Carbon storage and marine invertebrate behaviour

NFR CLIMIT Program - Project 215637
Project leader: Dr Shaw Bamber - IRIS Environment

Can seepage of CO2 stored in sub-sea geological formations disrupt important behavioural traits of benthic invertebrates?

Summary of activities and some results from experiments conducted during 2013:

This project was designed to provide important information that can contribute towards the debate regarding the sub-sea geological storage of CO2. It is important to be able to gauge possible consequences for the surrounding marine area in the event of unplanned leakages. Dissolved CO2 at concentrations higher than natural levels can affect the wellbeing of animals in a number of ways. Many marine invertebrates rely on calcium and other minerals for growth and shell building and decreases in seawater pH due to increased dissolved CO2 in seawater can interfere with this process. Increased levels of dissolved CO2 can also directly affect physiology associated with respiration in these animals. Most ocean acidification studies focus on the relatively small decreases in pH that will arise from the interaction between higher atmospheric concentrations of the gas and the ocean surface. Current predictions indicate that oceanic pH may drop from the current 8,1 to 7,6 by the end of the  century. This process is gradual, but in the event of accidental discharge of CO2 from a concentrated store, a rapid reduction in pH could occur in the immediate area of the leakage.

This project set out to discover the response of selected animals when exposed to these acute substantial falls in pH.  The endpoints chosen for the study were based on behavioural responses of mussels, shrimps and crabs. Such responses can provide a rapid indication of whether the organisms can detect the fall in pH and also give some idea on the nature of the response, be it attempting to isolate itself from the problem by closure of the shells, as in the case of bivalve molluscs, or by migrating away from the problem area, in the case of more mobile organisms such as shrimps and crabs. Mortality and migration of key species from within an ocean floor habitat would have serious consequences for the wellbeing of the affected ecosystem if such actions resulted from a sustained leak of CO2.
Islandic cyprine (Arctica islandica)
Sub-sea geological storage of CO2 has been underway in the North Sea for some years and it is likely that further sites will be chosen in this region in the event of further expansion of this method. For this reason the animals selected for the study represented key species that either could be found in the area of the central North Sea or were representative of groups of animals found there. The Islandic cyprine (Arctica islandica) is a bivalve mollusc found throughout the North sea. It lies buried in the sediment, using short siphons to draw in and expel seawater containing both food and oxygen. A simple method based on a proximity sensor was used to continuously measure valve gape movements throughout the test periods.  The valve gape behaviour during reducing pH conditions was compared against that recorded in animals maintained in normal pH water over a similar period of time.

The brown shrimp (Crangon crangon) lives in close association with sediment and is found throughout the North Sea region. Two methods are being used in the project to measure activity changes in shrimps exposed to seawater with reducing pH, with the goal of identifying the threshold level that initiates a change in activity level or pattern. The first method uses a quadropole impedance conversion technique capable of detecting electrical impulses arising from movements from the shrimps when contained within a coil. The generated signal can be used to identify different types of behaviour associated with various activities, such as gill ventilation with limbs, swimming and escape responses. This work has been carried out in conjunction with an international partner within the project, LimCo International, GmbH. The second method used to record activity patterns uses a simple infrared light beam array set out along the length of test tanks to continuously record beam breaks caused by the passage of the single shrimp contained within each tank. This technique allows a measure of the intensity of activity of the test animals over time. 
Brown shrimp (Crangon crangon)
The final animal being studied in the project is the shore crab (Carcinus maenas). Although typically found near shore, this crab is a useful model organism with which to study the potential influence of changes in seawater pH on  chemical communication and detection in decapod crustaceans. These animals have  chemical receptors capable of detecting both feeding cues and sexual pheromones released by conspecifics in the seawater about them. The question to be addressed  by the research within this project is whether pH reduction in seawater changes the sensitivity of chemical detection in the crabs and if so what is the threshold level that initiates changes in response. To answer these questions it is possible to use the very specific behavioural responses that crabs exhibit on detection of these cues within a behavioural bioassay. As an example, when a receptive male crab detects pheromone from a female ready to mate, it will adopt a characteristic stance in which it will extend its legs such that the main body is lifted, with the final pair of walking legs raised above the level of the carapace. It will then commence a slow forward walking motion seeking out the source of the signal. This type of behaviour only occurs after successful detection of pheromone, so it can be used within an observational bioassay to determine whether detection has taken place. The behavioural assays will be complemented by electrophysiological techniques used on the receptor structures themselves. Much of this work is being carried out in conjunction with the second international partner in this project, Dr Joerg Hardege, based at the University of Hull in the UK. 

