from about 75 m off 

 Cape Ann to about 30 m in the Wilkinson Basin 

 (Stations 3, 8, 14-16); the estimated zone of 

 twilight visibility extended to about 135 m. Lat- 

 eral visibility on most dives exceeded 5 m both 

 above the twilight zone and below it where the 

 lights on the submersible were used. 



Large numbers of A^. cara were observed at all 

 dive stations deeper than 125 m; they were also 

 present, though less dense, at two shallower sta- 

 tions, 28 (72 m) and 29 (98 m) (Table 1). During 

 daylight, siphonophores were observed only below 

 the thermocline. No dives were made at night so it 

 was not possible to predict if transthermocline 

 movement occurs during expected diurnal migra- 

 tions. They appeared to be distributed in patches 

 both horizontally and vertically. We estimated 

 that patch diameters ranged from 5 to 30 m. At 

 depths where N. cara was locally abundant, col- 

 onies could be seen out of every viewport (Figure 

 2). The densest concentrations often occurred be- 

 tween 3 and 45 m above the bottom where we 

 estimated that their densities ranged between 1 



and 7 colonies/m^. At Station 29 siphonophores 

 occurred in two distinct layers: sparsely distri- 

 buted from 76 to 85 m where concentrations were 

 usually <0.1 colony /m^, and more densely aggre- 

 gated above the bottom where concentrations 

 were about 1 colony/m^. We found no correlation 

 between colony density and substrate type. 



Colonies ranged in size from 0.2 to 3.7 m when 

 suspended in fishing posture with the stem and 

 tentacles relaxed. In this configuration the dis- 

 tance between adjacent stem groups ranged from 

 10 to 15 mm. The largest colonies had over 200 

 salmon-colored feeding polyps (gastrozooids) and 

 30 to 40 swimming bells (nectophores). Unless 

 swimming, most colonies oriented with the apical 

 gas-filled float (pneumatophore) and nectophores 

 upward. The rest of the flexible stem, which ap- 

 peared neutrally buoyant, hung in three- 

 dimensional series of loops and arcs. 



In high density localizations of A^. cara, colonies 

 of several different sizes were often present. In 

 areas of the aggregation peripheral to the highest 

 densities of siphonophores, however, colonies were 

 generally small, i.e., 20 to 40 cm long. Smaller 

 colonies were also found higher in the water col- 

 umn than the larger ones, or occurred singly. All 



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