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  Larval feeding & growth
  Larval feeding is considered in this section, while GONAD GROWTH, SPAWNING, & FERTILISATION, LARVAL DEVELOPMENT & BEHAVIOUR, and SETTLEMENT & METAMORPHOSIS are considered in other sections.
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Research study 1

drawing of an echinopluteus larva of a sand dollar Dendraster excentricus showing ciliary driven water currents for feedingAs in the larvae of other echinoderms, echinoplutei of sand dollars Dendraster excentricus feed on phytoplankton which they catch and direct to the mouth in ciliary currents.  In this diagram of an 8-armed larva, the large blue arrows show major water currents generated by the beating of cilia on the ciliated bands.  These drive the larva through the water and also draw in phytoplankton cells, which are then directed by cilia to the mouth.  Food particles bounce along the ciliary bands in the general direction of the mouth. Around the mouth, other cilia sort the particles and direct edible bits into the mouth. The author notes that ciliary reversal is common in larval sand dollars and in larvae of other echinoderms, possibly aiding in the capture of food particles. An actively feeding larva will consume so many plant cells that its gut becomes full and green in colour.  Strathmann 1971 J Exper Mar Biol Ecol 6: 109.



In this diagram, ciliated bands are shown in brown, Water currents are shown on the Right side, while particle movements are shown on the Left side. The mouth or buccal cavity is much larger than shown here, as part of it is hidden behind some transverse cilitated bands

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Research study 2

Suspension feeding may not be the only mode of nutrition for echinoplutei and other echinoderm larvae. Studies on nutrition of Dendraster excentricus larvae in Victoria, British Columbia show that embryos and larvae in laboratory culture can take up several kinds of 14C-labeled amino acids.  This suggests that dissolved amino acids and other nutrients are available to the developmental stages in seawater.  The authors suggest that the uptake of dissolved amino acids by Dendraster embryos may provide substrate for protein synthesis prior to when the larvae become capable of feeding, and would be of obvious benefit later in larval life.  de Burgh & Burke 1983 Can J Zool 61: 349; Davis & Stephens 1984 Am J Physiol 247: 733.

NOTE  the uptake of DOM (dissolved organic matter) from seawater by larvae has been shown for many marine invertebrate groups including anthozoans, bivalves, gastropods, polychaetes, ophiuroids, and others

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Research study 3

drawing of a pluteus larva of a sand dollar Dendraster excentricus showing a plastic microsphere being caught as foodLaboratory culture of larvae of several species of echinoderms at Friday Harbor Laboratories, Washington, including sand dollars Dendraster excentricus, indicate a common pattern of suspension feeding.  The studies involve videotaping larvae as they capture 20µm diameter plastic spheres while simultaneously recording elapsed time, and later measuring and comparing lengths of bodies, arms, and ciliated bands.  The diagram illustrates a 6-armed Dendraster larva of 1.5mm diameter capturing a single sphere over a time span of 1.2sec. Note that the sphere is captured twice before being caught up in ciliary tract leading to the mouth.  Hart 1991 Biol Bull 180: 12.

NOTE the ciliated band of the larva is shown by the dark lines

photograph of a pluteus larva of a sand dollar Dendraster excentricus taken from a video of its feeding behaviour

CLICK HERE to see a video of an 8-arm pluteus larva of Dendraster excentricus capturing plastic microspheres on its arms and passing them to its mouth. What is extraordinary about this is the speed with which a particle is captured and eaten, and the fact that particles can actually be seen entering the mouth. Video courtesy Michael Hart, SFU.

NOTE the video replays automatically

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Research study 4

drawing of a larva of a sand dollar Dendraster excentricus showing the measurements taken in a study of ciliated band length under conditions of different food rationsdrawings of plutei larvae of a sand dollar Dendraster excentricus showing relative ciliated band lengths under different ratio levelsSeveral studies on sand dollars and other echinoids show that larvae grown in culture with scarce food grow longer ciliated bands than larvae grown with abundant food.  The first test of this in sand dollars appears to have been done at , Washington using larvae of Dendraster excentricus.  Larvae are raised in the lab for up to 25wk on 4 rations1: 6000, 2000, 500, and 0 cells . ml-1.  Measurements taken on the larvae are post-oral arm length and mid-body length (drawing on Left).  By 15wk the adult rudiment has appeared in all treatments but the zero-ration one and by 17wk metamorphosis has commenced in all treatments but this one.  At 15wk the echinoplutei in the high-ration treatment have the shortest relative2 arm lengths, while those in the mid-ration 500 cells . ml-1) and zero-ration treatments have the longest relative arm lengths. The results show that larvae in the mid- and zero-ration treatments3 are increasing their feeding effectiveness by increasing the relative lengths of the ciliated bands on their arms.  Boidron-Metairon 1988 J Exp Mar Biol Ecol 119: 31.

