A Hydrodynamic Interpretation of Sand Dollar Morphology
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BULLETIN OF MARINE SCIENCE, 31(3): 605-622, 1981 A HYDRODYNAMIC INTERPRETATION OF SAND DOLLAR MORPHOLOGY Malcolm Telford ABSTRACT Sand dollars are shallow-domed echinoids which act as lifting bodies in water currents, Coefficients of lift (CI) were determined experimentally in air for a lunulate species, Mel/ita qllinqlliesperforllta (0,040) and for a nonlunulate species, Echinarachllius parma (0.077). The difference in CI., which was shown to be statistically significant (P = 0.001), can be attributed partly to differences in camber and partly to the presence of lunules. Observations of water flow at 10 cm' sec-1 confirmed that flow was attached and of a lift- generating pattern. Flow around E. parma was hindered by a relatively large standing vortex at the anterior margin. For M. sexiesperforata, which is thinner edged and less steeply cambered, the flow was more perfectly attached; flow from oral to aboral surfaces via the lunules was distinct. Similarly, Encope emarginata had a closely attached flow with some passage through the anal lunule and ambital notches. Calculations from weight-length relationships and coefficients of lift indicated that M. qllillqlliesperfora({l should be able to maintain its position in more severe current regimes than could E. parma. Excess pressure on the oral surface of lunulate sand dollars is relieved by flow along pressure drainage channels which lead from the central region of the disc into the lunules and ambital notches. Scanning electron micrographs show that these lack the podia char- acteristic of food grooves and that podia in adjacent food-gathering areas sweep parallel to or away from the pressure drainage channels. Several earlier theories of lunule function appear to be either insufficiently general or untenable. That lunules shorten the food-path is not fully supported by observation; that they aid in righting movements is contrary to observations of Mel/ita species and that they might strengthen the test is shown by both observation and theoretical considerations to be incorrect. In the preface to her monograph on the Echinodermata, the late Libbie Hyman (1955) said: "I also here salute the echinoderms as a noble group especially designed to puzzle the zoologist." This is perhaps most true of clypeasteroids which include the familiar sand dollars, sea biscuits and key-hole urchins. Many of these are of bizarre form: the African rotulids have numerous slender inden- tations of the posterior margin and some also have anterior perforations or lu- nules; the American genus Mellita (Fig. 1, left) has five or six slit-like lunules piercing the test and the related genus Encope (Fig. 1, right) has a single posterior (anal) lunule and five notches around the margin; the Indo-Pacific genus Echi- nodiscus has two posterior slit-like lunules whilst the fossil genus Amphiope (Miocene, France) had a round port hole in each of the two posterior ambulacra. In Durham's (1966) classification the Clypeasteroida includes twenty-four Recent genera and of these, eight (Mellita, Encope, Leodia, Mellitella, Astriclypeus, Echinodiscus, Rotu/a and Heliophora) have lunules and/or ambital indentations. In addition, at least three fossil genera (Monophoraster, Amphiope and Scutas- ter) had lunules and a further three dubious genera appeared to have notches or broad indentations of the margin. Seilacher (1979) has suggested that lunules have appeared independently in six different groups. The role of these assorted inden- tations and perforations is speculative and no satisfactory, unifying explanation has yet been offered. However, their repeated occurrence suggests that they 605 606 BULLETIN OF MARINE SCIENCE, VOL. 31, NO.3, 1981 Figure I. (Left) Mel/ita sexiesperjorata: Aboral surface showing pressure drainage channels (black arrow) and food grooves (white arrow). (Right) Encope emarginata: has pressure drainage channels (black arrow) and food grooves (white arrow) very similar to M. sexiesperjorata. confer significant adaptive advantages on these organisms, suiting them to their special environments. Sand dollars are generally shallow burrowers or exposed, epibenthic organisms. Most species occur in shallow water, some are intertidal, some inhabit deep water and a few, such as Echinarachnius parma, extend from the intertidal zone to considerable depths (Stanley and James, 1971). For burrowing their thin-edged discoidal form is ideal. Equally important, as echinoids living on an unstable sediment surface swept by waves and currents, their form minimizes drag and facilitates maintenance of position. A consequence of their low, domed profile is that they act as hydrofoils and generate lift. Chia (1973) reported the accumulation of high density sand grains which appear to act as a weight belt in Dendraster exentricus. O'Neill (1978) has provided an explanation of the peculiar upright feeding position of D. excentricus which exploits its lift characteristics in a group facilitation of feeding. Also, in the more typical horizontal posture, sand dollars generate lift, in a similar manner to flounders (Arnold and Weihs, 1978) which are able to regulate the amount of lift they generate by adjustments of their fins, thus altering their curvature. I propose the hypothesis that notches and lunules evolved as adaptive modi- fications to reduce lift, without significantly increasing drag nor disrupting water flow over the surface of horizontally oriented sand dollars exposed on the surface of the sediment. MATERIALS AND METHODS Sand Dol/ars.