176 



Fishery Bulletin 104(2) 



Similarly, high-density aggregations of brittle stars 

 and brachiopods in boulder-cobble areas and fields 

 of sea pens and sea urchins in sand and mud habi- 

 tats also may provide space and structure for other 

 organisms (e.g., Brodeur, 2001). 



With the exception of black corals and sea pens, 

 the largest structure-forming invertebrates, such as 

 sponges, Metridium spp., and crinoids, were most 

 common on high- to moderate-relief rocky habi- 

 tats. These long-lived organisms are likely to be 

 favored in stable habitats that are more insulated 

 from sediment transport and high particle loads 

 than low-relief, mud-dominated areas (Lissner and 

 Benech, 1993). Large invertebrates add structure 

 and micro-scale complexity to these rocky habitats 

 that already contain high-to-moderate amounts of 

 relief Sponges, with their broad distributions, may 

 also provide structure for flatfishes in low-relief 

 mud habitats (Ryer et al., 2004) 



Black corals and gorgonians, in contrast, are 

 more commonly found in current-swept areas near 

 drop-offs and under ledges (Grigg, 1974; Parrish, 

 2004). In our study, these invertebrates were found 

 in low-relief mixed cobble-boulder-sand habitats at 

 100-225 m depths, providing significant vertical 

 structure for potential use by a wide variety of 

 organisms. 



Aggregations of sea pens and sea urchins may 

 provide important structure in low-relief sand and 

 mud habitats where there is little physical habi- 

 tat complexity. In addition, these organisms may 

 provide refuge for small planktonic and benthic 

 invertebrates, which in turn may be preyed upon 

 by fishes. They also may alter water current fiow, 

 thereby retaining nutrients and entraining plank- 

 ton near the sediment. Urchins rapidly respond 

 to patches of drift kelp (Harrold and Reed, 1985), 

 which provide organic material to deep sea habitats 

 (Harrold et al., 1998). 



One of the central issues currently relevant to 

 structure-forming invertebrates is the degree to 

 which these species contribute to the spawning, 

 breeding, feeding, or growth-to-maturity of eco- 

 nomically important fishes. Although there are several 

 studies that report fish-invertebrate associations within 

 common habitats (Hixon et al.^), or make anecdotal or 

 general observations on fish-invertebrate associations 

 (e.g., Krieger and Wing, 2002), few studies have sys- 

 temically quantified these relationships. In our study. 

 for 9105 observations on the larger invertebrates found 

 on southern California rocky banks only 1.8% of indi- 

 viduals had other organisms lying on or attached to 

 them. Moreover, the vast majority of these organisms 

 were other invertebrates, including crinoids, sponges, 

 crabs, basket stars, brittle stars, seastars, anemones 

 and salps. Less than 1% of the observations of organ- 

 isms actually sheltering in or located on invertebrates 

 involved fishes (a total of five individuals and one egg 

 case), and most were observed on large black corals 

 (Table 3). This result implies that fishes are not strong- 



10 



05  



00 



0,5 



0,0 



10 



0,6 - 



0,0 

 4 





 10 



Gorgonians 



n=27 



Blacl< corals 



1 



A 



^^ 



-ia 



Flat sponges 



n 



nll^nHn^ 



n^ 



Barrel sponges 



on 



A 



r;i r^ r^-i n ^ ,J^ r^ 



Stielf sponges 



^^^n 



1 



Habitat Code 



n=139 



i 



htJx 



M- 



Vase sponges 



1 



A 



I, 



m 



Oii 



r^ ,^ ^ r-p 



Foliose sponges 



1 



M 



n r;n ri rp I I r-1 



i 



XU- 



T l X -r -I- T 



TT RR RM BB EC BS CB MB SB CS SC SP I^G SG MM MS SS 



Habitat code 



Figure 7 



Mean density of gorgonians, black corals, and sponges in 

 habitat patches of each substratum type. Vertical bars are 

 ± one standard error. See page 169 for definitions for the 

 substrate abbreviations along the .v axis. 



ly associated with structure-forming invertebrates in 

 the areas we surveyed off southern California. 



However, we should note that our observations were 

 limited to daylight hours and that the viewing angle 

 from the submersible generally precluded seeing inside 

 some of the sponges (especially vase and barrel types). 

 Moreover, our analyses focused on associations between 

 fishes and individual solitary invertebrates, most of 

 which were <0.5 m in height. We did not examine as- 

 sociations between all structure-forming invertebrates, 

 nor did we examine associations between invertebrates 

 and assemblages of fishes at the level of discrete habitat 

 patches (100-1000 m scale) (e.g., Tissot et al.^) 



■• Tissot, B. N., M. A. Hixon, D. L Stein. Unpubl. manu- 

 script. Habitat-base submersible assessment of groundfish 

 assemblages at Heceta Bank, Oregon from 1988-1990. 