NOTE1  the food is the flagellate Dunaliella teriolecta and feeding is every 2-3d

NOTE2  relative to mid-body length

NOTE3  interestingly, an earlier study at Friday Harbor Laboratories, Washington shows that relative ciliated-band lengths do not change significantly if the plutei of D. excentricus are grown at higher temperatures.  By increasing metabolic energy needs, a higher temperature would be predicted to have the same effect on relative ciliated-band lengths as would a lower food ration.  McEdward 1985 J Exp Mar Biol Ecol 93: 169.

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Research study 5

graph of ciliated band lengths with age in sand dollars Dendraster excentricus under conditions of small and large rationsThe relationship of food level and ciliated-band length is further tested at Friday Harbor Laboratories, Washington with larvae of sand dollars Dendraster excentricus maintained in cultures on small1 and large rations of the single-celled alga Rhodomenas sp.  Results show that ciliated band lengths2 are actually slightly greater in larvae grown at high food ration than in larvae grown in low food ration up to about 10d of age (graph upper Left). However, if younger larvae are examined, for example, at 3d of age, then ciliated band lengths are, indeed, significantly longer in larvae fed on low rations than in larvae on high rations (2.5mm vs. 2.3mm, respectively). Ciliated band length increases as arm length increases, and the growth is at the tips of the arms. 

photographof a microsphere being captured by the pluteus larva of a sand dollar Dendraster excentricusThe authors show through analysis of fast-frame videography that particles (in this case, plastic microspheres of 15µm diameter) are  captured by these new arm tips and passed as food to the mouth (photo on Right; see also Research Study 3 above). 

If clearance rates of Rhodomenas cells are compared between larvae on the different rations at different ages then, once again, it is apparent that larvae on the high rations grow more quickly and have a higher absolute clearance rate3 than the slower-growing larvae on the low rations (graph lower Left). Eventually, however, larvae on the small rations grow longer bands and their clearance rates surpass those of the largest larvae fed on the large rations.  Also, if relative clearance rates are compared as a function of development of postlarval structures, for example, the juvenile rudiment diameter, then it is clear that larvae on small rations have significantly higher clearance rates (based on greater ciliated band lengths) than larvae on large rations at a given size of rudiment.  The authors note that the phenotypic plasticity of arm growth and ciliated band length is functionally important to Dendraster larvae in that it increases feeding rate when food is scarce.  The plutei grow longer arms and ciliated bands at the expense of growth of postlarval parts.  Both ration-groups undergo metamorphosis, but individuals on the small ration take longer (40 vs. 13 days), are less successful in completing metamorphosis (27 vs. 47%), and produce smaller-sized juveniles (330 vs. 343µm test diameter) than larvae on the large rations.  Hart & Strathmann 1994 Biol Bull 186: 291.

NOTE1  the small ration has 250-300 cells . ml-1, while the large ration has 5000 cells . ml-1

NOTE2  the authors include 3 other morphometric measurements in their study (stomach length, midline body length, and juvenile rudiment diameter), but these data are not included here

NOTE3 this is a measure of the rate that particles are removed from solution. If, for example, a seawater solution has 10 particles per µl, and 100 particles are removed per min, then clearance rate would be 10µl per min. Because clearance rate is directly dependent on ciliated band length, the 2 graphs shown here should, and do, look similar

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Research study 6

Through use of flavoured and unflavoured polystyrene beads1 a researcher from the University of Southern California, Los Angeles shows that Dendraster excentricus larvae can preferentially select particles at both ciliated band and mouth locations. The beads used are 10 and 20µm in diameter and are flavoured by soaking them in an exudate of Dunaliella tertiolecta culture solution for a 3-h period.  Tests involve presenting larvae with flavoured and unflavoured beads, of different sizes2.  Results from gut analyses show that the larvae preferentially select flavoured particles.  Particles are observed3 to be selected or rejected at both sites, but with the mouth being the dominant site.  Particle loss from ciliated band to mouth is less than 5%.  The author concludes with the statement, “my study provides a basis for future investigations designed to determine how and to what extent selective feeding can improve the nutritional intake of larvae”.  Appelmans 1994 Limnol Oceanogr 39: 404.

