Figure 4.

Thyroxine accelerates rudiment development in S. purpuratus larvae. Pictured are late stage larvae kept either without T4 (A) or with 100 nM T4 for 5 days (B). T4 exposed larvae have significantly more developed skeletal elements in the rudiment, as well as shortened larval arms. Larvae at soft tissue stage 1 (C) or skeletal stage 0 (D) were exposed to T4 for 5 days [n = 12, for staging scheme, see (21)]. T4 drastically accelerated skeletogenesis in the rudiment (Mann–Whitney, p < 0.001) as well as accelerating other markers of metamorphic competence, including arm retraction and tube feet protrusion, but did not accelerate early soft tissue development (Mann–Whitney, p = 0.63). This suggests that the rudiment of late stage larvae may become responsive to T4 only as skeletal development begins. r, rudiment; t.f, tube foot.