To settle or not to settle? Oyster larvae settlement cues investigated

Oysters as ecosystem engineers

The inclusion of ecosystem engineers in coastal planning is an approach being considered to mitigate coastal erosion in different environments worldwide. Oysters are reef building and can alter and improve their habitat acting as a barrier for coastal protection by sediment stabilization and wave attenuation. In addition, they enrich the local environment, increasing biodiversity as a food source and nursery. However, there is a need for more research into the requirements/preferences of the early life stages of ecosystem engineering bivalves. During their free-swimming larval stage, oysters, like many other marine invertebrates, respond to cues in the water column and at the substrate surface which indicate ideal habitats. Understanding how these species respond to cues in natural dynamic environments will help inform best practices for new reef-building initiatives.

"As part of her PhD study, Sarah Schmidlin, early career researcher at VLIZ, investigates how suitability of a site for survival steer settlement of oysters, specifically using the pacific oyster (Magallana gigas)."

Objectives

Settlement is a critical step in the life history of marine invertebrates with a planktonic larval stage, particularly for sessile organisms that have limited to no relocating ability. Settling larvae, therefore, screen their environment for cues that will indicate the suitability of a site for survival. As part of her PhD study, Sarah Schmidlin, early career researcher at VLIZ, investigates how these cues steer settlement of oysters, specifically using the pacific oyster (Magallana gigas). This includes, substrate properties and/or environmental conditions, focusing on effect of the interactions from multiple settlement cues. In the course of the PhD, several lab-based experiments have been conducted.

In one experiment the interaction of natural cues originating from marine biofilms, conspecific adults, and predator species were assessed to better understand how larvae interpret and modify behavior under interacting positive and negative signals.

Another experiment focused on how larvae settlement changes in response to habitat associated sounds from conspecific reefs, as well as vessel noises from the BPNS.

Methodology

To study settlement cues in a laboratory setting, it is necessary to have a source of oyster larvae in their pediveliger stage (the stage of their development immediately preceding settlement), which takes place approximately 2-3 weeks after fertilization. A small-scale hatchery was created for the purpose of suppling these experiments. Once at the appropriate development, larvae are exposed to different cues and cue combinations. After exposure larvae are then manually inspected for metamorphosis. In parallel to manual assessment of settlement success trials, a low-cost video analysis setup using Raspberry pi HQ cameras and computers was used to film larvae reaction to cues during settlement prior to metamorphosis.

Used components of the LifeWatch Infrastructure

During the study, state-of-the-art LifeWatch imaging tools (e.g. FlowCam) have been used to count phytoplankton food stock cultures before feeding oyster larvae to determine culture concentration and appropriate dilution necessary. Furthermore, these imaging tools have been used in approximating densities of oyster embryos for stocking cultures at appropriate densities.

Recordings from the LifeWatch broadband acoustic network were used in the sound experiment and the sound within the lab set-up was recorded using LifeWatch equipment.