Spermatophore of Phoronopsis harmeri

The first image shows the spermatophore of a phoronid worm Phoronopsis harmeri. A spermatophore is a package that contains multiple sperm. Sperm is produced by males inside the body coelom, then released through the nephridiopores at the anterior end, and shaped into these packages by the special spermatophoral gland located inside the crown of tentacles (called the lophophore) which surrounds the mouth. The spermatophores of this species are round, about 300 micron in diameter, and equipped with a cork-screw shaped transparent “sail”. The sail is easily dislodged from the capsule.

The second picture shows two spermatozoa (or sperm). The sperm in this species is unusual in that it is V-shaped. The nucleus is in one arm of the “V”, and the flagellum forms the other arm, the acrosome is at the apex of the “V”. After the spermatophore is released it floats in the water and lands on the lophophore of a female. Once there, the spermatophore somehow makes its way into the tentacular coelom of the female, and eventually into the body coelom where internal fertilization occurs. Typically development is arrested until the female releases the fertilized eggs into the water. Occasionally development is initiated inside the female coelom, and one can find gastrulae or even more advanced developmental stages. See a post by Phillip Warner showing a 6-day old actinotroch larva of this species reared in the lab, and a post by Svetlana Maslakova showing an advanced actinotroch of this species collected from plankton.