Triploidy induction in shellfish aims to obtain faster growth and sterility of reared individuals. Triploid Pacific
oysters are most often not totally sterile, but have greatly reduced reproductive capacities compared to those
recorded in diploid individuals. The description of gamete output in triploid aquatic animals is most often
confined to the assessment of fecundity in females. The present work aims at further describing gamete quality
of triploid Pacific oysters and comparing it to values observed in diploid individuals. Diploid and triploid oysters
(produced bymating tetraploidmales ×diploid females) were reared in standard conditions and then transferred to
the wild to allow the offspring to attain reproductive maturity. At the end of gametogenesis, the reproductive
characteristics of both groups were estimated.
After gonad sampling, gametes could be observed in 92.9% diploid oysters comparedwith 42.0% in triploids. A higher
number of spermatozoa was produced in diploids (344 ± 106 × 109 spermatozoa) compared with triploids
(5±7 × 109 spermatozoa). Furthermore, the percentage ofmotile spermand sperm swimming speed were
higher in diploids compared with triploids. In contrast, a higher intracellular ATP content was found in triploids
(99.1±34.0 nmole 10−9 sperm) than in diploids (63.6±20.7 nmole 10−9 sperm). A higher number of oocytes
was collected from diploid oysters (19.1±3.8 × 106 oocytes), than fromtriploid ones (0.1±0.1 × 106 oocytes).
The D-larval yield was 45% higher for crosses (female × male) triploid × diploid, than the control (diploid ×
diploid). Furthermore, the lowest D-larval yield was measured for triploid × triploid crosses.
Considering the present data, a partial estimation of the reproductive potential of triploid Pacific oysters
(triploid × triploid crosses) could be close to 0.06% of that of diploid individuals. However, this estimation is
probably over-evaluated because it does not take into account the low sperm production of triploids (1.5%
compared to diploids), the unknown frequency of spontaneous spawning in triploids and the low viability of the
progeny. The consequences of the low reproductive potential of triploid Pacific oysters on natural populations and
on hatchery practices are discussed. In conclusion, the present work confirms that triploidy leads to a limited
reproductive potential as estimated by gamete characteristics and embryo developmental success.
Statement of relevance: This study is relevant to aquaculture because triploids are produced by aquaculture and the
consequences of their low gamete quality, described in this paper, on the natural environment but also on hatchery
practices are discussed.