4.7.2 Macroalgae 



Andrews (1977) and Goff and Glasgow 

 (1980) recently summarized information on 

 seaweed pathogens, and the latter 

 publication is a particularly compre- 

 hensive account. Perhaps the most 

 notorious disease is "black rot" of 

 Macrocystis , which is visible as dark 

 areas on the margins of the blades. These 

 become lesions, and the blades eventually 

 disintegrate. The symptoms may occur 

 throughout large kelp stands, suggesting 

 that the disease may cause large-scale 

 loss of plants (ZoBell 1946). Scotten 

 (1971), however, could not find evidence 

 of a bacterial or fungal cause, and 

 suggested that black rot is not caused by 

 a pathogen, but may simply be 

 deterioration associated with elevated 

 water temperatures. Dean (pers. comm.) 

 observed extensive black rot and numerous 

 sinking fronds in southern California kelp 

 forests in October 1983, coincident with 

 the warm water associated with the current 

 "El Nino" (see Section 2.3). 



Recent observations of adult 

 Macrocystis pyrifera isolated in a large 

 container at Santa Catalina Island 

 revealed another potential disease (Gerard 

 pers. comm.). Blades (especially those 

 near the holdfast) growing in warm, high 

 nutrient water developed numerous small 

 holes and eventually deteriorated. Dean 

 (pers. comm.) noted similar symptoms in 

 small Pterygophora cal ifornica off San 

 Onofre in 1983. The symptoms in 

 Macrocystis led to the tentative name of 

 "shot hole disease," but the cause is 

 unknown. However, just as in monocultures 

 of terrestrial plants, the common 

 occurrence in culture of this hitherto 

 rarely observed symptom suggests that 

 other unanticipated problems with disease 

 may occur in future attempts to isolate 

 Macrocystis in culture (see Goff and 

 Glasgow 1980 for a review of pathogens in 

 other currently-cultivated seaweeds). 



Macrocystis as well as other 

 macroalgae are inhabited by a diverse 

 group of potential pathogenic organisms 

 such as nematodes, algal and animal epi- 

 growths, algal parasites, bacteria, and 

 fungi. Plants may be found with tumors 

 and galls (Andrews 1977, Goff and Glasgow 

 1980). Few of these have been observed to 



cause severe damage to individuals in kelp 

 forests, and none has been observed to 

 cause damage to populations. 



4.7.3 Invertebrates 



Invertebrate pathogens and parasites 

 are also common, and some have significant 

 impacts on populations. Pearse et al . 

 (1977) reported a mass mortality of red 

 sea urchins ( Strongylocentrotus 

 franciscanus ) near Santa Cruz, and 

 reviewed other occurrences of this 

 phenomenon in California. When affected, 

 the urchins' spines are no longer held 

 upright and are eventually lost, the 

 epidermis degenerates, and the animal 

 dies. Similar symptoms and widespread 

 mortality have occurred recently in sea 

 urchin populations in Nova Scotia (Miller 

 and Colodey 1983). The cause of the 

 disease in California is unknown; a 

 protozoan may be the cause in Nova Scotia 

 (Miller and Colodey 1983). If sea urchin 

 grazing limits kelp distribution, then 

 such a disease can ultimately result in 

 kelp forest enlargement, as it did at the 

 site near Santa Cruz (Pearse and Hines 

 1979), and as it is doing in Nova Scotia 

 (Pearse pers. comm.). Observations in 

 these and other areas suggest that disease 

 may be an important factor in the 

 regulation of sea urchin populations 

 (Pearse pers. comm. ) . 



Mortality of other echinoderms, 

 particularly the bat star ( Patiria 

 miniata ) has been observed in southern 

 California, both along the mainland 

 (Schroeter and Dixon pers. comm.) and at 

 San Nicolas (Harrold pers. comm.) and 

 Santa Catalina Island (Gerard pers. 

 comm.). Similar mortality has occurred in 

 the Gulf of California (Dungan et al. 

 1982). When the water is abnormally warm, 

 bat stars become covered with a white, 

 mold-like film and eventually die. 

 Affected bat stars that fell down steep 

 slopes into colder water at Santa Catalina 

 Island apparently recovered. Schroeter et 

 al. (1983) have shown that P_. miniata 

 predation may significantly alter the dis- 

 tribution and abundance of white sea 

 urchins ( Lytechinus anamesus ), and feeding 

 by the star may also significantly affect 

 other populations, including plants (see 

 Section 4.4.3 above). Bat star 

 populations have been nearly destroyed by 



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