Selective cnida sequestration in the aeolid nudibranch Hermissenda crassicornis: ecology and mechanism of defense acquisition

• Author / Creator

  Anthony, Susan E

• The aeolid nudibranch Hermissenda crassicornis (Eschscholtz, 1831) (Mollusca, Gastropoda, Opisthobranchia, Cladobranchia) belongs to a special group of shell-less gastropods that sequester cnidae stolen from cnidarian prey in their cerata (dorsal papillae). Cnidarians produce over 30 morphological types of cnidae (harpoon-like subcellular capsules), a particular subset of which are present in a given species. Cnidae are used by cnidarians for prey capture, defense, and substrate adhesion. The aeolid Flabellina verrucosa (M. Sars, 1829) sequesters different cnidae in the presence of seastar and fish predators vs no predator, by prey switching, thereby potentially gaining cnida types more apt at combatting that particular predator. I repeated such an experiment with H. crassicornis, and found that this species does not switch prey in the presence of predators. I also found that H. crassicornis from various locations in Barkley Sound, BC have similar cnida complements. This indicates that prey abundance and predator pressure are either similar at each site, or have no influence on sequestered cnidae. Prior to the discovery of prey switching in F. verrucosa, aeolids were assumed to selectively sequester cnidae most useful to them by the dissolution of unwanted cnidae. This hypothesis was based upon observations that aeolids do not sequester all cnida types produced by their prey; but some of these observations were based on the examination of only a single ceras. By collecting several cerata from different locations on the body, I found that rare cnida types – those produced in low numbers by the cnidarian prey – were present in only a few cerata, and may have been missed in previous studies due to small sample sizes. Sequestered cnidae can be switched over to newly collected cnidae within two weeks, but cnida retention time without replacement is unknown. Cnidae are stored in a functional state within cells (cnidophages) at the tips of the cerata. When attacked by a predator, the aeolids forcibly eject the cells through a small pore, rupturing the cell membrane, and releasing the cnidae. I found that without attack, or ability to replace cnidae from prey, the cells containing cnidae degraded and diminished over time, leaving H. crassicornis bare of cnidae after 44 days. With an unlimited supply of cnidarian prey, I found that H. crassicornis maintained a constant supply of cnidae in their cnidosacs. These experiments show that H. crassicornis does not selectively sequester cnidae under the conditions I exposed them to, and that previous observations of cnida selectivity may have had flawed sampling methods. In H. crassicornis, cnidae are replaced to maintain a constant complement of cnidae in the cnidosac, but are not retained indefinitely.

• Subjects / Keywords

  • Hermissenda crassicornis
  • Cnida sequestration
  • Aeolid
  • Defense

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R35Q4RV3X

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.