Is it safe to come out? Fish responses to changes in predator density

Overfishing can cause changes in fish populations over time by removing the big fish (often predators) from marine ecosystems. Reducing predator density can have consequences for ecosystem function, including movement of prey and reproduction of the predators themselves (Madin et al. 2012). Often marine parks are established to protect marine ecosystems by preventing these changes or allowing communities to recover from fishing.

Example of a food web and the responses of lower trophic levels to a reduction in the number of top-level predators  (Cury et al. 2001).

Example of a food web and the responses of lower trophic levels to a reduction in the number of top-level predators
(Cury et al. 2001).

Professor Robert Warner from the University of California, Santa Barbara and his team of researchers study fish behaviour. Professor Warner spoke last week at Macquarie University about fish behavioural responses to predators and what happens when predators are removed from ecosystems.

Removing predators can cause a trophic cascade, which is a change in the abundance of animals and plants in the lower levels of the food chain (Baum & Worm 2009). For example, the removal of sharks means predatory fish can increase in numbers which can lead to a decrease in smaller herbivorous fish population sizes and a boom in the amount of seaweed and/or algae on the reef.

As well as these direct changes to the food chain there are also indirect changes in the ecosystem, such as the behaviour of fish (Madin et al. 2010). Prey fish change their behaviour to avoid predators and reduce their risk of being eaten. This avoidance behaviour changes in response to their environment, so when there are more predators around the prey fish are more risk averse. The behaviour can change both temporally (over time) and spatially (over an area).

The extent to which prey fish will range from shelter (blue line) in fished (lower predator density) and unfished (higher predator density) areas. Photo: Belinda Fabian.

The extent to which prey fish will range from shelter (blue line) in fished (lower predator density) and unfished (higher predator density) areas. Photo: Belinda Fabian.

The researchers compared the distances prey fish ventured from shelter and the density of predators in both fished and unfished areas. The predators in fished areas are smaller and a lower density compared to the predators in unfished areas. In unfished areas they found the prey fish ranged over a shorter distance from shelter than in fished areas (Madin et al. 2012).

Another change in fish behaviour is their foraging patterns. When predators are present, fish can change the location of their foraging and/or the time when they forage. For example, a fish that normally feeds on the reef can avoid a predator by moving to the mangroves or feed at night to avoid a predator which is active during the day.

These changes in prey fish behaviour can have flow on effects for other parts of the reef. The restriction in the distance the fish are willing to range from shelter during feeding can have an impact on the distribution pattern of algae (food of the prey fish). When there is low predator density prey fish are willing to range far and wide which leads to even consumption of algae over the reef. In contrast when there are more predators the prey fish are more risk averse and only forage close to shelter (Madin et al. 2012). This means the algae is heavily cropped close to shelter and there is low cropping at further distances from shelter. This uneven distribution and overgrowth of algae can negatively impact other organisms on the reef such as coral (Coyer et al. 1993). The heavy cropping close to shelter means some of food the fish is consuming may be less than ideal and their growth and reproduction may be limited due to energy and/or nutrient deficiencies (Heithaus et al. 2008).

Algae growing over coral in Suva, Fiji. Photo: Belinda Fabian.

Algae growing over coral in Suva, Fiji.
Photo: Belinda Fabian.

Understanding the impacts of predator density in marine ecosystems is important for fisheries management and the establishment of marine sanctuaries. The sites used in these studies include currently fished, long established protected areas (no previous fishing) and new protected areas (recently fished). The researchers included these types of areas in the study as they wanted to determine the impacts of predator removal on prey behaviour and if these effects can be reversed through the cessation of fishing and a resulting increase in predator density (Madin et al. 2012).

Reef environments have a delicate balance of species, interactions and environmental variables. Professor Warner and his team have shown that a change such as overfishing of a predator species could have far-reaching impacts on the distribution and abundance of organisms on the reef. If the interactions are permanently changed then there could be negative impacts on the functioning of the reef, especially in the current context where there are many other challenges for reefs such as pollution and climate change.

To learn more:

Baum JK and Worm B (2009). Cascading top-down effects of changing oceanic predator abundances. Journal of Animal Ecology, 78, 699-714.

Coyer JA, Ambrose RF, Engle JM and Carroll JC (1993). Interactions between corals and algae on a temperate zone rocky reef: mediation by sea urchins. Journal of Experimental Marine Biology and Ecology, 167 (1): 21-37.

Cury P, Shannon L and Shin Y-J (2001). ‘The Functioning of Marine Ecosystems’, Reykjavik Conference on Responsible Fisheries in the Marine Ecosystem, Reykjavik, Iceland, 1-4 October.

Heithaus MR, Frid A, Wirsing AJ and Worm B (2008). Predicting ecological consequences of marine top predator declines. Trends in Ecology and Evolution, 23 (4), 202-210.

Madin EMP, Gaines SD and Warner RR (2010) Field evidence for pervasive indirect effects of fishing on prey foraging behaviour. Ecology, 91 (12), 3563-3571.

Madin EMP, Gaines SD, Madin JS, Link A-K, Lubchenco PJ, Selden RL and Warner RR (2012). Do Behavioral Foraging Responses of Prey to Predators Function Similarly in Restored and Pristine Foodwebs? PLoS ONE, doi: 10.1371/journal.pone.0032390.

