Measurement and Interpretation of Rates of Shell Repair Due To Predation

• Author / Creator

  Mendonca, Steven

• Repair scars, traces left by a failed attack on a prey item, provide direct evidence of predation in the fossil record modern systems without preservation or observation of predation events themselves. Counting repair scars and calculating repair frequencies, the number of scars in an assemblage relative to the number of individuals, allows a comparison of predation between assemblages in the fossil record and modern systems. However, because repair scars represent unsuccessful predation and not mortality, there are still questions concerning the measurement and interpretation of repair frequencies. Here, this thesis answers two of these questions relevant to taxa that are commonly used in repair scar studies: 1) can accurate repair frequencies can be measured samples of disarticulated bivalved prey and 2) how do predator size, prey size and prey species affect the likelihood of crab-gastropod encounters resulting in repair scars?
  At present, repair scar studies only use articulated specimens but, because of this, repair scar studies are likely biased against taxa and environments that tend to be represented by disarticulated (single-valve) specimens. Using a temporally extensive dataset of articulated Paleozoic concavo-convex brachiopods this study investigates whether this exclusion is justified by measuring articulated and single-valve repair rates from the same specimens. I found that single-valve repair frequencies closely tracked true repair frequencies with an average of 93% of scars represented by the single valve (dorsal or ventral) with the greatest repair frequency. This high degree of accuracy was largely because the majority of scars (54%) occurred on both valves as the result of a single attack.
  In modern temperate intertidal crab-gastropod systems, repair frequencies have been demonstrated to reflect predator density. However, while the interactions between crabs and gastropods are well understood in a mechanical sense, it is not clear under what conditions these encounters lead to the generation of repair scars. Here, this study uses arena experiments to investigate how the predator size, prey size and prey species affect the likelihood of predator success (prey is killed) or failure (prey survives, and a repair scar could be generated). These experiments involved the crab, Cancer productus, and two of its common gastropod prey, Nucella ostrina and Tegula funebralis. Regression models indicate that predator size has an effect on crab-gastropod encounters with encounters between larger predators being more likely to be successful. It was found that C. productus was more likely to fail in encounters with T funebralis than those with N. ostrina. In addition, encounters with T. funebralis were much longer in duration than those with N. ostrina. Field data show that T. funebralis populations tend to have greater repair frequencies than coexisting N. ostrina populations.
  Considering the main objectives of this study: 1) single-valve repair frequencies were found to record the same trends in repair frequency as do articulated repair rates supporting that repair frequencies from disarticulated samples can be used in repair scars studies. 2) it was found that larger predators are more likely to be successful in crab-gastropod encounters and that T. funebralis was more likely to survive encounters than N. ostrina. Considering this, the lower repair frequencies measured in N. ostrina populations relative to T. funebralis encounters are likely due to the much higher rates of predator success in N. ostrina encounters.
   

• Subjects / Keywords

  • brachiopoda
  • repair
  • arena
  • predation

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-bjy9-pv40

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.