NOTE1  the idea for this paper appears to originate with a study published a decade earlier, one in which flavoured and unflavoured plastic spheres, and larvae of sand dollars Dendraster excentricus, are also employed.  Although the authors of the earlier paper do not present the results for Dendraster separately from ones for other echinoid species, the same pattern of preference by the plutei for flavoured particles is assumed to be present in the data.  The earlier publication is therefore the first to show that an attractive factor, presumed to be chemical, associated with living phytoplankton, can be transferred to other particles, and that this transferable attractive property can modify or override effects of other factors, such as size, on the preferential ingestion of particles.  Rassoulzadegan et al. 1984  Limnol Oceanogr 29: 357.    

NOTE2  in one experiment, the large particles are flavoured; in another, the small particles.  The particles selected can then be identified later by their sizes, as to whether they are flavoured or unflavoured

NOTE3  a CCD video camera is used in conjunction with a dissecting microscope to observe details of feeding

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Research study 7

drawing of pluteus larva of a sand dollar Dendraster excentricus showing measurements taken for growth studyhistogram comparing growth of pluteus larvae of sand dollars Dendraster excentricus on different dietsAn interesting experiment at Friday Harbor Laboratories, Washington involving feeding and growth in larval sand dollars Dendraster excentricus employs larvae in cultures reared in natural seawater from 2 depths, 1 and 20m, with their performance being compared with that of larvae in the laboratory feeding on a monoculture of algae.  The researchers are interested in the extent to which larvae in the water column may be limited by food quality and/or quantity.  Eggs are spawned from laboratory adults and larvae reared over a period of 8d at 11oC.  The cultures are monitored on Days 0, 5, and 8 post-fertilisation for differences in size and extent of development of juvenile rudiment (see drawing on Left). 

Results at 5d post-fertilisation reveal best growth and development of plutei on the laboratory diet of Dunaliella, followed by those reared in surface water (1m depth) and in deep water (20m), in that order. By 8d post-fertilisation the surface-water and laboratory-diet larvae are similar in size, but the former have significantly smaller juvenile rudiments (data not shown here).  Analyses of the seawater media reveal significantly higher chlorophyll concentrations in the surface seawater as compared with deep seawater, as expected.  However, these enhanced levels likely owe to greater concentrations of diatoms and dinoflagellates, typically known to be poor-quality food items for echinoid larvae.  The authors’ results support other findings of food limitation in the natural environment for development of larvae, but do so in a more convincing way through direct comparison of field and laboratory diets.   Reitzel et al. 2004 J Plankton Res 26: 901.

NOTE  the seawater is filtered to remove competing zooplankton and algae too large for the plutei larvae to ingest

NOTE  the alga is Dunaliella tertiolecta at a concentration of 6 cells . µl-1

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Research study 8

series of photographs showing a pluteus larva of a sand dollar Dendraster excentricus capturing a phytoplankton cell, courtesy Stranthmann 2007 Biol Bull 212: 93.Suspension-feeding larvae must concentrate food particles before ingesting them.   Plutei of sand dollars Dendraster excentricus do this not by filtering the particles, but by capturing them individually.  A particle first strikes the main swimming/feeding ciliary band and causes a reversal of beat in a small section of these cilia.  The reversal, lasting for only a fraction of a second, redirects the particle towards the part of the ciliary band that encircles the mouth, where it and other particles are concentrated before being taken into the mouth.  Oftentimes a particle may be lost then recaptured. The author uses video-recording techniques with a microscope at the Friday Harbor Laboratories Washington to observe details of feeding of larvae constrained between a microscope slide and coverslip.  The sequence of video frames shows the capture of a cell of Rhodomonas sp.   Time taken from first contact with the ciliated band to ingestion is less than a second. The author discusses the merits of several processes that could be involved in causing the beat reversal. Strathmann 2007 Biol Bull 212: 93.

NOTE  ciliary reversal is described for sea-urchin plutei in an earlier paper by the same research group. Strathmann et al. 1972 Biol Bull 142: 505.

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