What are invasive species and why should we care?

Animals and plants that are transported or travel to locations outside of their natural habitat range can cause major shifts in the community structure of their new home. You may be familiar with the story of the lionfish: a fish that made its way to the Atlantic likely due to the aquarium trade and is now causing major destruction to other local fish communities. Or maybe you have heard of  the invasive zebra mussel: Eurasian mussels that have cost the US billions of dollars to try and remove from water treatment and power facilities. Invasive species are a serious issue, not just because of the ecosystem imbalance they can cause, but they also can have huge economic impacts.

The tale of an invasive air-breathing fish

One invasive fish, the Northern Snakehead, is particularly challenging to manage because it does something most fishes cannot: it can breathe and “walk” on land. Northern snakeheads are native to swamps in Southeast Asia (where they can grow up to 6 feet long!) where water conditions are considered very poor for most other fishes- low oxygen, high acidity. But the snakehead can tolerate most of these harsh conditions, by breathing air, and can even emerge from the water and hop onto land. Managing a land-traversing fish is tricky, but understanding their biology and behavior can help inform management and contain the population. New research published in IOB by N. Bressman et al. explores exactly that. Bressman and colleagues looked at the environmental conditions that cause the snakeheads to flee their aquatic homes and how exactly they use their fishy-form to traverse across land.

What causes these fish to leave water?

While it has been assumed poor water quality will encourage these fish to leave water, the causal mechanisms behind the fleeing response had not been examined before. To test this, Bressman et al. set up different laboratory treatments where they were able to regulate the water conditions. To mimic the harsh environment of the Asian swamps, Bressman et al. regulated the acidity (pH), salinity, temperature, oxygen/CO2 levels, and even simulated rain. With increasing stressful conditions, the fish generally swam less and gulped air from the surface more, but it took the highest treatment level for the fishes to emerge from the water and onto “land” (a ramp within the cage). Bressman et al. found that high salinity, high acidity and high dissolved CO2 in the water was the only treatment that caused this emersion to land.

Why is this interesting?

This finding that Northern Snakehead will remain in water unless conditions are very poor is a promising conclusion for management. Eradication efforts can focus on managing the spread of snakehead by focusing on regions with poor water quality and can be hopeful that there is less purposeful emersion of snakehead in higher water quality areas. However, accidental emersion onto land by fisherman and predators is still an issue that needs to be addressed.

So, how does this fish actually move around on land?

Bressman et al. explored how these fish move on land- the kinematics of their terrestrial locomotion as they grow and develop. To understand their movements, Bressman et al. removed the fish from water and recorded the movements using Gopro cameras. In order to look at the effects of different terrains, Bressman used different substrates for the fish to move across including: real grass, turf, no-slip boat deck, and smooth bench liner. To track body movements, Bressman placed points on the digitized videos, frame by frame; one point on the head, one on the tail, and one on the center of mass (region where most of the fish’s weight is). Using these data and length measurements, Bressman was then able to get information about the fish’s stride length, speed, and other kinematic data.

Electrodes were also attached to various fin and body muscles of the fish, to determine when the muscle was being activated during movement. Bressman was able to sync up the muscle activation with the kinematic data in order to reveal a complete picture of their terrestrial locomotion.

What did they find? (1 of 3)

1. Northern snakeheads are effective (and fast) land-walkers that use axial-appendage locomotion

Most fish that have the unique ability to move out of water and onto land move like an eel, this is called axial-based locomotion. This movement is common because fish don’t have feet and they have to slither on land moving their whole bodies back and forth. Some fish however, with strong fins that can support their body weight, can use their fins like feet and this movement is called appendage-based locomotion (the appendage being the fin). An example of a fish with appendage based locomotion is the mudskipper, one of the more well-known air breathing fishes.

Snakeheads were found to move in short, fast bursts and had a combination of these locomotor methods- hence “axial-appendage locomotion.” Snakeheads mostly move by twisting their whole bodies back and forth and their pectoral (side) fins moved together to provide extra thrust and stability (preventing the fish from rolling on its side). This was especially unique because no other amphibious fish with this locomotion method is known to use their pectoral fins in sequence like this. Lastly, the electrodes recording muscle activity also found there was an alternating left-right activation of muscles along the fishes body.

2. How they move on land as juveniles is completely different from adults

Bressman looked at both juveniles and adults and found that their locomotion was so different they had to be categorized differently. The adults displayed axial-appendage locomotion as discussed above, but juveniles employed a different method that Bressman termed “tail flip jumps.” All individuals smaller than 3.5 cm used this method of flipping their tail and jumping forward. All individuals larger than this size used the axial-appendage locomotion, therefore these fish display an ontogenetic shift (a change with growth and development) in locomotion. This complete shift was a surprising find because shifts this extreme are often accompanied by drastic changes in body form (metamorphosis- like a butterfly) and the snakehead does not have a large alteration in body form.

3. It’s easier for snakeheads to move on grass than smoother substrates

Snakeheads were tested on a range of different substrates and it was found that they could move the quickest over grass. This is not too surprising as grass may more closely mimic their natural habitat, but grass is also a more complex substrate for the snakehead to traverse. Snakehead were found to move their bodies less dramatically (smaller amplitudes) on grass which may have helped to increase their stride frequency. Bressman et al. concluded that the grass likely allowed for additional traction and support for the snakehead to use to push against.

Summary:

Northern snakeheads are an invasive species of fish that can travel from lake to lake via terrestrial locomotion. Snakehead only emerge from water in very poor water quality conditions. Snakehead have a very unique way of moving on land that changes as they grow. Adults display axial-appendage based locomotion with most of the movement from their whole body but with assistance from their unique in-phase pectoral fins. Juveniles move by flipping their tail and jumping. Snakeheads move in short bursts and can travel fastest on grass-like terrains. This study is useful and interesting to: 1) help management efforts to target and constrain snakehead populations 2) better understand how different terrestrial based locomotion in fish evolved.

Kelsi Rutledge is 2nd year Ph.D. student at UCLA studying the functional morphology and fluid dynamics of olfaction in batoid fishes. Check out her website here or follow her on Twitter here.