36 . SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



FIGURE 28. Working model of calcification in C. compactum. Inner-wall crystals (short, prismatic crystals perpendicular to the filament axis) 

 form in the meristem (fracture) plane within an organic matrix and form year-round at the same dimensions; increasing numbers of calcite 

 crystals and their organic matrix (longer cells) and production of a greater number of cells produce increased growth. Note the organic wall 

 extending across the inner- wall crystal zone but not extending into interfilament space. Interfilament crystals (large, angular crystals parallel or 

 diagonal to filament axes) formed, primarily in summer, in nonorganic space between filaments. Interfilament crystals extend upward into the 

 epithallium and are thinner and flatter. 



The seasonal formation of these crystals, narrows the cell lu- 

 mina (since the inner-wall crystals remain the same length), and 

 typically creates the summer high carbonate density and character- 

 istic seasonal banding obvious in many Clathromorphum plants. 

 However, development of this banding, which is partly dependent 

 upon available sunlight, is complex. In the southernmost locality 

 known for C. compactum in the Gulf of Maine, where sunlight 

 is present in winter, seasonal banding tends to be weak because 

 some interfilament crystals form year-round. In northern New- 

 foundland, with less winter sunlight, the banding tends to be more 

 pronounced. However, in the western Labrador Sea, where sea ice 

 forms in midwinter and remains through a large part of the spring. 



the return of sunlight is greatly delayed, and the summer band of 

 high density is very narrow (Figure 20). On the other hand, in 

 deeper water in the same region, at the lower extent of the C. com- 

 pactum range, peak summer temperature and maximum growth 

 occurs in October, whereas light peaks earlier in June, at nearly the 

 minimum water temperature. Thus, whereas high carbonate den- 

 sity corresponds approximately to high growth rates and long, nar- 

 row cells over most of the range of both Clathromorphum species, 

 under some environmental conditions, the reverse occurs, with 

 maximum density (due to interfilament formation) coinciding with 

 short cells formed under low temperature. In the Aleutian Islands, 

 C. itereostratum often has very strong seasonal banding. 



