Jonathan P. Davis

The Kumamoto Oyster Crassostrea sikamea is Neither Rare nor Threatened by Hybridization in the Northern Ariake Sea, Japan

Abstract The status of the Kumamoto oyster Crassostrea sikamea in its native Japan is uncertain because of a lack of information about its abundance and distribution and a suggestion that C. sikamea and the Pacific oyster C. gigas hybridize in the northern Ariake Sea. Furthermore, broodstock populations on the United States Pacific coast have been hybridized with C. gigas in the past and may suffer inbreeding depression from multiple generations of hatchery-propagation. As a result, Japanese conservationists and United States oyster growers share an interest in the status of this species in the wild. We collected wild oysters from three sites in Saga Prefecture located in the northern portion of the Ariake Sea, Kyushu, Japan, in September 2006 and used molecular methods (species-specific PCR of the mitochondrial COI gene and PCR-RFLP of the nuclear ribosomal ITS1 gene) to assign 628 sampled oysters to one of three species found in this region. C. sikamea proved to be the dominant organism on artificial hard substrates, comprising 91% of the oysters sampled and typed. Many individuals confirmed as C. sikamea by diagnostic DNA markers had C. gigas-like phenotypes, such as striped shells. Crassostrea ariakensis was present (8% of typed oysters) but only at the lowest intertidal levels, and C. gigas was rare (1%) at these sites. We found no evidence of hybridization between any of the species and were unable to repeat a previous study, which suggested hybridization between C. sikamea and C. gigas based on sharing of a calmodulin allele. We conducted gamete compatibility tests among all combinations of Japanese (Ariake Sea) and United States C. sikamea and C. gigas broodstocks and found strong one-way gamete incompatibility (male C. sikamea × female C. gigas) between species of Japanese stocks, supporting the molecular diagnosis of C. sikamea. However, this one-way incompatibility was less evident in United States stocks, indicating lower barriers to potential hybridization in commercially cultured stocks.

Optimal chemical triploid induction in geoduck clams, Panopea abrupta, by 6-dimethylaminopurine

Abstract The development of geoduck culture techniques coupled with increased market demand during the past decade led to cultured intertidal geoduck beds, over which concerns have arisen regarding the potential genetic risk posed by the reproductive contribution of hatchery outplants to wild stocks. Although an ongoing study to determine the genetic stock structure of Puget Sound geoducks has yet to yield definitive results, sterile triploids would enable geoduck culture to proceed irrespective of any genetic differences found, and may increase rate of growth. At 15 °C and salinity of 30?,we evaluated the suitability of cytochalasin B (CB) and 6-dimethylaminopurine (6-DMAP) for triploid induction in geoducks. Parallel tests of induction agent, start time, duration, and concentration were conducted on the same gamete pool. We found optimal triploid induction (92%) and suitable survivorship (30%) resulted from a 600 µM 6-DMAP treatment. This work enables the future examination of the efficacy of triploid induction for conferring sterility and increasing growth rate in this commercially valuable species.

Distribution, Abundance, and Habitat Associations of a Large Bivalve (Panopea generosa) in a Eutrophic Fjord Estuary

ABSTRACT Marine bivalves are important ecosystem constituents and frequently support valuable fisheries. In many nearshore areas, human disturbance—including declining habitat and water quality—can affect the distribution and abundance of bivalve populations, and complicate ecosystem and fishery management assessments. Infaunal bivalves, in particular, are frequently cryptic and difficult to detect; thus, assessing potential impacts on their populations requires suitable, scalable methods for estimating abundance and distribution. In this study, population size of a common benthic bivalve (the geoduck Panopea generosa) is estimated with a Bayesian habitat-based model fit to scuba and tethered camera data in Hood Canal, a fjord basin in Washington state. Densities declined more than two orders of magnitude along a north—south gradient, concomitant with patterns of deepwater dissolved oxygen, and intensity and duration of seasonal hypoxia. Across the basin, geoducks were most abundant in loose, unconsolidated, sand substrate. The current study demonstrates the utility of using scuba, tethered video, and habitat models to estimate the abundance and distribution of a large infaunal bivalve at a regional (385-km2) scale.

