About a month ago, I presented my first post on the marine shell-bearing protists known as foraminiferans. That post was on forams that constructed their shell by gluing together sand but I mentioned that there were other families that secreted their shells themselves. Some of these families can reach sizes large enough to be seen with the naked eye (over a millimetre and sometimes well over a centimetre in diameter); the Indo-Pacific calcareous Calcarinidae are one of these larger families. Such large forams live in association with endosymbiotic algae and have been given the evocative name of 'living sands' (Lee, 1995).
Among the living sands, the shape of Calcarinidae is distinctive. Their basic form is trochoid (i.e. shaped like a top shell, a Trochus) and similar to species of the related family Rotaliidae (Cushman, 1940) but extra shell material and chambers are laid down over and around the central trochoid structure. Large blunt external spines radiate from the sides of the foram, often giving the whole a star-like appearance when viewed from above. A system of canals runs between the chambers and along the spines allowing for the passage of cytoplasm between the chambers and the outside world. The chosen symbionts of calcarinids are diatoms which are held in special vacuoles inside the chambers. Symbiotic forams do still feed on other micro-organisms as well as deriving nutrients from their endosymbionts (Lee, 1995) but laboratory studies have shown that calcarinids are capable of living solely on nutrients derived from their diatoms in the absence of another food supply (Röttger & Krüger, 1990). If the algal symbionts are killed, the forams cannot live long without them though they may replace them if the opportunity arises in time (Lee, 1995).
Calcarinids can make up a significant part of coastal calcareous deposits in the Indo-Pacific region; one of the most fun expressions of this abundance was made by Lee (1995) who commented that calcarinid deposits in one part of Japan were "so abundant that they can be scraped together by hand to build sand castles". Because of their dependence on their diatom symbionts, calcarinids are only found in shallow waters, mostly preferring high energy environments (Lobegeier, 2002). Many calcarinids live as epiphytes on macroalgae and seagrasses where they attach themselves to their substrate by means of cytoplasm extended through the spine canals (Röttger & Krüger, 1990); among filamentous algae the spines themselves may form a mechanical anchor among tangled filaments (Lobegeier, 2002).
Cushman, J. A. 1940. Foraminifera: their classification and economic use, 3rd ed. Harvard University Press: Cambridge (Massachusetts).
Lee, J. T. 1995. Living sands. BioScience 45 (4): 252-261.
Lobegeier, M. K. 2002. Benthic foraminifera of the family Calcarinidae from Green Island Reef, Great Barrier Reef province. Journal of Foraminiferal Research 32 (3): 201-216.
Röttger, R., & R. Krüger. 1990. Observations on the biology of Calcarinidae (Foraminiferida). Marine Biology 106 (3): 419-425.