Hodin J , Lutek K , Heyland A .

	Figure 1. Larvae of the sand dollar D. excentricus show directional asymmetry in postoral arms. (a) Representative D. excentricus larva at 9 days postfertilization, with four pairs of larval arms. Note that the left side of the larva is the side of the well-developed juvenile rudiment (dark region labelled ‘Rud’ in this image); as this is a ventral view, the ‘left’ side of the larvae is seen here on the right side of the image, and vice-versa; scale bar: 100 µm. (b) Schematic of a sand dollar pluteus larva with four pairs of larval arms, with the coloured lines indicating the measurements taken for this study, oriented as in (a). (c) Mean larval arm length in µm for all four larval arms at day 9; n = 20 larvae. Lighter bars: right arms; darker bars: left arms. (d) Mean index of asymmetry [ln(R/L)] for all arm pairs; positive values indicate right-based asymmetry (i.e. longer arms on the right side); negative values indicate left-biased asymmetry. PO: postoral arms; PD: posterodorsal arms; ALA: anterolateral arms; PRO: preoral arms. Asterisks in (d) indicate significant differences (p < 0.05) between left and right side, and therefore directional asymmetry (DA). Error bars are one standard error of the mean.
	Figure 2. Directional asymmetry in purple urchin (Str. purpuratus) larvae as a function of age. We analysed left–right asymmetry in larval arms as a function of age during late-larval development for the (a) postoral, (b) anterolateral, (c) posterodorsal and (d) preoral arm pairs (on day 15, these larvae had only six arms; hence the absence of data for PRO arms on that day). Graphs in (a–d) have two y-axes: the primary (left) axis shows larval arm length in micrometres for all four larval arms, as is seen in the data points connected by dark solid (left arm) and dashed grey (right arm) lines. The secondary (right) axis shows the mean index of asymmetry [ln(R/L)], as is seen in the grey bars. (e) Schematic as in figure 1b repeated for convenience. The cross-polarized light micrograph in (f') shows a representative early stage larva (day 15, stage bin A; see figure 3), and in (f''), a representative late-stage larva (day 39, stage bin F; see figure 3). Note the visible juvenile skeleton (on the left side; pictured in the right side of these ventral views) and clear L–R arm asymmetry in (f'') but not (f'). Time is in days postfertilization (PF), Scale bars in (e,f), 150 µm. Abbreviations, asterisks and error bars as in figure 1. See table 1 for statistics.
	Figure 3. Directional asymmetry in purple urchin larvae as a function of juvenile rudiment stage. We analysed left–right asymmetry in larval arms as a function of juvenile rudiment stage for the (a) postoral, (b) anterolateral, (c) posterodorsal and (d) preoral arm pairs. We binned juvenile rudiment stages after [22] as follows: bin A, skeletogenic stage 0; bin B, skeletogenic stages 1–2; bin C, skeletogenic stages 3–4; bin D, skeletogenic stages 5–6; bin E, skeletogenic stages 7–8; bin F, skeletogenic stages 9–10. Double axes and line colours as in figure 2. Abbreviations, asterisks and error bars as in figure 1. See table 2 for statistics.
	Figure 4. Directional asymmetry in total arm length of purple urchin larvae as a function of age and juvenile rudiment stage. We summed the length of the four pairs of arms on the right and left sides, and compared them by age (a) and juvenile rudiment stage (b). Note the lack of a monotonic increase in asymmetry as development proceeds, especially in (b). Double axes and line colours as in figure 2. Abbreviations, asterisks and error bars as in figure 1. Stage bins as in figure 3. See tables 1 and 2 for statistics.
	Figure 5. Reduced larval food does not alter directional asymmetries in arm length across stages. (a–d) We detected asymmetry in arm lengths with stage under both high food (HF; upper graphs in each panel) and low food (LF; lower graphs) conditions in purple urchins, with no detectable differences in any arm pair between high and low food (see the text). Double axes and line colours as in figure 2. Abbreviations, asterisks and error bars as in figure 1. Stage bins as in figure 3. Numbers of larvae at each stage are as follows. Stage bin A: HF, n = 0; LF, n = 4. Stage bin B: HF, n = 3; LF, n = 18. Stage bin C: HF, n = 6; LF, n = 9. Stage bin D: HF, n = 13; LF, n = 17. Stage bin E: HF, n = 8; LF, n = 18. Stage bin F: HF, n = 15; LF, n = 8. Note that there were no HF larvae in stage bin A, presumably due to their more rapid development than in the corresponding low food larval cohort.
	Figure 6. Larval arms of anomalous purple urchin larvae with double rudiments are more symmetrical. We analysed the index of arm-length asymmetry (see figure 1 legend) in naturally occurring larvae with juvenile rudiments on both the left and right sides (double rudiments; (b), as compared to their full siblings with single rudiments (c). Cartoons along bottom of figure indicate single and double rudiments. (a) Whereas larvae with single rudiments (n = 9 larvae) in this experiment showed right-biased directional asymmetry (positive values) in postoral (PO) and posterodorsal (PD) arms (asterisks in right half of (a)), stage-matched larvae with double rudiments (n = 8 larvae) did not show right-biased asymmetry in any of their arm pairs; in fact PO arms in double-rudiment larvae showed left-biased asymmetry (negative values; asterisk in left half of (a)). Scale bars in (b,c): 150 µm. Abbreviations, asterisks, error bars and orientation of larvae as in figure 1.

