Larval development of the northern horse mussel, Modiolus modiolus (L.), including a comparison with the larvae of Mytilus edulis L. as an aid in planktonic identification.

1. Spontaneous spawning of Modiolus modiolus occurred under laboratory conditions at a temperature of approximately 14° C, and larvae were successfully cultured through settlement and metamorphosis.2. Fertilized egg diameters varied from 78 to 90 µ with a mean of 85 µ.3. The smallest shelled veliger observed had length, height, and straight-hinge dimensions of 105, 90, and 95 µ, respectively. The distribution of hinge-line lengths remained constant over time, with a mean at 100 µ and extremes of 90 and 110 µ.4. As total lengths reached 150 to 165 µ the hinge-line was obscured by the appearance of a low, rounded umbo which subsequently developed into a knobby protrusion.5. Pigmented eyespots began appearang as larvae exceeded 270 µ in size, and the full-grown foot of the pediveliger became functional as larvae surpassed 295 µ in length.6. Discrimination of Modiolus modiolus and Mytilus edulis veligers is generally possible through a comparison of the following larval characteristics: hinge-line lengths, to...