Table 2 Skeletogenic stages in 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. |
