REEVE and BAKER: PRODUCTION OF A CHAETOGNATH AND CTENOPHORE 



were not encountered. Zooplankton were thus 

 collected from both locations using two nets 

 (which were towed simultaneously), one of which 

 (the 200-ju,m mesh) was common to both locations. 



An extensive series of samples was collected in 

 Card Sound to check on the adequacy of the 64- and 

 200-ju.m mesh Va-m mouth diameter nets in 

 sampling the entire size range of the population of 

 Sagitta hispida. In comparisons between a 64- and 

 35-ju.m mesh, the size-frequency distribution of the 

 population was not significantly different. Ab- 

 solute numbers often differed, but this was at- 

 tributable to the rapid clogging of 35-/xm mesh, 

 which rendered flowmeter readings unreliable, 

 and was why this mesh was not used routinely. The 

 64-^m mesh net, which filtered less than 50% of the 

 volume of water of the 200-/xm mesh in the same 

 time, collected fewer of the larger size chaetog- 

 naths than the 200-]u,m mesh, indicating that a 

 greater proportion of the larger animals were 

 avoiding the smaller meshed net. Comparative 

 tests between the 200-iU,m V2-m diameter net and a 

 200-/im 1-m net (which filtered 3 times more water) 

 did not indicate that the larger net caught either a 

 larger absolute number, or a higher percentage, of 

 the larger size classes per volume filtered. These 

 data are available by writing to the first author. It 

 appeared, therefore, that the two standard V2-m 

 nets utilized in the sampling program quantita- 

 tively collected the entire size range of this species 

 in the surface water. 



Vertical distribution of S. hispida Conant in the 

 3-m water column was investigated on six dates 

 during the year using both towed nets and a pump 

 as described by Reeve and Cosper (1973). There 

 was considerable variability in vertical distribu- 

 tion between sampling dates, due in part to 

 variability in incident radiation and water tur- 

 bidity, but it was estimated that the numbers per 

 cubic meter from surface hauls should be mul- 

 tiplied by a factor of 1.54 to obtain a mean water 

 column density per cubic meter in the 3-m deep 

 water column. This factor was very close to the 

 1.45 calculated for the plankton as a whole, by 

 Reeve and Cosper (1973). As noted previously 

 (Reeve and Walter 1972), S. hispida has the ability 

 to attach itself to substrates in the laboratory and 

 lays its eggs on surfaces in clumps. It does not 

 attach significantly until near maturity and even 

 then, most of the population is usually to be found 

 swimming in the water column in aquaria. We 

 believe that the biomass estimates of our plankton 

 samples were not biassed downwards due to this 



behavioral pattern, as eggs are usually laid at 

 night while the plankton samples were taken dur- 

 ing the day, and the vertical sample series gave no 

 indication of a higher proportion of older animals 

 nearer the bottom. On the other hand, comparisons 

 of the size-frequency distribution of a population 

 sampled with a towed net and with an Okelmann 

 sledge lightly skimmed across the bottom, which is 

 an effective means of sampling the benthic 

 Spadella, usually yielded a few mature individuals 

 in the larger size classes which were absent from 

 the net. The sledge, however, only provided a 

 qualitative sample and it was not possible to ad- 

 just the biomass of Table 1 to take these few 

 animals into account. Our biomass estimates are, 

 therefore, slightly underestimated on this ac- 

 count. No estimates of egg numbers were made, 

 since Sagitta hispida does not deposit them in the 

 water column, but attaches them to objects on the 

 bottom. 



Ctenophores presented different sampling 

 problems. Lobate ctenophores, such as the genus 

 Mnemiopsis, tend to break up easily in nets and 

 are rapidly disintegrated in the usual fixatives. 

 Baker (1973) reported that transference of in- 

 dividual, newly hatched larvae by pipette from one 

 beaker to another would result in the disap- 

 pearance without a trace of over 90% of these 

 200-/im diameter animals. It was futile, therefore, 

 to attempt to assess the numbers of eggs or the 

 smallest larvae from net tows, and probably some 

 of size class A (0.8-4.4 mm) were also fragmented 

 beyond recognition. Even so, the pattern of dis- 

 tribution of biomass between the size classes (Ta- 

 ble 1) suggests that the fraction contributed by the 

 smallest unsampled or inadequately sampled 

 members of the population is small. It may be 

 presumed that animals in the larger size classes 

 were not avoiding nets, since Mnemiopsis is a 

 weak swimmer with no rapid escape behavior, and 

 hence were sampled adequately. No feasible 

 method was devised of making tows near the bot- 

 tom of this shallow water column with a 1-m 

 mouth diameter net, and pumps were impractical 

 for sampling ctenophores. The only indication we 

 have that Mnemiopsis does not exhibit any 

 marked vertical layering are observations by 

 scuba. 



Analysis of Samples 



The chaetognaths of the preserved samples (all 

 of which belonged to the species S. hispida) were 



239 



