Growth and heterosis of hybrids of two closely related species of Pacific sea urchins (Genus Echinometra) in Okinawa

Hybridization between Echinometra sp. A (Ea) and Echinometra mathaei (Em) was examined through cross fertilization and rearing experiments to evaluate the aquaculture potential of hybrids. Mean performance traits of fertilization, larval survival, metamorphosis, and recovery of juveniles Em (ova)×Ea (sperm) and Ea (ova)×Em (sperm) hybrids were not significantly different from each other but were significantly lower than either of their conspecific controls, Ea×Ea and Em×Em. Percent survival from metamorphosis to 3-month-old juveniles at recovery was approximately 63% for hybrids and approximately 74% for parents. Postzygotic isolating mechanisms were not large enough to cause developmental incompatibility or hybrid survival. Growth of postmetamorphic hybrids was significantly higher than that of conspecific controls. After culture for 2 years, the mean weight gained by Ea×Ea, Em×Ea, Ea×Em, Em×Em was 41.1±0.8, 50.5±0.4, 49.8±0.3, and 34.3±0.2 g, respectively. Both reciprocal hybrids, Em×Ea and Ea×Em had significantly faster growth than the superior parent (Ea×Ea) and inferior parent (Em×Em). Other growth parameters such as final weight, weight gain (%), specific growth rate (SGR) (%/day) and test size had the same trends as weight gain. Production of gonads was significantly lower in slow-growing Em×Em (7.5±0.1 g) than the fast-growing Ea×Ea (9.1±0.4 g). Reciprocal hybrids contained significantly larger amount of gonads (12.1±0.1 g in Em×Ea and 11.9±0.2 g in Ea×Em) than conspecific controls. Gonad production showed an increase of 45% in F1 hybrids (mean of both hybrids) over mid parents and an increase of 34%, 63% and 47% in F1 hybrid of Em×Ea and 31%, 60%, and 44% in F1 hybrid of Ea×Em over the superior (Ea×Ea), inferior (Em×Em) and mid parents, respectively. Survival was similar in Em×Em (87%) and Ea×Ea (86%), followed by Em×Ea (79%), and Ea×Em (78%). Despite this, survival was lower in the reciprocal hybrids; the values are very close to their conspecific controls. Therefore, body growth, gonad production and survival indicate hybrids in either direction were viable in laboratory conditions and showed parental heterosis. Mean heterosis for final body weight, weight gain, percent weight gain and test size of Em×Ea and Ea×Em hybrids was not differed significantly between hybrid groups, but a significant difference in heterosis for SGR values of Em×Ea and Ea×Em was found. Mean heterosis for gonad weight and gonad index of Em×Ea (1.5±0.03 and 1.1±0.02) and Ea×Em (1.4±0.06 and 1.1±0.03) was not significantly different. The superiority of these growth traits of the hybrid groups over their parental values indicates positive heterosis (hybrid vigor). Hence hybrids in both directions appear to have considerable potential for use in aquaculture.