Growth and Burrowing Rates of Juvenile Geoducks Panopea generosa and Panopea globosa under Laboratory Conditions

ABSTRACT The burrowing behavior, growth, and size-dependent burrowing rates of two geoduck species—Panopea globosa and Panopea generosa—from the Mexican coast were assessed during early juvenile development (6–7 mo) using 30-L aquariums with a muddy or sandy sediment substrate. Preburial response time, digging rates, and growth were recorded monthly for organisms with an initial shell length of 5 mm (P. globosa) or 10mm (P. generosa), and continued for 6 or 7 mo. Organisms were fed Isochrysis sp. (clone T-ISO) at libitum on a daily basis during the experimental period. Digging behaviors included the inflation of the geoduck siphon, followed by expulsion of a water jet through the pedal orifice, in conjunction with coordinated muscular contractions of the foot. Both species exhibited a lag in their burial response time that was related linearly to shell length, with rates of 4.5 min/mm (P. globosa) and 3.4 min/mm (P. generosa). Burrowing rates were independent of shell length and were similar in both species (∼0.9–1.4 mm/min) for a size range of 7–36 mm, but were slower in 5-mm seed (0.4 mm/min). Shell growth rate increased linearly with time and was faster in P. globosa (162 µm/day) than P. generosa (61 µm/day). These results suggest that short-term temporary enclosures may be useful for restocking natural populations of geoducks with laboratory-produced seed, at a size as small as 7 mm.

Maturation, Spawning, and Fecundity of the Farmed Pacific Geoduck Panopea generosa in Puget Sound, Washington

ABSTRACT Among the challenges facing aquaculture of native species are potential negative effects of gene flow from cultured to wild populations. Estimates of gene flow are based in large part on the capacity for gamete exchange between individuals, and make estimates of reproductive output and timing of gametogenesis in adjacent cultured and wild populations important to assess. Farmed geoducks of known age from each of five year classes and from nearby wild populations were sampled for reproductive development and other morphometric parameters in March, April, and May 2007 from three Puget Sound, Washington, locations. Results indicate that, at all three locations, cultured geoducks began to mature during year 2 and were fully mature by year 3, with males maturing earlier and at a smaller size than females. It was estimated that 50% maturation occurs at 64 mm in shell length. The gender ratio in 2–5-y-old geoducks was male biased relative to the 1:1 sex ratio observed in wild populations (P << 0.05), providing evidence for facultative protandric dioecy. Rates of maturation in cultured populations were synchronous with nearby wild populations. Overall, mean relative fecundity of cultured 3-, 4-, and 5-y-old clams was approximately 25% that of mean wild relative fecundity. These results suggest that reproductive interactions between cultured and wild geoducks can potentially occur through two mechanisms. First, when farmed geoducks are in proximity to wild geoduck aggregations, spawning may be synchronized, with subsequent gametic interaction occurring. Second, planktonic larvae produced from cultured populations may subsequently settle and mature to propagate with wild conspecifics. Interactions between cultured and wild conspecifics are important to assess especially in cases when domestication selection is proceeding via hatchery-based breeding and other approaches.

REDUCED GENETIC VARIATION AND DECREASED EFFECTIVE NUMBER OF BREEDERS IN FIVE YEAR-CLASSES OF CULTURED GEODUCKS (PANOPEA GENEROSA)

ABSTRACT Aquaculture for the Pacific geoduck (Panopea generosa) is a small but expanding industry in Washington state, where geoducks are native and genetic interactions between wild and cultured geoducks are likely. To examine the potential genetic implications of geoduck aquaculture, genetic diversity, and effective number of breeders (Nb), five contiguous year-classes of cultured geoducks were compared with a wild population. The results from five microsatellite loci indicate the cultured year-classes exhibited reduced allelic richness and Nb as well as increased mean pairwise genetic relatedness. However, examination of relationships within year-classes using sibship assignment revealed that many parents contributed progeny to each year-class. The geoducks in each year-class were comprised of 9 to 25 full-sib groups as well as a large number of individuals unrelated to others at the full-sib level. No clear pattern emerged regarding changes in genetic diversity during the 5-y time span of this study. To decrease the genetic risk to wild geoducks, the results suggest that hatcheries should increase the genetic diversity of cultured geoducks by adopting a partial factorial mating scheme, or they should minimize gene flow from cultured to wild populations by culturing sterile triploid geoducks.