Larval settlement of the Pacific oyster Crassostrea gigas on microbial biofilms obtained by immersing half-size glass slides in the sea (1.0 m depth) off Taira-cho, Nagasaki, Japan for 1 to 24 days during the period between May 2009 and January 2010 was investigated. Settlement inducing activities of biofilms that were subjected to heat (80°C), formalin (FA) and antibiotic mixture (AM) treatments were also investigated. Moreover, the settlement inducing activities of 4 bacterial strains isolated from the biofilm were investigated. C. gigas larvae settled in response to microbial biofilms. The percentage of post larvae increased with immersion period in biofilms obtained during Jan to Mar and Oct to Dec. Larval settlement also increased with the bacterial density of biofilms in Jan to Jun and Oct to Dec, and with the diatom density in Apr to Jun. By contrast, larval settlement did not linearly correlate with the dry weight of the biofilms. FA treatment did not affect the activity of the biofilm but heat and AM treatments of the biofilm resulted in significantly low percentage of post larvae. Of the 4 bacterial isolates tested, Pseudoalteromonas tetraodonis and Pseudoalteromonas sp. induced the highest percentage of post larvae but their activities were reduced with formalin treatment. Thus, microbial biofilms may possess a cue that remains intact even after killing the components of the film by FA treatment, and this cue may be distinct from the water soluble metabolite produced by specific bacterial species, such as Pseudoalteromonas sp.
With five large and several small ports, Osaka Bay is heavily industrialized rendering the introduction of marine species that are not native to Japan inevitable. It presents, therefore, an ideal model locality to investigate the presence and persistence of introduced marine and estuarine species. We used the results obtained from surveys by citizen scientists undertaken between 2010 and 2013 to investigate the temporal and spatial patterns in numbers of introduced species. The results show remarkable temporal consistency in numerical diversity with between 11 and 15 introduced species being recorded every year. There were, however, strong spatial differences with tendencies for greater numbers of introduced species in the inner Bay and for lower numbers in the outer Bay. We analyzed the factors responsible for these patterns statistically using a generalized linear model (GLM). The optimal model implicated salinity and the number of native species. Because the salinity of Osaka Bay lies within the tolerance range of all the introduced species, it was discounted as a factor limiting introductions into the Bay. This implies that, interestingly, the actual number of native species is the most important limiting factor. This is then a real-world validation of the ecosystem resistance hypothesis.