HOBSON and CHESS: TROPHIC INTERACTIONS 



column after dark. We made no attempt to iden- 

 tify our specimens to species, but probably many 

 are larvae of the two species discussed as adults 

 and juveniles, below. 



Caridean Adults and Juveniles 



Caridean adults and juveniles were observed in 

 the water column only at night. On the one even- 

 ing that the event was noted, the first individual 

 seen rising as much as 1 m above the bottom 

 attained this level 39 min after sunset. Adult and 

 juvenile carideans are absent in the daytime 

 collections, but Hippohjte clarki (Figure 5W), was 

 sometimes numerous in collections made after 

 dark. The nighttime middepth collections (n = 6) 

 took .r = 25.2 H. clarki, and the near-bottom 

 collections {n = 6) took x = 10. Only one other 

 adult caridean was collected, this a single Eualus 

 herdmani in a nighttime near-bottom sample. The 

 specimens of H. clarki were 4 to 10 mm long, the 

 single E. herdmani 12 mm. Hippohjte clarki is 

 numerous during the day in the kelp forest bor- 

 dering the study area, where it concentrated in the 

 dense surface canopy and upper regions of these 

 massive plants. At the same time E. herdmani 

 predominated in the lower regions of the same 

 plants (Hobson and Chess in prep.). 



The stomach contents of 20 H. clarki collected 

 during day and night were examined. DAY- 

 TIME-Of 10 (8-16 mm, .f = 10.8) collected from 

 giant kelp plants during midafternoon, 4 were 

 empty, and the other 6, whose stomachs averaged 

 17% full, contained only extensively macerated 

 fragments. NIGHTTIME-Of 10 (8-17 mm,. r = 12) 

 collected close to giant kelp 1 h before first morn- 

 ing light, 1 was empty, and the other 9, whose 

 stomachs averaged 34% full, contained a variety of 

 prey, some of it fresh: mollusk veligers in 4 (28% of 

 total volume); foraminiferans in 3 (9% of total 

 volume); shrimp larvae in 1 (11% of total volume); 

 and extensively macerated material in 7 (52% of 

 total volume). These limited data indicate this 

 animal is primarily a nocturnal predator, but only 

 a relatively few seem to swim far from algal cover. 



Reptantian Zoea 



Based on the collections (Tables 1, 2), zoea were 

 consistently present in moderate numbers at all 

 levels of the water column during both day and 

 night, but were most numerous there after dark. 

 Usually we failed to notice zoea in the water, but 



one night observed them in dense swarms close to 

 the bottom. 



Brachyuran Megalops 



Our plankton collections (Tables 1, 2) indicate 

 that brachyuran megalops were frequently pres- 

 ent, if not numerous, in the water column at night, 

 but only infrequently present during the day. 



Bryozoan Larvae 



The cyphonautes larvae of bryozoans (Figure 

 5B) were consistently taken in substantial 

 numbers by middepth and near-bottom collections 

 both day and night. 



Chaetognaths 



Our collections regularly took chaetognaths both 

 day and night (Tables 1, 2), but even though these 

 animals are relatively large, we failed to see them 

 in the water, presumably because they are largely 

 transparent. Chaetognaths probably were more 

 numerous in the study area than our collection 

 data indicate, owing to a mobility that would 

 permit many to evade our net. 



Larvaceans 



We collected larvaceans in our plankton net both 

 day and night, but only in small numbers (Tables 1, 

 2). It became clear that these numbers far under- 

 represented the numbers present, however, when 

 we examined the gut contents of the blacksmith 

 (recounted below). Most larvaceans in the area are 

 less than 0.5 mm in diameter, and apparently their 

 pliable bodies readily squeeze through the 

 0.333-mm mesh of our net. So we made a midday 

 tow in the study area using a 0.25-m net with a 

 0.253-mm mesh. Significantly, larvaceans, most 

 being of the genus Oikopleura (Figure 5A), made 

 up 20% of the sample. There was one larvacean to 

 about every six copepods (calanoids and cy- 

 clopoids), and they ranged from 1 to 3 mm long, 

 with a diameter of about 0.2 to 0.5 mm. Sig- 

 nificantly, active individuals throughout much of 

 this size range were observed passing through a 

 piece of 0.333-mm mesh net placed among them in 

 a petri dish. Because these animals are transpar- 

 ent, and so small, we failed to see them in the 

 water. 



579 



