Date of Award

Spring 5-13-2017

Level of Access

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Biology

Advisor

Richard Wahle

Second Committee Member

Robert Steneck

Third Committee Member

Damian Brady

Additional Committee Members

Pete Jumars

Chris Petersen

Kevin Stokesbury

Abstract

Most marine organisms are broadcast spawners, releasing their sperm and eggs into the water column. Methods of measuring in situ fertilization have proven successful with a few model species, which are reviewed in my introductory chapter. However, many commercially exploited species, such as the sea scallop Placopecten magellanicus, have been neglected. Sea scallop populations have greatly increased from fishing closures, but the mechanism behind this response is uncertain, particularly in regard to fertilization. In this dissertation I developed a methodology of measuring fertilization success and spawning events of P. magellanicus, tested it in laboratory and field settings, and developed a novel genetic probe to detect and quantify scallop gametes.

Chapter 2 describes laboratory experiments and field results from our development of nylon mesh chambers used to measure fertilization success (percent of eggs fertilized) in situ. In dilution-series experiments, maximum fertilization success occurred at sperm concentrations >107 sperm ml–1 . Between 8 and 24 h at ambient temperature, egg viability fell to zero. Sperm half-life shortened from 2 h to 9 min when sperm concentrations diluted by 10-fold from 107 cells ml–1 to 106 cells ml–1 . Flume trials demonstrated chamber artifacts: fertilization was lower inside the chamber than outside, and the effect was greater at higher flow rates, but chamber orientation to flow had no effect on fertilization. Increasing the numbers of eggs tended to reduce fertilization success. In dockside tests, a 30-fold difference in spawner numbers had a significant effect on fertilization success.

In Chapter 3, I analyzed video surveys of scallop aggregations on western Atlantic fishery grounds to determine whether population density, degree of aggregation, and shell size were correlated with fishing closures. Based on these data, I created experimental benthic populations to measure fertilization success in situ. Fertilization success in these experiments did not vary significantly across a 10-fold difference in population density, a result which was inconsistent with the outcome predicted by a current fertilization model. This likely underscores the extreme variability in fertilization success in the field that is not captured by models.

In Chapter 4 I developed and tested a genetic probe (Pmag_304F) and primer set (Pmag_282F, Pmag_492R) to detect and quantify P. magellanicus gametes in the water column. I used a TaqMan fluorescent probe and primer set to target the intergenic spacer region (ITS) in the scallop genome. To verify this probe works on scallop gametes, I tested it on replicate sperm dilution series. This method may be applied to field samples to detect and quantify spawning events for this species and other important invertebrates.

This dissertation presents empirical data on the relationship between spawner abundance and fertilization success in P. magellanicus, evidence for a possible component Allee effect, some form of compensation at low densities and the development of two methods to detect spawning events in the field. These new tools and data improve our understanding of a previously poorly studied aspect of scallop reproduction, and may provide insight into their resilience to fishing pressure.