FISHERY BULLETIN: VOL. 74, NO. 3 



On four evenings we noted the first gammar- 

 idean seen as much as 1 m above the bottom, and 

 found this level attained 27 to 39 (.r = 34) min after 

 sunset. On each occasion, individuals had been 

 visible close among the bottom algae for about 5 

 min before any of them rose to the 1-m level. The 

 final return to the sea floor at daybreak was 

 monitored on four mornings, when the last in- 

 dividual was seen 1 m above the bottom 26 to 41 (.f 

 = 35) min before sunrise. Similar to the evening 

 situation, individuals continued to be visible close 

 above the bottom algae for an additional 5 min, or 

 so. 



To roughly determine the proportion of gam- 

 marideans that rise from the sea floor at night, we 

 compared the amphipods in a sample of benthic 

 algae at night, with a similar sample taken in the 

 same place the following day (both samples, 

 loosely packed in a 2.3-liter plastic bag, were taken 

 immediately after plankton collections). Both 

 samples contained 2.5 ml of animals (including 

 other forms besides amphipods). Nevertheless, the 

 limited data (Table 8) indicate that the numbers of 

 some gammarideans on the algae, notably B. 

 transverm, dropped sharply after dark, those of 

 others, including Ericthonias braziliensis (Figure 

 5U), experienced a lesser decline, and those of still 

 others remained essentially unchanged. Data 

 from the collections (Table 7) and direct observa- 

 tions indicate that there were fewer amphipods on 

 benthic algae after dark because many have risen 

 into the water column. But the tendency to leave 

 the sea floor clearly varies between species and in 

 perhaps no species is it absolute. Probably at least 



Table 8.-Gammaridean amphipods collected in samples of 

 benthic algae, day and night. 



'At least some of the unidenitfied forms probably are juveniles 

 of tfie species listed above. 



many individuals make only short excursions into 

 the water column. 



Caprellid Amphipods 



We never saw caprellids above the bottom 

 during the day, but saw them, though infrequent- 

 ly, in the water column at night. Consistent with 

 these observations, caprellids were collected in the 

 plankton net at night, but never during the day. 

 The nighttime middepth collections (n = 6) took x 

 = 3.2 Caprella pilidigita and x = 0.2 C. califor- 

 nica, whereas the near-bottom collections {n = 6) 

 took X = 9.7 C. pilidigita and x = 5.2 C. califor- 

 nica. In addition, a single unidentified juvenile 

 was taken in one nighttime middepth collection. 

 Size ranges of specimens: C. pilidigita 4 to 18 mm, 

 C. californica 6 to 10 mm, and the unidentified 

 juvenile 3 mm. Both C. californica and C. pilidigi- 

 ta (Figure 5V) were at all times numerous amid 

 the low benthic algae that floors most of the study 

 area. 



Euphausid Larvae 



The calyptopis larvae of euphausids occurred 

 regularly in both day and night collections. 



Euphausid Adults and Juveniles 



Euphasid adults and juveniles were neither seen 

 nor collected in the water column during the day, 

 but occasionally swarmed around our lights at 

 night. The few individuals collected in the plank- 

 ton net (Tables 1, 2) are of one species: Thijsan- 

 oessa spinifera. The numbers collected, however, 

 underrepresent the numbers we saw in the water 

 (all of which appeared to be T. spinifera), probably 

 because this relatively large, motile animal effec- 

 tively evaded our net. Rather than rising from the 

 sea floor at nightfall, as do so many other noctur- 

 nal components of the plankton discussed above, 

 this euphausid seems to move in from deeper 

 water. Unlike the other forms, euphausids were 

 not taken in our extensive diurnal sampling of the 

 benthos (Hobson and Chess in prep.). 



Caridean Larvae 



Based on the collections (Tables 1, 2), caridean 

 larvae are numerous in the plankton during both 

 day and night, but more so at night. Furthermore, 

 there are more larger individuals in the water 



578 