Preliminary results from 2013 experiments.

Islandic Cyprine (Arctica islandica)

Acute exposure to stepwise reductions in pH in successive 24 hour periods indicated�� that the critical pH that triggered a significant difference in valve gape behaviour was between 6.2 and 6.0 (see plot 1 below). 
Plot 1
Plot 1. Acute response of cyprine valve movements under reduced pH conditions.
This experiment was followed up by a chronic exposure of cyprines to a pH of 6.2 to observe whether they would acclimate to these conditions. The test animals were held in pH 6.2 continuously for 7 days.  As seen in plot 2 below, the level of valve gape activity climbed significantly compared against the control animals and was sustained for 2 days before falling back towards the level of the control animals.

Tests at the end of the exposure period demonstrated that burrowing ability in the exposed cyprines was not impaired when compared against the control animals. There was no mortality during the tests and the cyprines survived for several months in the laboratory before their eventual return to the ocean. 
Plot 2
Plot 2. Response of cyprine valve movements to chronic exposure to pH 6.2

Brown shrimp (Crangon crangon)

Two sequences of experiments were conducted using the brown shrimp this summer. The first used the quadrapole method to gather information on the type and intensity of behaviour exhibited by the shrimp when exposed to a range of pH conditions. The data from these experiments are currently being analysed with results expected soon.

Experiments with the second method, using infrared light beams to record emergence and swimming behaviour, are now nearly complete. In this approach brown shrimp, freshly collected from a beach adjacent to the laboratory, are transferred to individual tanks. Each tank is fed with a continuous flow of seawater and has a layer of sand in its base that allows the shrimp to burrow. Light beams played across the width of the tank record each passage of the shrimp during periods of emergence and activity. For the latest sequence of experiments, treated shrimp were held at natural seawater pH for 24 hours and then continuously exposed to reduced pH conditions for 5 days. Control shrimp, held at natural seawater pH throughout the 6 days, showed a clear endogenous circadian rhythm, with activity levels peeking during expected hours of darkness (plot 3). All tests were conducted under constant low light conditions so it appears these patterns of activity arise from an inbuilt clock within the shrimp capable of predicting light level changes. 
Plot 3
Plot 3. Activity patterns in brown shrimp held under natural seawater conditions.
Preliminary findings have shown that pH reduction to 7.0 is sufficient to cause an increase in activity throughout the length of the exposure and cause the circadian pattern to be modified (plot 4). Such a shift in behaviour would suggest an attempt to move away from the reduced pH environment but this would also leave the shrimp at a greater risk of predation. These behavioural changes therefore, could have significant consequences for both the shrimp themselves and the wider ecosystem in which they play an important part.  Analysis of this data is continuing. 
Plot 4
Plot 4. Activity patterns in brown shrimp held in seawater at pH 7.

Shore crab (Carcinus maenas)

Work at the University of Hull has continued examining the effects of reduced pH on the ability of the crabs to detect both food and sex pheromone. Preliminary findings suggest that a modest reduction in pH down to 7.7 resulted in a significant reduction in sensitivity to both of the test cues. The experiments will be concluded during this autumn.
Shore crab (Carcinus maenas)
For further information on the project, contact Shaw Bamber.
Last updated  18/10/2013

All photos: IRIS