5.6.1 Invertebrate Grazers (Other Than 

 Sea Urchins) 



There are few published experimental 

 studies that have assessed the effects of 

 gastropods on algae in subtidal regions. 

 A recent study by Watanabe (1983, 1984a) 

 in central California assessed some of the 

 effects of three species of Tegula in a 

 shallow Macrocystis forest. These gas- 

 tropods normally live and feed on the 

 fronds and laminae of Macrocystis . During 

 and after storms they are abundant on sub- 

 strata below the plants, but quickly 

 occupy fronds again when calmer conditions 

 ensue. Their grazing activities, however, 

 had no discernible effect on algal distri- 

 bution or abundance. 



Schiel and Foster (unpublished data) 

 noted an increase in abundance of Tegula 

 on the substratum of reefs in central 

 California after winter storms removed the 

 fronds of most Macrocystis plants. Tegula 

 grazed heavily on the broken ends of old 

 fronds and also on younger fronds, 

 preventing them from growing. Tegula also 

 grazed the ends of blades on Pterygophora 

 which were damaged in the same storms. 

 These effects were not long-lasting, 

 however, as the vegetative blades and 

 sporophylls re-grew in the spring. 



Schiel (1981) used both exclusion and 

 inclusion cages to assess the effects of 

 limpets and turbinid and trochid gastro- 

 pods in the shallow subtidal of northern 

 New Zealand. Limpets and turbinids could 

 prevent the establishment of large brown 

 algae on a small scale (25 x 25 cm 

 patches), presumably by grazing algal 

 spores. These grazers had greater effects 

 at higher densities. 



Other invertebrate grazers such as 

 abalone ( Hal iotis spp.) and sea stars 

 ( Patiria spp. ) may have small-scale 

 effects on algal abundances in Macrocystis 

 forests, but their effects have not been 

 assessed. Small Crustacea can be very 

 abundant in algal turfs (cf. Kennelly 

 1983) and may be major grazers on algal 

 spores. Experiments assessing their 

 effects, and the interactions of grazers 

 that co-occur on areas of substratum, have 

 yet to be done. There also may be 

 indirect effects of invertebrates on algal 

 assemblages. For example, Santelices et 



al. (1983) have shown that spores of many 

 seaweeds can survive digestion by sea 

 urchins, and suggest that this may affect 

 the abundance of opportunistic plants in 

 grazed areas and perhaps the dispersal of 

 species that occur later in succession. 

 Schroeter et al . (1983) found that the sea 

 star Patiria was an abundant predator of 

 Lytechinus anamesus in the San Onofre kelp 

 forest, affecting the local distribution 

 of the sea urchin. This echinoid can be 

 an important grazer of juvenile 

 laminarians in local patches (Dean et al. 

 1984), and an alteration in its dispersion 

 patterns could allow successful 

 recruitment of kelps. Tegner and Dayton 

 (1981) suggested that the spiny lobster, 

 Panul irus interruptus , may be a major 

 predator of sea urchins in southern 

 California. This again could have effects 

 on algal assemblages by reducing the 

 incidence of grazing. Laboratory 

 experiments indicated that lobsters would 

 eat echinoids, but no data were presented 

 on lobster abundances in the kelp forest, 

 making it difficult to assess their 

 present effects. 



5.6.2 Effects of Fish 



A few species of fish in Macrocystis 

 forests are known to include algae in 

 their diets (Quast 1968). A recent study 

 by Harris et al. (1984) suggested that 

 fish, particularly the halfmoon, Medial una 

 cal iforniensis , and the opal eye, Girella 

 nigricans , can be important grazers of 

 small Macrocystis sporophytes on a local 

 scale. At Naples Reef off Santa Barbara, 

 fishes grazed about 59% of sporophytes 

 (< 10 cm tall) that were concealed in a 

 turf of ephemeral algae, while 94% of 

 those on open reef quadrats were grazed. 

 They reported that plants > 10 cm in 

 height were not grazed, suggesting a size 

 refuge from fish grazing. The result of 

 this grazing was a small-scale change in 

 the dispersion pattern of juvenile 

 Macrocystis on the reef. They did not 

 report the abundances of the fish present 

 over the reef, however, and no 

 observations of fish feeding behavior were 

 mentioned. 



There are no studies that demonstrate 

 extensive modification of the biota by 

 grazing fishes in Macrocystis forests. 

 Indirect effects are reported in some 



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