Conspecific and rocky intertidal biofilm metamorphic cues for the Asian shore crab, <i>Hemigrapsus sanguineus</i>
The Asian shore crab, Hemigrapsus sanguineus, has become one of the most abundant invasive crabs along the east coast of the United States. The research described in this dissertation will increase the information on the chemical characteristics of the cues encouraging the Asian shore crab’s spread to new areas. The purpose of my investigation was to compare and characterize the biofilm and adult conspecific cues that induce metamorphosis in Hemigrapsus sanguineus. The specific objectives were: (1) Investigate the properties of the conspecific metamorphic cues, (2) Explore the properties of the biofilm cue, (3) Compare the conspecific and biofilm metamorphic cues in the laboratory, and (4) Investigate the metamorphic cues in situ .
Results from the initial set of experiments showed that H. sanguineus megalopae respond to a water-soluble cue produced by conspecific adults. My results show that this cue is not only present in the adult exudate but also is produced by stage I zoeae and early juveniles. The cue produced by adult H. sanguineus is a small molecule (< 14 kDa) that has limited thermal stability. Results of my bioassay experiment indicate H. sanguineus megalopae can detect cue in solutions produced at 0.3 g live crab per liter, but not at 0.03 g 1-1. My results show that protein concentrations in standard exudate solutions (3 g live crab per liter) contain approximately 1 μg of protein per ml of seawater. Thus, it appears that the lower limit for activity of the cue in seawater lies between 0.1 and 0.01 μg of protein per ml. In my investigation, a TCA precipitation of the proteins in the exudate results in a loss of activity in the cue. My results also show that treating the adult exudate with broad-spectrum serine proteinase K and serine protease trypsin to digest proteins resulted in a loss of metamorphic activity. Therefore, it appears that the metamorphic cue produced by adult H. sanguineus is a small protein or peptide.
Results of a second set of experiments showed that H. sanguineus megalopae respond to a biofilm cue from the rocky intertidal. This experiment found that H. sanguineus responds to newly established biofilms that are least 8 days old, but that established natural biofilms elicit the strongest response. While beach and rocky intertidal biofilms both induced metamorphosis, rocky intertidal rock yielded the stronger response. Actual contact with the biofilm seems to be necessary to trigger a metamorphic response as a water-soluble component was not found. Additionally, the biofilm cue has limited thermal stability. Exposure to freezing overnight had no impact on the response, while boiling the biofilm inactivated the cue.
A third group of experiments showed that naturally occurring biofilm can enhance metamorphosis of H. sanguineus megalopae more than exudate or structure alone, but all three of these cues have positive additive effects. For H. sanguineus, the combined treatment of established biofilms, adult exudate, and structure produced the shortest mean time to metamorphosis (MTM) values of any treatment previously tested in the laboratory, and biofilmed rocks by themselves resulted in shorter MTM values than adult exudate or abiotic rocks alone. In contrast to the strong effects of natural substratum, water collected from natural adult habitat did not have a significant effect on MTM, and this lack of activity was significantly correlated with low levels of dissolved total nitrogen in the water. Results of my experiments also showed that the water-soluble cue produced by adult conspecifics is associated with exudate produced by the crabs and is not associated with any biofilm that may be growing on the carapace of the adult crab.
In the related field experiment, the proximity to an adult population of H. sanguineus in the rocky intertidal zone (RIT) was investigated in regards to metamorphosis with the hypothesis that H. sanguineus would respond positively closer to the RIT. Results of my investigation support this hypothesis and show higher percentage metamorphosis among H. sanguineus megalopae in cages deployed within 2 m of the RIT. (Abstract shortened by UMI.)