NUMBER 40 . 33 



algal food, can survive by severely slowing their growth and can 

 even become reproductive on overgrazed coralline bottoms (see 

 Adey and Hayek, 201 1, for a review of this topic). 



In the northwestern Atlantic, when larger fleshy algae are 

 not available, large populations of very small green sea urchins 

 can subsist in crevices, without growth, on diatom and filamen- 

 tous algal mats (Himmelman, 1986). Coralline-dominated bot- 

 toms, including clathrostromes, usually harbor populations of 

 young urchins that can survive for many years awaiting more 

 favorable feeding conditions. This is an unstable situation, and 

 although very small "standby" urchins are not likely to signifi- 

 cantly affect the growth and anatomy of Clathromorphum coni- 

 pactum, population explosions leading to larger urchins, with 

 transitory fleshy algae for support, can do considerable damage 

 to C. compactum plants. Occasionally, adult green sea urchins 

 will graze surfaces of C. compactum. Although the sea urchins 

 typically remove only scattered patches or grooves of coralline 

 surface that will be regenerated, this grazing can create an irregu- 

 lar terrain of secondary tissue. When sea urchin grazing is very 

 extensive, reaching below the regenerative surface band, an un- 

 conformity will develop. Usually, this surface is overgrown again 

 from the side, beginning with new basal, or hypothallial, tissue. 

 The result can be several years of missing carbonate. Sometimes 

 this is obvious (Figure 2B) because a layer of epiphytes and en- 

 dozoic green algal borers, or even other corallines, have settled; 

 however, if the missing periods are not recognized from SEM ob- 

 servations, the resulting archive can be seriously misinterpreted. 



Concepiacle Breakout 



Typically, during post reproduction and after spores are re- 

 leased, cells at the margins of the conceptacle roofs, or the roof 

 itself, will regenerate meristem cells (Figures 9, 10, 12B). This 

 leaves conceptacle cavities often with secondary calcification in 

 the sporangium walls; these are quite visible and easily avoided 

 during geochemical analysis. However, in some cases, a con- 

 ceptacle roof can break out, either autonomously or because of 

 grazing. When that happens, secondary meristems form, and 

 new tissue from the sides and bases of the conceptacle cavities 

 develops (Figure 12B). The accompanying cellular carbonate, 

 although showing a minimum pattern disruption, is formed 

 many months later than the surrounding carbonate. This can 

 be hard to recognize without SEM examination. 



Death Due to Disease 



Killing disease has been documented in corallines (Littler and 

 Littler, 1994) and has long been recognized in Subarctic waters. 

 Typically seen as white, expanding patches with green centers (as 

 epiphytic and endophytic algae occupy the dead crust), these patches 

 are usually no more than a few to 10 cm in diameter. However, they 

 can expand to a meter or more. Generally, when the patches are 

 small, they are regrown laterally by secondary hypothallium; in the 

 case of larger patches, resettlement probably occurs. Sometimes, 



grazing by green sea urchins on the epiphytic and endophytic sec- 

 ondary algae in the dead patch can remove several years of Clathro- 

 morphum carbonate accretion. This can develop an unconformity, 

 with new growth layers not being parallel to the pre-disruption lay- 

 ers; the number of missing layers would have to be documented by 

 matching of cyclic patterns or cross dating. 



DISCUSSION 



We have described the patterns of growth, reproduction, 

 and layered carbonate formation, the intricacies of calcifica- 

 tion, and the basic ecological patterns for the most abundant 

 Subarctic-Arctic Clathromorphum spp. This description provides 

 a framework for increasing our understanding of a key element 

 of a widespread and characteristic ecosystem and its basal bio- 

 genic carbonate component. The structure of the overlying fleshy 

 seaweed community in the northwestern Atlantic, characterized 

 with quantitative data and supported by many published studies 

 of plant-animal interactions, has been summarized by Adey and 

 Hayek (201 1). Although ecological studies of the Subarctic fringe 

 occurrence of this ecosystem in the Aleutian Islands have been 

 published (e.g., Steneck et al., 2002; Springer and Estes, 2003; 

 Estes et al., 2005; Trites et al., 2007; Chenelot et al., 2011), the 

 information for the northwestern North Pacific and for C. nereo- 

 stratum is more fragmentary. A better understanding of the aut- 

 ecology of C. nereostratum in the Aleutian Islands and the island 

 and mainland coasts of the Bering and Okhotsk Seas is necessary. 



Climate Archives 



The information we have presented can also provide criti- 

 cal support for further development of already promising Arctic- 

 Subarctic climate archives from both C. compactum and 

 C. nereostratum. In the skeletons of these species, there is po- 

 tentially a wealth of ecological and water state (climate) infor- 

 mation available in the complex formation of yearly layers of 

 carbonate-encased cells and reproductive structures. Also, there 

 is no known biological limitation, inherent in the coralline struc- 

 ture itself, to the preservation of the information built into the 

 carbonate, although the high-magnesium calcite itself is relatively 

 unstable and subject to later diagenesis. Given geographic iden- 

 tification of sites with the geomorphological and oceanographic 

 conditions that produce long-term continuous accumulation of 

 carbonate, more than a thousand years of detailed subannual 

 ecological and climate information can be obtained. Data de- 

 rived solely from anatomical features not degraded by chemical 

 alteration (e.g., thickness of yearly bands) could extend the ar- 

 chive considerably further. Recovery of specimens transported to 

 sedimentary environments could also produce records of longer 

 duration. However, as we have described, the complexity of the 

 Clathromorphum skeleton is such that considerable care needs 

 to be taken to understand the details inherent in the anatomy of 

 this genus if analytical variation is to be separated from environ- 

 mental variation. 



