20 



SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



FIGURE 14A. CLithromorphuiii coiupactuin variation in cell dimen- 

 sions with depth in perithailium: mean perithallial cell dimensions, 

 as measured in two crusts, from meristem, with depth (summer col- 

 lected in southern Labrador; mean of two consecutive years, three 

 cell rows). LAV = length/width. 



FIGURE 14B. Clatbrojjiorphuiu coiiiltactwn variation in cell di- 

 mensions with depth in perithailium: mean dimensions from five 

 continuous years, matched peak to peak (summer collected from 

 northern Labrador). LAV = length/width; Tk = thickness 



(reviewed by Adey, 1998; see also Adey et al., 2005). However, 

 in Clathroinorpbum species, especially during summer and au- 

 tumn, the space between the filaments becomes filled with much 

 larger, often spear-shaped, vertically or diagonally oriented cal- 

 cite crystals (interfilament crystals). As shown in Figure 17C, the 

 inner-wall crystals are formed within the organic membranes of 

 the cell, whereas the deltoid interfilament crystals are formed 

 in the cavity between the filaments (Figures 16A, 17B,C). This 

 phenomenon is coincident with the narrowing of cell lumens. 

 Interfilament crystals tend to be present year-round. However, 

 in winter they are far less abundant, the cell lumens being larger 

 in diameter. Thus, summer and fall tissue is considerably denser 

 than winter tissue. 



Unlike the closely related, sheet-forming species Clathromor- 

 phiim circumscriptum (which occurs in shallower water and tide 

 pools), mature C. compactum tends to be dome shaped. Large 

 sheetlike (clathrostrome) formations often develop, especially in 

 the northern Subarctic, but they form from the coalescing of ad- 

 jacent C. compactum plants. Although smooth, dome-shaped, 



individual plants are common, especially in the warmer part of 

 the C. compactum range, domes featuring more or less flat facets 

 separated by slightly grooved ridges are more typical (Figure 18). 

 Facets arise when the scattered filament branching required to 

 form the domed morphology of C. compactum occurs in local- 

 ized vertical planes. Facets can shift with time or split in two 

 for several years. Conceptacles generally do not form near facet 

 boundaries; when a facet does develop, previous nearby concep- 

 tacle areas can become nonreproductive. Also, different facets 

 on the same plant can grow at different rates. For that reason, 

 growth rates in mound-forming C. compactum are taken in the 

 middle of the dome. 



Clathromorphuni nereostratum can develop into a much 

 thicker crust (up to 50 cm) than C. compactum (up to 12 cm). 

 This is partly due to the much slower growth of C. compactum 

 in the colder water of the core Subarctic and Arctic (see below). 

 The crust thickness achieved by C. compactum in warmer Lab- 

 rador waters, for example, is comparable to that of C. nereo- 

 stratum, taking into account the difference in the yearly growth 



