Optimizing sperm cryopreservation in great scallop (Pecten maximus)

A basic cryopreservation protocol for great scallop sperm was recently published, with which thawed sperm survival remained limited to 25% of control values (Suquet et al. 2014). We designed here a set of five experiments to test ways of improving thawed sperm survival and storage by studying the effects of (1) cryoprotectant concentration; (2) sugar incorporation in the extender; (3) egg yolk addition to the extender; (4) sperm dilution rate; and (5) storage capacity of thawed sperm at 4 degrees C. A sixth experiment was then performed to compare rearing performances of larvae produced using thawed sperm from the optimal protocol defined in the present study with those produced using fresh sperm. During the first experiment, a significantly higher percentage of motile spermatozoa was recorded after thawing when the treatment had included 15 or 20% polyethylene glycol (PEG). In the second experiment, no significant improvement in post-thaw sperm motility was recorded when trehalose or glucose was added to the extender. In the third experiment, adding 10 or 20% egg yolk to the extender significantly decreased the percentage of motile sperm. During the fourth experiment, significantly higher percentages of motile sperm were observed for dilution rates ranging from 3 : 1 to 1 : 1 than for higher dilution rates. In the fifth experiment, a significant decrease in the percentage of post-thawed sperm was observed for storage durations of 30 and 60 min, relative to shorter durations. During the last experiment, similar rearing performances were observed between larvae produced using fresh and thawed sperm, up to 9 days post fertilization. In conclusion, the high survival of sperm recorded after thawing (67% of the control level) and the high rearing performances of progenies obtained using thawed sperm validate the use of the present improved cryopreservation protocol for sperm cryobanking in great scallop.