30 . SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



FIGURE 24. Meristem fracture and calcification zone in a specimen of C. compactitm from southern Labrador for comparison with 

 C. nereostratwn (Figure 17). (A) The meristem (arrow to the right) shows fracturing in the calcification zone. The conceptacle formed the 

 previous fall and winter, with expansion down into the previous summer band. A new meristem formed over roof but has not yet filled 

 the depression in the epithallium (see Figure 9B). (B) Fracture zone in the meristem. (C) Fracture zone in the meristem opened up to reveal 

 distinct interfilament grooves (precipitation cavities?) between meristem cells (arrows). (D) Meristem fracture zone (in summer) looking 

 into the perithallium. Dense organic material covers the inner-wall crystal band, with interfilament crystals having no organic cover. 



Hiatella arctica, a boring bivalve, is the chief biotic com- 

 ponent responsible for degrading and limiting Clathromorphnm 

 compactum buildup in the northwestern Atlantic. Hiatella arc- 

 tica also occurs in C. nereostratitm in the North Pacific (Che- 

 nelot et al., 2011), but it is overshadowed by an abundance of 

 other invertebrate infauna. Hiatella arctica must gain access to 

 the C. compactum crust during settlement at a growing coralline 

 margin or following sea urchin grazing or other injury. The inter- 

 calary meristem, with an epithallium subject to constant chiton 

 grazing (and more occasionally sea urchin and limpet grazing), 

 prevents most invertebrate and seaweed recruitment. When re- 

 cruitment of several coralline species has been optimal, entirely 

 covering a rock surface with coralline carbonate, and when sea 

 urchin grazing is limited, H. arctica recruitment is minimal. 



Thus, there is an inverse relationship between the success of 

 C. compactmn and H. arctica; when the factors supporting strong 

 clathrostrome development are optimal, boring by H. arctica is 

 most limited. Burial of the base of old C. compactum mounds by 

 accumulating fine sediment can also create anaerobic zones that 

 prevent further Hiatella boring. Under these conditions, continu- 

 ous carbonate accumulations exceeding 1000 years can develop 

 (Adey, unpublished). 



Lithothamnion lemoineae, a branching coralline species, 

 has an ecology similar to that of Clathromorphnm compactum. 

 Although generally less abundant, it is an integral component of 

 northwestern Atlantic clathrostromes. Lithothamnion lemoineae 

 is a particularly important component of clathrostromes in that 

 it often "seals" the sides of C. compactum mounds, thereby 



