A marine mollusc provides the first example of in vivo storage of prostaglandins: prostaglandin-1,15-lactones.

Prostaglandin-(PG) 1,15-lactones and, in smaller amounts, free acids, were isolated from both the mantle and the dorso-lateral appendices of the opisthobranch mollusc Tethys fimbria. In vivo conversion of PGs into the corresponding lactones and accumulation of PGE2- and PGE3-1,15-lactones in the appendages were shown. The detachment of these appendages from the molested mollusc caused the in vivo conversion of PGE2- and PGE3-lactones back to PGE2 and PGE3 respectively, thus providing the first example of a mechanism by which prostaglandins can be stored and, when needed, released.