32 



SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



collection; these mostly nocturnal invertebrates crawl out of nu- 

 merous crevices and borings in the drying crusts. The shallow, 

 parallel, scraped groove patterns of chiton grazing are abundant 

 and easily visible in SEM images of C. compactum surfaces (Fig- 

 ure 9). However, chitons rarely affect the regularity of carbonate 

 buildup since they graze only the surficial part of the epithal- 

 lial tissue; indeed, like grazers of grasslands, chitons are prob- 

 ably nearly symbiotic in their relationship to Clathromorphum 

 plants, preventing settling by organisms, algal or invertebrate, 

 that otherwise might cover the surface (Steneck, 1992). 



Sea Urchin Crazing 



Strongylocentrotus droebachiensis, the green sea urchin, is 

 an often-abundant species on most shallow rocky shores in the 

 northwestern North Atlantic. Like Clathromorphum, the echi- 

 noid S. droebachiensis is a member of a North Pacific group (the 

 Strongylocentrotidae) and is circumboreal in distribution. North 

 Atlantic and North Pacific. In the Bering Sea, additional species 



of Strongylocentrotidae join with S. droebachiensis. These spe- 

 cies are typically grazers of brown seaweeds, including Alaria, 

 Saccharina, Laminaria, and Chordaria, where they are avail- 

 able. Green sea urchin grazing can be strongly limited by wave 

 action in exposed, shallow waters, and they are known to move 

 to deeper waters in winter to avoid the more destructive wave 

 action common at that time of the year. Controlled by shallow- 

 water wave action, they will aggregate into "fronts" or lines and 

 can graze into the margins of shallow kelp beds, reducing the kelps 

 to stubble or "barren grounds." However, S. droebachiensis will 

 actively avoid some common algae of northern Atlantic regions 

 (e.g., Agarum, Desmarestia, Ptilota) because they are protected 

 by distasteful or toxic chemical compounds. At the depths where 

 C. compactum is most abundant and well-developed clath- 

 rostromes are most common, it is these protected species of fleshy 

 algae that form the overstory; these species create a patchy "sa- 

 vanna" of coralline alternating with fleshy algal forests that have 

 been mistakenly referred to as coralline-urchin barrens. Strongylo- 

 centrotus droebachiensis, at moderate density but with insufficient 



Clathrostrome index (N x mn + max) 



WAVE EXPOSURE 



<^ EXTREME ZZZZ=ZZZZZZ: MODERATE ' LOW IlC> 



2. . Balanus balanoides 



P^""y' gravel mud 



FIGURE 26. Clathrostrome index for the southern Labrador coast (see text for explanation of formula) from the 2010 

 cruise of the RA^ Alca i. The base diagram is a community structure model for western Atlantic Subarctic rocky shore 

 from Adey and Hayek (201 1). Abundance and thickness of C. compactum crusts peak from 12 to 20 m on moderately 

 exposed shores. LWST = low water spring tides. 