-Echinarachnius parma was collected intertidally and in shallow water at 81. An- drews, N.B.; Mel/ita (Leodia) sexiesperjorata in Barbados; M. quinqlliesperjorata at Beaufort, N.C.; D. excentricus near Victoria, B.C. and E. emarginata was purchased in Florida. For most of this study clean, dry tests were used, prepared in commercial bleach (sodium hypochlorite) and washed in distilled water. Measurements.-Lengths,.widths and thicknesses were obtained to the nearest 0.1 mm with vernier calipers; weights were recorded to the nearest 0.1 g,"Densities of whole specimens were determined TELFORD: SAND DOLLAR MORPHOLOGY 607 a Figure 2. (Left) Installation of pressure ports in Echinarachnius parma. a. Longitudinal section: capillaries installed to monitor pressure on aboral (upper) surface. b. Arrangement of pressure ports along anterior-posterior diameter. Figure 3. (Right) Diagram of lift balance. Specimens were poised in the center of air stream (shown by arrow) in a horizontal position (zero angle of attack) with lift indicator needle centered. As air stream velocity increased and the specimen lifted, weights were added to the balance pan returning lift indicator to the center position. by displacement. Plan areas were estimated by exposing the sand dollars on photographic paper, then cutting out and weighing the silhouettes. Determination of Lift.- Two techniques of measuring lift, using the Philip Harris Ltd. (Birmingham, England) Air Stream Generator P10600, were employed. In the first method, the specimens were mounted horizontally in the center of the air stream with the balance arm immobilized so that the sand dollars were unable to lift. Pressures over the surface were measured by means of individual pressure ports. These consisted of lengths of 1.5 mm glass tubing connected to small manometers filled with Krebs' modification of Brodie's solution (Dawson et aI., 1959). The sand dollars were perforated by a dental drill and the capillary tubes glued into place under a microscope, with their open ends minutely recessed below the surface (Fig. 2). By combining data from three individuals of E. parma and two of M. quinquiesperforata, with pressure ports arranged in different positions, composite isobars were prepared for both species. Areas within isobars were estimated by superimposing the diagrams on squared paper. These determinations were made with the specimens poised in the center of the air stream. A simple disc of Plexiglas, flat on each side and a Plexiglas model of E. pamUl domed on the upper surface, were made. Both were tested in the free air stream and again, glued to a flat board which simulated a substrate. For the second method, the Harris Drag and Lift Ba]ance No. P]0602 was used. The specimens were again mounted horizontally in the center of the air stream but with the arm counter-balanced and free to lift. As air velocity was increased and the specimens lifted the starting horizontal position was restored by adding weights to the balance pan (Fig. 3). Balance measurements of lift were obtained for twenty specimens each of E. parma and M. quinquiesperforata with the air flow passing exactly anterior to posterior and again with the flow reversed, posterior to anterior. For M. quin- quiesperforatll both of these measurements were repeated with the ]unules smoothly filled with par- affin wax. Water Tunnel Experiments.-Specimens of E. parnUl, M. sexiesperforata and E. emarginata were placed flat on the floor of the observation chamber, simulating their natural position on the substrate. Depending on size, the specimens were 10-]5 em from the chamber walls (a full description of the facility is given by Dobrodzicki, 1972). Using a fine jet of fluorescein for visualization, observations were made at a flow rate of ]0 cm·sec-t• Sand dollars were sprayed with black paint to enhance contrast. 608 BULLETIN OF MARINE SCIENCE. VOL. 31. NO.3. 1981 Figure 4. Isobars on aboral surface of Mellita quinquiesperforata (left) and Echinaraclmius parma (right) in air stream of 1,437 em' sec-I. Pressures, indicated in mm Kreb's solution, are relative to stagnation pressure of oral surface. Data combined from three specimens of E. parma and two of M. quinquiesperforata. Scanning Electron Microscopy.-Specimens of both M. sexiesperforata and M. quinquiesperforara were cleaned and washed as described, air dried, mounted on stubs and sputter coated with gold in a SEMPREP 2 (Nanotech (Thin Films) Ltd., Cambridge, England) before scanning with a Cambridge SI80 SEM. Righting Experiments.-Laboratory and field experiments using M. sexiesperforata were conducted in Barbados. The sphaeridia (Fig.