Weaker Mussels in Warm Water?

You’d be forgiven for thinking that mussels attach to rocks and other substrates using a muscular foot. After all, that’s what their name implies. But mussels actually hang on using byssal threads – small fibres constructed by the mussel that are very strong while also being highly flexible.

MusselFoot300x267Researchers at the University of Washington are looking at the impacts of environmental conditions on the strength of byssal threads. They found the strength and flexibility of the threads varies with temperature and ocean pH, which could have far reaching consequences in the not too distant future.

Mussel foot (right) and byssal thread (left). Photo: Laura Coutts

The researchers compared the strength of byssal threads at 10oC and 25oC. In warmer water the mussels produced fewer threads and those that were produced were weaker than the corresponding threads created in cooler conditions. These changes were seen even as a result of short-term variations in temperature.

The warming of the ocean due to climate change could impact mussel populations by reducing their attachment strength. They may not be able to hold on as tightly to the substrate and could be washed away by waves and currents. Existing sites may no longer be habitable by mussels and there could be increased mortality if feeding is impacted by the inability to remain attached to the substrate.

Byssus threads 300x271When these temperature impacts are combined with other environmental stressors, such as ocean acidification and a change in the frequency and intensity of storms, mussels could be detrimentally affected. Mussels have a larval stage at the beginning of their life cycle, so the colonisation of cooler and calmer environments is theoretically possible.

Mussels attaching to substrate using byssal threads. Photo: Emily Carrington

Mussel migration due to changes in ocean temperature has the potential to dramatically impact intertidal ecosystem composition and dynamics. Changes in water temperature and mussel attachment strength will also have ramifications for the aquaculture industry as mussel attachment to ropes is important for productive mussel farming.

The mussel species in these experiments was Mytilus trossulus which lives mainly in the intertidal zone of the northern Pacific Ocean. Mussels are also found in warmer environments around the world, but these findings seem to imply that they may not be able to hang on to the substrate in turbulent conditions as well as their counterparts in cooler environments.

Maybe mussels in warmer environments may be more successful in habitats with calmer conditions? It would be interesting to extend these experiments to warmer conditions and possibly freshwater mussels to see if the same limitations apply to their byssal threads.

To find out more:

Newcomb LA, Carrington E, George MN & O’Donnell MJ (2014). Short−term exposure to elevated temperature and low pH alters mussel attachment strength. Abstract of presentation to The Society of Integrative & Comparative Biology, Austin, Texas, 3-7 January.

O’Donnell MJ, George MN & Carrington E (2013). Mussel Byssus Attachment Weakened by Ocean Acidification. Nature Climate Change. doi: 10.1038/nclimate1846.

Hear Professor Emily Carrington discussing this research and Professor Phillip Messersmith talking about the applications of mussel attachment for medical research on the ‘The Science Show’ Radio National podcast here.

Seeing-eye sponges? Phototaxis and eye development in sponge larvae

This week I attended a seminar by Professor Bernard Degnan from the University of Queensland on the early evolution of animal complexity. He discussed many genetic aspects of the evolution of multicellularity, the similarities between sponges and more complex organisms and the differences between sponges and their unicellular ancestors. The presentation was captivating (even if some of the genetic bits did go over my head) and I loved the passion he had for the topic.

But the thing that stuck with me is his argument that sponges have organs which they use to see. This absolutely goes against the grain of everything I’ve learnt about sponges in the last 20 years. No more believing that sponges are ‘simple’ (and on a side note: no more believing that just because it’s in a textbook means it’s infallible – this part of the scientific method has been drilled in to me intellectually, but this is my first experience of having my biological world rocked in reality).

Only a tiny part of the seminar related to sponge (Amphimedon queenslandica) response to light. Opsins (photo-sensitive proteins involved in light responses of all animals with nervous systems) are not found in sponges so how do sponges respond to light? During A. queenslandica larval development (cleavage) a cryptochrome gene is expressed before any cells become pigmented. Pigmented cells then form throughout the blastula and they migrate to form a pigment ring at the posterior pole of the larvae. This pigment ring is complemented by long cilia which move in response to blue-light stimulus (440nm) possibly using cryptochrome proteins. These long cilia dictate the swimming direction of the larvae and act as a rudder.


Pigment and cilia ring at A. queenslandica larval posterior pole

From 0-24 hours old the larvae are negatively phototactic (avoid light) and from 48 hours old they lose their ability to respond to light. This negative phototaxis early in the larvae’s life could be a response to an environmental cue (light) that indicates the presence (or absence) of an appropriate location for the larvae to settle and/or a timing cue for metamorphosis.

So the pigment ring structures in a sponge fit the minimum definition of an eye – pigment adjacent to photoreceptors. The evolution the sponge eye (albeit a neuron-less, opsin-less eye that loses function as the larvae develop) is a fascinating example of convergent evolution.

Further reading:

Rivera AS, Ozturk N, Fahey B, Plachetzki DC, Degnan BM, Sancar A and Oakley T (2012) Blue-light-receptive cryptochrome is expressed in a sponge eye lacking neurons and opsin. The Journal of Experimental Biology, 215, 1278-1286.