Raphides in the Uncalcified Siphonous Green Seaweed, Codium minus (Schmidt) P. C. Silva

The vacuole of utricles, the outermost cell layer of the siphonous green seaweed, Codium minus, had numerous single needles and needle bundles. The crystals composing each needle appeared arranged in a twisted configuration, both ends were pointed, and each needle was contained in a matrix or membrane; bundles of needles appeared enclosed by a matrix. Chemical and electron diffraction analysis indicated that the needles consisted of calcium oxalate. This is the first paper on terrestrial plant-like raphides in an alga. 1. Introduction Bundles of acicular crystals of calcium oxalate formed in specialized cells, idioblasts, are termed raphides in embryophytes [1–3]. Only one bundle of needles occurs per idioblast; the needles are long; needle length from several key pacific economic plants had a mean minimum length of 43？μm [4]. Each needle in a bundle is enclosed in a membrane, the crystal chamber [5–7], and bundles are enclosed by a water-soluble organic matrix, termed the vacuolar matrix [5, 8]. Raphides can burst through mature idioblasts due to swelling of the large amounts of mucilage contained in the cell [9]. Single needles do not occur in terrestrial plants but only as a bundle of needles [9, 10]. Individual needles, raphide, are found as microfossils in soils [4]. Abundant evidence supports the role of these needles in deterring vertebrate and invertebrate herbivory [1, 3, 7, 11, 12]. Raphides abrade the mouth and digestive tract of terrestrial herbivores causing edema; in addition, grooves in the needles may inject noxious plant metabolites and bacteria into these grazers [2]. Defense against herbivory in the marine environment generally involves calcification of the outer surface of green, red, and brown seaweeds [13]. This hard outer surface of calcium carbonate not only increases the difficulty of getting to the soft inner tissue, but consumption of the hard outer matrix can also alter the digestive pH, a deterrent for several herbivores [1, 13]. No alga has been found to have raphides. Where needles (acicular crystals) have been found, they appear singly, not in bundles of needles; they are surrounded by a vacuolar membrane or crystal chamber but not both, are generally very small, or reside individually in the cytoplasm and not the cell vacuole [14–18]. Single needles also occur in the cell vacuole of both lightly calcified parts of otherwise heavily calcified seaweeds [19–21]. We provide the first report of calcium oxalate crystals with all the characteristics that typify raphides of terrestrial plants. Large numbers of single needles and