Genetic Heterogeneity in Hymeniacidon sinapium from Elkhorn Slough Supports Multiple Introductions from Japan and Korea

A large number of invasive marine invertebrates are reported from Elkhorn Slough, California. One of the more conspicuous invasive species is a bright orange colored sponge, recently identified as Hymeniacidon sinapium (Family Halichondriidae), it is native to Japan and Korea. Preliminary DNA sequence analysis of H. sinapium from the slough determined it was heterogenetic, containing two unique nuclear rDNA internal transcribed spacer sequences within a single individual. The purpose of this study was to determine the extent of the intragenomic variation in populations of H. sinapium in Elkhorn Slough and the origin of the invading population. To address these pursuits, the ITS regions were analyzed in 27 specimens distributed throughout the slough and compared to sequences from Korea and Japan. Five distinct genotypes were discovered: 3 heterogenetic; 2 single-copies. One of the sequences matched another in the database: the invasive specimen from San Diego Mission Bay, California. The other collected specimens emulated characteristics from southeast Korea and Japan. Mapping of the genetic diversity of H. sinapium in the slough showed that the greatest diversity occurred near an area of historical disturbance, a long ceased oyster culturing facility that obtained oyster spat from Japan. Based on the high degree of diversity, it appears as though multiple introductory events of H. sinapium radiated outward from the central portion of the slough. Hybridization may also be responsible for the high levels of intragenomic variation.