N U M B E R 4 0 . 2 1 



Clathromtxphum nereostratum 

 Amchitka AM-KR-80 Spec B, Sample 1 



03-05-24. Amchitka 

 Fall Sufnmei Spring 



Posrtcn From Start of Wffilet 



FIGURE 15. Clatbromorphimi nereostmtitni cellular anatomy. Cell dimensions with depth m 

 perithallial tissue, (left) Amchitka plant taken in the 1970s. Mean cell dimensions, two successive 

 years, (right) Plant from Amchitka, taken in 2008. Mean, three parallel filaments, single year. 



rates. However, even in the Gulf of Maine, the southern limit 

 of C. compactunu where the yearly mean temperature and 

 growth rates are similar to those of C. nereostratum, C. com- 

 pactum plants remain much thinner. This is partly due to the 

 "leafy" growth morphology of the hypothallium (the basal tis- 

 sue that grows parallel to the substrate) of C. iwrcostratmii. \n 

 C. compactufji, the hypothallium has a mean thickness of 3.5 

 cells (maximum 6 cells) and 22 pm (maximum 43 pm), and 

 the cells are tightly adherent to the substrate (Adey, 1965). In 

 C. nereostratum, on the other hand, the hypothallium has a mean 

 thickness of 16.8 cells (maximum 24) and 135 pm (maximum 

 206 pm; Lebednik, 1976); the growing margin is thick enough to 

 be structurally freestanding and often becomes cantilevered off 

 the substrate for a considerable distance. 



Clatbromorphum nereostratum is thus capable of grow- 

 ing over obstacles, and if the growing margin is damaged, a 

 secondary hypothallium, formed from perithalliimi, grows out 

 over the damaged area (primary hypothallium develops initially 

 from a settled spore on the rock or shell substrate). Individual 

 C. nereostratum plants can thus be much broader than those 

 of C. compactum (Figure 2F) and have the capability to 

 achieve greater thickness in a single plant. Clathromorplnim 

 compactum produces a clathrostrome largely by the fusion of 

 many individual small plants. These can be any size, depend- 

 ing upon the recruitment density. At the warmer margin of its 

 temperature limits in the Gulf of Maine, perhaps because of the 

 warmer summer temperatures, which allow greater invertebrate 



damage to crusts, and likely because of less successful recruitment, 

 C. compactum plants are more scattered and rarely exceed sev- 

 eral centimeters in thickness and 100 years of age. As noted 

 above, C. nereostratum likely evolved from a C. compactum- 

 like ancestor (Adey, unpublished data); the primary innovation 

 was the development of a thicker hypothallium allowing a free- 

 standing cantilevered morphology. 



Biologically Induced Etching as a Source of Information 



Hiatella arctica is a common Subarctic bivalve that estab- 

 lishes burrows within the calcihed perithallium tissue of Clatlj- 

 romorphum crusts. Apparently, H. arctica larvae settle at the 

 margin or on a damaged surface of a young plant and are buried 

 by plant growth. The bivalve then grows along with the perithal- 

 lium, enlarging its burrow by gradually decalcifying the coral- 

 line walls surrounding it. If a coralline sample is collected with 

 the animal still alive, the burrow wall can be seen to be etched 

 into ridges and valleys corresponding to the yearly layers of C. 

 compactum growth (Figures 19, 20). The etching is probably ac- 

 complished with an acid secretion, although localized CO, pro- 

 duction by the bivalve, accompanied by pH reduction within the 

 conhned space of the burrow, could also be a factor. The cell wall 

 carbonate formed in summer, being about two times denser than 

 that in winter, forms the ridges, and the less dense wmter cell 

 walls form the valleys. Also, particularly as seen in longitudinal 



