NUMBER 4 0 • 31 



limiting Hiatella access. In the more protected bays and fjords of 

 inner coasts, C. compactiim is considerately less abundant and 

 often absent. Where lower salinities and higher sedimentation 

 are present, Lithothamnion glaciale becomes the dominant cor- 

 alline species of northern rocky bottoms. 



In the northwestern Atlantic, Clathromorpbum com- 

 pactiim is most abundant, relative to other corallines, at mid- 

 photic depths (10-20 m; Adey 1965, 1966b). We have developed 

 a clathrostrome index utilizing the number of samples from 

 each station depth, the mean thickness of collected samples, 

 and the maximum single-sample thickness as an indicator of 



FIGURE 25. (A) Typical boulder slope on low islands in northern 

 Labrador (at 25 m depth). Rock surfaces covered with coralline 

 crust dominated by C. compactum. Large, angular, sheet-fracture- 

 produced boulders protect glacial cobbles and small boulders from 

 downslope movement. (B) Diver with 105-mm-thick clathrostrome 

 extracted from the same locality. At a summer temperature of 5°C- 

 8°C, the growth rate of this crust was about 130 pm yr^', with an 

 age exceeding 800 years. 



clathrostrome development (Figure 26). Smcc all collections 

 (2010-201 1) were taken by a single individual and dives were of 

 a uniform length, the number of samples (N) taken at each sta- 

 tion depth are an indicator of the abundance of thick material. 

 As shown in Figure 26, for southern Labrador, clathrostrome 

 buildup also tends to peak at optimum abundance depths and 

 at mid-wave exposures. This is generally below the shallower 

 Alaria kelp zone but within the Agantm savanna (Adey and 

 Hayek, 2011), from 10 to 25 m. In northeastern Newfound- 

 land and the northern Gulf of St. Lawrence, where the water 

 is generally more turbid, peak clathrostrome development oc- 

 curs at shallower depths (5-10 m). The thickest C. compactum 

 collected in the northwest North Atlantic to date were taken 

 off Hopedale, Labrador, in the summer of 2011. Several crusts 

 exceeding 100 mm in thickness were found. 



Tissue Irregularity 



Under some conditions, the perithallial tissues of Clath- 

 romorphum compactum and C. nereostratimi, as produced 

 by an essentially planar (faceted) or domed meristem, can be 

 quite regular. However, grazing by sea urchins, death due to 

 disease, and grow-out from damaged postmature conceptacles 

 can change not just cellular orientation but also planar tem- 

 poral variations in seasonality and therefore calcite crystal 

 chemistry. When the meristem and upper perithallial tissue 

 are removed by grazing or other abrasion, meristem regenera- 

 tion is possible, approximately to level of the base of the most 

 recent layer of conceptacles (200-500 pm deep). While wound 

 tissue tends to repair these irregularities, in large part, it is this 

 meristem regeneration on an irregular surface, rather than on 

 a planar or domed surface, that renders archiving potentially 

 problematic. 



The most difficult scenarios for climate archiving are the 

 removal of calcified tissue and the death of the meristem and 

 upper perithallium to below the level at which cells are capable 

 of meristem regeneration. In the clathrostrome environment, 

 this "dead" surface is for a time occupied by other crustose or 

 filamentous species of algae, boring algae, and encrusting inver- 

 tebrates. Fventually, the dead surface is likely to be overgrown 

 by new Clathromorpbum tissue from the sides or in some cases 

 by new spore settlement. However, several years may intervene, 

 creating a disconformity or even an unconformity of imknown 

 longevity. By matching several seasonal bands or patterns in ad- 

 jacent perithallium or other plants or by dating, it may be pos- 

 sible to fill in such unconformities. 



Chiton Grazing 



In the northwestern North Atlantic Subarctic, chitons and 

 limpets are prominent grazers of Clatbromorpbum compactum 

 surfaces. Chitons, particularly the species Tonicella rubrum^ are 

 ubic]uitous on coralline surfaces, especially those of C. compac- 

 tum. Collected specimens of C. compactum, left to dry, usually 

 reveal man\' small animals that were not visible at the time of 



