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Table 2 Skeletogenic stages in rudiment

From: A detailed staging scheme for late larval development in Strongylocentrotus purpuratus focused on readily-visible juvenile structures within the rudiment

Stage Shorthand Days Drawings Smith et al. 2008 [[37]] Chino et al. 1994 [[38]] Description
0 no skeleton < 21   I-V a-j No spicule dot (or any other rudiment skeleton) is visible; “soft tissue” Stage vi or earlier (see Table 1).
1 spicule dot 22 VI k At least one spicule dot is visible in the rudiment (first spicule dot is usually at the anterior edge of the rudiment, near the stomach/esophagus border); this occurs at about “soft tissue” Stage vi-vii (see Table 1).
2 spicule 22-23 VI k At least one triradiate spicule present in the rudiment.
3 MB/TF spic dots 23 VI k Multi-branched spicule (MB) and/or tube foot (TF) spicule dots present in the rudiment. The first MB spicules seen are the de novo ocular plate primordia of the urchin test, as well as the interambulacral plate primordia that will articulate with adult spines after Stage 8.
4 TF spicules 24 VI k TF triradiate spicule present in at least one primary podium.
5 sp primord/incomplete ring 25 VI k-l Adult spine primordium (6-prong spicule) present and/or TF spicules elongating but not forming complete ring in any primary podia.
6 spine primord + base/1st TF ring complete 26 VI l Adult spine primordium element elongating in direction orthogonal to 6-sided primordium (i.e., along the axis of the incipient spine) and/or 1st TF ring has fused to form a complete ring in at least one primary podium. 2nd TF ring may be beginning to form as bifurcating extensions from the 1st TF ring, but none of these bifurcations have yet started to fuse back with the 1st TF ring.
7 pre-spine/2nd TF ring < ½ complete 27 VI l At least one adult spine primordium has a complete base + 6 fronds, but no cross hatches have formed between the fronds and/or the 2nd TF ring has bifurcating spicules that may have started to fuse in at least one primary podium, forming part of the incipient second ring, but this ring is at most half complete.
8 spines 28 VI l At least one cross hatch is now present in at least one forming adult (6-sided) spine. Spine growth can be further subdivided by counting the average and maximum number of cross hatches (see Figure 5).
9 2nd TF ring > ½ complete 29 VI? l Bifurcating bits of the 2nd TF ring have continued to fuse in at least one primary podium, so that more than half or the 2nd TF ring is complete, but the 2nd TF ring is not yet complete in any podium.
10 2nd TF ring complete 30 VII? m 2nd TF ring is now complete in at least one primary podium. Larvae start becoming competent to settle at around this stage, though timing of competence seems to vary with skeletogenic stage.
  1. Description and illustration of stages presented in the new staging scheme as well as approximate timing in days after fertilization at 14°C. Note that roman numerals in the column “Smith et al. [37]” and the letters in the column “Chino et al. [38]” are the stage designations proposed in those studies (see Figure eleven in Smith et al. [37], and Figure two in Chino et al. [38]) as they relate approximately to the stages used in our new skeletogenic staging scheme here. Question marks indicate ambiguities in the comparison of staging schemes.