Article: Size, body plan and respiration in the Ostracoda
Ostracodes are small (0.3-32 mm) bivalved crustaceans with an exceptional fossil record covering the last 540 million years of aquatic life; the group is still represented by nearly 8000 species (Podocopa and Myodoeopa). Only two major body plans prevail in the modern fauna if we except the enigmatic punciids. 'Body plan 1' corresponds to ostracodes (Myodocopida, Halocyprida) wilh a bilateral symmetry, a frontal polarity, a high activity level, an ellipsoidal shape, well designed for moving through water or soft muddy sediment, and (primarily) a circulatory system. 'Body plan 2' corresponds to ostracodes with a ventral or lateral polarity (left/right valve asymmetry). Such organisms (Podocopa) are mostly crawlers (on thoracic legs] on a substratum or within flocculent material, and occasionally remain motionless, simply resting on one valve The carapace is heavily calcified, bearing strengthening features, and no circulatory features (e.g. a dorsal heart) are present. The living ostracode fauna as a whole has a high diversity at low taxonomic level and conversely a low disparity of body plans. The respiratory and circulatory processes with relation to animal size have been investigated using a scanning electron microscope, microtomized sections, and high-resolution video observations of live specimens. Small ostracodcs (0.3 to less than 3 mm) rely exclusively on integumental gaseous diffusion. Larger ones (up to 32 mm) have a dorsal heart, with haemolymph circulation supplementing diffusion. Fewer (e.g. cylindroleberidids) develop additional paired gill-like features, also present in the Triassic. Integumental circulation, evidence for which is preserved as anastomozing features in fossils, is shown to have occurred early in the evolution of Ostracoda. among Ordovician to Devonian leperditicopes. through the myodocope lineage (Silurian to Recent) and also possibly in bivalved arthropods such as the Bradoriida, The post-Cambrian evolution of Ostracoda is believed to have been influenced strongly by either the loss or the conservation of the circulatory system, leading (respectively) to minute organisms confined within a narrow size range (e.g. all living podocopc ostracodes) and to larger crustaceans (e.g. living myodocope oslracodes] capable of higher metabolism and free swimming life styles.