Adomako-Ankomah, P58-A and P58-B: novel proteins that mediate skeletogenesis in the sea urchin embryo. 2011, Pubmed, Echinobase

Adomako-Ankomah, P58-A and P58-B: novel proteins that mediate skeletogenesis in the sea urchin embryo. 2011, Pubmed , Echinobase
Aihara, Left-right positioning of the adult rudiment in sea urchin larvae is directed by the right side. 2001, Pubmed , Echinobase
Balser, Ultrastructure of the Coeloms of Auricularia Larvae (Holothuroidea: Echinodermata): Evidence for the Presence of an Axocoel. 1993, Pubmed , Echinobase
Chan, Biomechanics of larval morphology affect swimming: insights from the sand dollars Dendraster excentricus. 2012, Pubmed , Echinobase
Cheers, P16 is an essential regulator of skeletogenesis in the sea urchin embryo. 2005, Pubmed , Echinobase
Chino, Formation of the adult rudiment of sea urchins is influenced by thyroid hormones. 1994, Pubmed , Echinobase
Clay, Morphology-flow interactions lead to stage-selective vertical transport of larval sand dollars in shear flow. 2010, Pubmed , Echinobase
Collin, ONTOGENY OF SUBTLE SKELETAL ASYMMETRIES IN INDIVIDUAL LARVAE OF THE SAND DOLLAR DENDRASTER EXCENTRICUS. 1997, Pubmed , Echinobase
Duboc, Left-right asymmetry in the sea urchin embryo is regulated by nodal signaling on the right side. 2005, Pubmed , Echinobase
Emlen, Costs and the diversification of exaggerated animal structures. 2001, Pubmed
Emlet, The bilaterally asymmetrical larval form of Stomopneustes variolaris (Lamarck). 2009, Pubmed , Echinobase
Ettensohn, Lessons from a gene regulatory network: echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis. 2009, Pubmed , Echinobase
Hart, Functional Consequences of Phenotypic Plasticity in Echinoid Larvae. 1994, Pubmed , Echinobase
Heyland, Heterochronic developmental shift caused by thyroid hormone in larval sand dollars and its implications for phenotypic plasticity and the evolution of nonfeeding development. 2004, Pubmed , Echinobase
Heyland, Hormone signaling in evolution and development: a non-model system approach. 2005, Pubmed
Heyland, Manipulation of developing juvenile structures in purple sea urchins (Strongylocentrotus purpuratus) by morpholino injection into late stage larvae. 2014, Pubmed , Echinobase
Heyland, Thyroid hormones determine developmental mode in sand dollars (Echinodermata: Echinoidea). 2004, Pubmed , Echinobase
Heyland, A detailed staging scheme for late larval development in Strongylocentrotus purpuratus focused on readily-visible juvenile structures within the rudiment. 2014, Pubmed , Echinobase
Hibino, Phylogenetic correspondence of the body axes in bilaterians is revealed by the right-sided expression of Pitx genes in echinoderm larvae. 2006, Pubmed , Echinobase
Hodin, Expanding networks: Signaling components in and a hypothesis for the evolution of metamorphosis. 2006, Pubmed , Echinobase
Ji, Echinoderms have bilateral tendencies. 2012, Pubmed , Echinobase
Luo, Opposing nodal and BMP signals regulate left-right asymmetry in the sea urchin larva. 2012, Pubmed , Echinobase
McAlister, Evolutionary responses to environmental heterogeneity in central american echinoid larvae: plastic versus constant phenotypes. 2008, Pubmed , Echinobase
McDonald, Abrupt change in food environment induces cloning in plutei of Dendraster excentricus. 2010, Pubmed , Echinobase
Morris, Origins of radial symmetry identified in an echinoderm during adult development and the inferred axes of ancestral bilateral symmetry. 2007, Pubmed , Echinobase
Nijhout, Competition among body parts in the development and evolution of insect morphology. 1998, Pubmed
Pennington, Consequences of the Calcite Skeletons of Planktonic Echinoderm Larvae for Orientation, Swimming, and Shape. 1990, Pubmed
Smith, Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. 2013, Pubmed , Echinobase
Strathmann, Good eaters, poor swimmers: compromises in larval form. 2006, Pubmed , Echinobase
Strathmann, THE EVOLUTION AND LOSS OF FEEDING LARVAL STAGES OF MARINE INVERTEBRATES. 1978, Pubmed
Strathmann, HETEROCHRONIC DEVELOPMENTAL PLASTICITY IN LARVAL SEA URCHINS AND ITS IMPLICATIONS FOR EVOLUTION OF NONFEEDING LARVAE. 1992, Pubmed , Echinobase
Tomkins, Phenotypic plasticity in the developmental integration of morphological trade-offs and secondary sexual trait compensation. 2005, Pubmed
Vickery, Effects of food concentration and availability on the incidence of cloning in planktotrophic larvae of the sea star Pisaster ochraceus. 2000, Pubmed , Echinobase
Warner, Left-right asymmetry in the sea urchin embryo: BMP and the asymmetrical origins of the adult. 2012, Pubmed , Echinobase