Table 1.

Morphological stages at which Baltic Mytilus edulis‐like larvae were sampled during the experiment

Stage	Description	Hours postfertilization
		Control families	Reared under low C T conditions
			F001	F002	F003	F004
Early trochophore	Precalcification	20	20	20	20	20
Trochophore	Larvae exhibit dorsal flattening at the region of the shell field which marks the onset of calcification	22	22	22	22	22
Trochophore	First trace of mineralization is observed by the presence of a small (ca. 20 µm) shell and birefringence at the hinge area. Presence of an early trochophore shell has previously been observed at 22 hpf (Ramesh et al., 2017)	24	25	25	24	25
Trochophore	49% ± 7.7% of the larval shell is covered by a mineralized shell	27	29	29	28	28
Late trochophore	76.9% ± 7.9% of the larval shell is covered by a mineralized shell	30	33	32	31	31
D‐veliger	Larvae have secreted the PD I shell and exhibit a distinctive “D” shape	35	41	35	36	36

Changes in shell cover are quantified from N = 20 larvae and are reported as mean ± SD. Under control conditions, larval development across families was relatively uniform (maximum standard deviation = ca. +7% shell cover, Table 1) and sampled at six identical time points.