“Where are you?” asked the drowsy scud: ocean acidification can impair mate detection and reduces metabolism in a marine amphipod

Figure 1. Gammarus locusta, photo by Francisco O. Borges (2017). 

Gammarus locusta, photo by Francisco O. Borges (2017).

Climate change is a persistent subject nowadays, both at the scientific and political dinner tables. Indeed, since the beginning of the industrial revolution, the continuous injection of carbon dioxide (CO2) into the atmosphere, as a by-product of humanity’s activities, has been disrupting climatic systems around the globe.

Of the vast array of potentially negative effects, ocean acidification poses one of the emergent risks to marine life, on par with ocean warming and hypoxia (i.e. the reduction of seawater oxygen levels). With rising concentrations of this greenhouse gas in the atmosphere, chemistry dictates that the dissolution of carbon dioxide into surface seawater will also increase. It is this global rise in oceanic carbon dioxide concentrations which creates the phenomenon of ocean acidification by effectively increasing the acidity of seawater (disrupting organism’s acid-base balances) and reducing the availability of certain elements which are necessary for the build-up of calcium carbonate structures (e.g. the shells in bivalves and the exoskeletons of crustaceans).

Some of the main effects of this phenomenon to marine biodiversity can affect survival, growth, reproduction, and behavior, as has been demonstrated by numerous studies across taxa. Indeed, a high concentration of carbon dioxide can induce severe negative effects both on the physiological (e.g. metabolism) and neurologic (e.g. sensory systems) levels. Regarding physiology, many organisms have exhibited a reduction in energy stores (i.e. the energy an individual has available to ensure its biological processes) and decrease in metabolic rates (also known as metabolic depression) when exposed to increased CO2. The latter constitutes a condition considered to be a time-limited adaptation, in which an organism will reduce the energy requirements for the regulation of the acid-base balance, consequently reducing oxygen consumption, as a strategy to enhance survival under physiologically unfavorable conditions – although for a limited amount of time.

Then, there are the potential effects on the neurologic level, which could affect chemosensory reception (i.e. the use of chemical information from the environment). Chemosensory reception is of extreme importance to marine organisms, in particular to crustaceans who depend on olfactory-mediated behaviors for a wide variety of processes (e.g. from feeding to habitat selection, mate-tracking, as well as reproduction, etc.). The potential hindering or impairing of this sensory system, either through the damaging of cues or impeding the correct reception and or processing in the receivers, could eventually lead to serious ecological effects. Surprisingly, there is relatively scarce information concerning the potential effects of climate change variables on this subject, particularly on what concerns the detection of sexual cues in crustaceans, and thus, this presented a rather interesting research subject.

The marine amphipod Gammarus locusta (Figure 1) is a widespread crustacean with key ecological importance for marine and estuarine food webs, being preyed upon a vast number of species: from seabirds to fish and other crustaceans; and actively preying on a wide variety of other organisms, such as algae and smaller invertebrates. In gammarid amphipods, the attraction of sexual mates is elicited on two levels: through long-distance pheromones, which elicit the search for a female in males; and through body-contact pheromones present in the female’s exoskeleton, which allow the recognition of a receptive female by the male, and elicits initiation of a specific pre-copula behaviour (precopulatory mate-guarding – which consists in the guarding and carrying of the female by the male until they can mate). These pheromones are detected by the male’s aesthetascs (antennae-like structures), allowing a male to be aware not only a female’s current location but also gather information on her internal processes.

Facing the current scenario of worldwide environmental change, we wondered what could be the potential effects of projected end-century acidification to the reproductive behavior of G. locusta males, as well as to the metabolic rates of both males and females. With this in mind, we exposed two generations of this species to a control and acidification scenario, adjusting carbon dioxide concentrations to current (~400 µatm) and projected end-century (~900 µatm) levels. When the last generation reached sexual maturity (which in this species and at 18ºC occurs at approximately 30 days of age), a sampling of the behavioral and metabolic traits was performed.

A complete description of the methodology employed for the behavioral and metabolic sampling is available in our paper, for those interested in further details. However, here follows a brief description of the methods we used. In order to measure the impacts on the reproductive behavior of males (specifically on the detection of long-distance female cues), a group of males was chosen at random and tested individually in a binary-choice experimental device (Figure 2).

Francisco O. Borges, Eduardo Sampaio, Cátia Figueiredo, Rui Rosa & Tiago F. Grilo, Science Trends, 18 January 2019. Full article.

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