HOBSON and CHESS: TROPHIC INTERACTIONS 



various larval forms (see Tables 1, 2)-tend to be 

 less than 2 mm in their greatest dimension. Forms 

 appreciably larger than this— including chaetog- 

 naths and some larvaceans-tend to be transpar- 

 ent. These organisms are equally numerous in the 

 water column at night, and none are residents of 

 the study area. The species are widespread in the 

 water columns of the various inshore habitats, and 

 also offshore. 



This assemblage resists a common label. Most of 

 the species have been considered holoplankton 

 (planktonic throughout the whole of their life 

 histories: Newell and Newell 1963), but this term 

 excludes the larval forms so prominent here. The 

 larval forms generally are considered meroplank- 

 ton (planktonic during some stage in their life 

 histories, but benthonic during some other: Newell 

 and Newell 1963), but this term has been used in 

 general reference to organisms that are plank- 

 tonic at night, but benthonic by day (e.g., Williams 

 and Bynum 1972). As noted above, we do not use 

 these terms because they fail to define categories 

 meaningful to the concepts developed in this 

 paper. 



Fishes that forage in the water column by day 

 have certain characteristics relating to the prob- 

 lems they face as diurnal planktivores. Sig- 

 nificantly, of the four diurnal planktivores studied 

 at Santa Catalina, three-the senorita, the kelp 

 perch, and the small juvenile olive 

 rockfish— outgrow this habit. Apparently as they 

 grow larger they find the tiny organisms in the 

 mid-waters increasingly inappropriate as prey. 

 We believe that each is limited in taking very 

 small prey by the size and structure of its mouth, a 

 problem solved by changing either feeding place, 

 or feeding time. Thus, the senorita and kelp perch 

 (noted by Hubbs and Hubbs 1954, to have similar 

 dentition and feeding habits) increasingly aban- 

 don the water column as a hunting ground as they 

 grow and shift to prey on organisms that live on 

 algae. The small juvenile olive rockfish, on the 

 other hand, continues to feed in the water column, 

 but assumes nocturnal habits that bring it into 

 contact with the larger organisms that rise above 

 the sea floor at night (see below). The senorita and 

 kelp perch, both relatively small-mouthed species, 

 generally shift their food habits when about 100 

 mm long; the olive rockfish, with a much larger 

 mouth (compare Figures 6, 11, 12), generally shifts 

 when under 65 mm long. 



The fourth diurnal planktivore studied at Santa 

 Catalina, the blacksmith, retains its planktivorous 



diet through adulthood. It does so despite growing 

 to a relatively large size because it has, among 

 other adaptive features, a small mouth specialized 

 for this habit. Judging from its numbers, the 

 blacksmith is highly successful in the warm tem- 

 perate waters of southern California. But it does 

 not range far into the colder waters northward, 

 and all its many congeners live in the tropics. The 

 blacksmith embodies morphological features un- 

 characteristic of temperate-zone fishes, but which 

 are widespread among tropical species. In writing 

 of reef fishes in the tropical Atlantic Ocean, Davis 

 and Birdsong (1973) described morphological 

 specializations adaptive for foraging on small 

 organisms in the mid-waters, and although they 

 do not make the point, all their examples are 

 species that feed by day. Especially striking are 

 the modifications of head and jaws, including 

 dentition, that permit even relatively large in- 

 dividuals to effectively capture tiny prey in open 

 water. 



The Mid-Waters at Night 



The nocturnal zooplankton include, in addition 

 to the organisms also present during the day, a 

 large array of forms that rise at the onset of 

 darkness from daytime shelters on, in, or close to 

 the sea floor or other cover. These nocturnal 

 additions to the zooplankton include various poly- 

 chaetes, mysids, cumaceans, gammaridean and 

 caprellid amphipods, isopods, tanaids, carideans, 

 and others (see Tables 1, 2). Most exceed 2 mm in 

 their greatest dimension, and many are 7 to 10 

 mm, and longer. Unlike the full-time zooplankters, 

 which have no particular relation to the study area, 

 these part-time zooplankters are local residents 

 that rise at night from substrata they are closely 

 associated with during the day. 



The nocturnal components of the zooplankton 

 seem to have reasons for rising into the water 

 column at night that are as diverse as their 

 morphologies. Because they have diverse habits 

 that are little known, we feel that terms defining 

 ecological categories among them are premature. 

 Bousfield (1973), and others, have referred to many 

 such forms as tychoplankton, but this term implies 

 that presence in the water column is by chance, or 

 accident-a description that fits very few, if any, of 

 the forms considered here. Many are nocturnal 

 feeders; e.g., when the mysid Siriella pacifica 

 moves into the mid-waters after dark, it feeds on 

 copepods and other smaller zooplankters. Similar- 



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