Genetic variation in two starfish, Acanthaster planci and Echinaster luzonicus, from Okinawa

Enzyme polymorphism was studied in the populations of two tropical starfish species, Acanthaster planci of the family Acanthasteridae and Echinaster luzonicus of the Echinasteridae from Ryukyu Islands (Okinawa) in Japan by allozyme analysis of 11 different enzymes. In 35 genetic loci scored, the proportion of polymorphic loci(P)was 12.5% and 22.9%, the average heterozygosity per locus(H)was 6.1% and 9.3% , for A. planci and E. luzonicus, respectively. These values were comparable to those observed in many other asteroids and echinoids living in shallow water as well as the two starfish studied here, and lower than those of echinoderms from deep-sea. The author suggested that the extent of genetic variation is closely related to the population size:Echinoderms from deep-sea have large population size, and thus they can maintain high genetic variability within population. Furthermore, the substrate specific enzymes (dehydrogenases)of strict functional constraints showed the lower genetic variability than the non-specific enzymes such as esterase(EST)or peroxidase(PO)of weak functional constraints. The result is consistent with the neutral theory of Kimura. The genetic identity(I)and genetic distance (D)between the two starfish species were I=0.417 and D=0.875. These values were comparable to those obtained between different genera in other asteroids and many animal groups. The maintenance mechanism of genetic variation in echinoderm populations was discussed in some detail from the viewpoints of population genetics and the neutral theory with putting the allozyme data of echinoderms obtained until now together.