This week we are talking about more ways that marine mammals help other species, and the environment. From commensalist organisms, to dolphins and tuna and the well-known sea-otter-urchin connection, we discuss a variety of ways that marine mammals help other species, ecosystems and even impact climate change.
Otters, Urchins & Kelp, Oh My!
You may already be familiar with how sea otters are critical to the health of kelp forests because their story is used as a prime example of what a keystone species is. All organisms are important in the ecosystem, but some exhibit a stronger effect on the ecological health of an area, and these are your keystone species. They are like the wedge-shaped stone that supports the top of an arch in a bridge, take that away and the bridge (or ecosystem) comes crashing down. Sea otters are a classic keystone species. Sea urchins eat kelp (voraciously). But Sea otters eat sea urchins (voraciously – they have to eat up to ¼ of their body weight per day!). What you get then, is where you have sea otters, you have few sea urchins, and thus large healthy kelp forests; without sea otters you have the exact opposite. These forests are important habitat for many other species including crustaceans, mollusks, fish and other marine mammals. Without the forests, these species decline and the ecosystem is altered, becoming a barren sea floor, teeming with urchins (called urchin barrens).
The story of the sea otter-kelp relationship was one of the first to describe what is called the trophic cascade – the effect of apex predators on lower trophic levels (species lower on the food chain) through top-down forcing. This has now been shown in many different ecosystems, most notably the wolves-elk-forest relationship in Yellowstone National Park, another story that many people may be familiar with. Thus these top predators have a strong indirect effect on lower trophic levels. The health of the otter population has a strong effect on the overall health of the ecosystem, and all the animals that rely on it. You can see this play out in areas where sea otters are no longer present (due to hunting), and where they survived or have been reintroduced. Where sea otters were present, kelp forests flourish, which provides a healthy, diverse ecosystem for the many other species that depend on it.
What you may not be aware of is how healthy sea otter populations can help to combat climate change. Kelp forests have high biomass (they are huge!) and extreme productivity (photosynthesis!), and thus are key controlling elements of coastal ecosystems. Photosynthesis takes in carbon dioxide, one of the main greenhouse gases responsible for climate change, and thus can help reduce the carbon in the atmosphere. A recent study in 2012 showed that in a 5.1×1010 m2 area, the effect of sea otter predation on living kelp biomass would mean 4.4-8.7 teragram (one trillion grams) of carbon storage, equal to $205-$408 million on the European Carbon Exchange (at 2012 prices). Healthy sea otter populations mean healthy kelp forests and the ability to store massive amounts of carbon, which may help to reduce the effects of climate change.
Thus sea otters have a strong influence not only on the health of the ecosystem and species that depend on it, but on the carbon cycle and carbon storage, which may impact the effects of climate change. So just remember how important sea otters are to the health of our oceans – they aren’t just another cute, adorable, furry face (thought they are ridiculously cute!).
As fisherman have long understood, schools of yellowfin tuna are often found with pods of dolphins around the tropical regions of the open ocean, particularly with spotted and spinner dolphins. It was previously thought these schools of tuna followed the dolphins in search of food, using the dolphins to locate a food source in an often barren open ocean. This theory, however, is now disputed, with evidence suggesting the tuna actually use the dolphins for means of protection against predators (primarily sharks) with strengths in numbers. Researchers identified when schools of tuna join pods of dolphins, they two species are actually feeding at different depths, different times, and sometimes even different prey.
With tuna so often associated with dolphins, tuna fisherman have used pods of dolphins to locate the schools to catch. Fisherman have transitioned from bait and hook fishing to purse-sein fishing, a method consisting of circling the school of tuna (and often the pods of dolphins) with a net and corralling them onto the boat. This practice can and has led to mass mortalities of dolphin bycatch. With this increase in mortality, many countries have implemented new fishing laws and “Dolphin Safe” food labels for tuna products. These labels indicate the tuna caught in the product were captured with minimal dolphin mortality, such as boats no longer chasing pods or allowing the dolphins to escape the nets before closing them. These labels can only be achieved if dolphin mortality is low according to an onboard observer. With these laws and labels, dolphin mortality from tuna fishing has drastically reduced, and the fishing industry has changed. Fewer boats now target tuna (as part of public outcry and lesser demand), and others use different methods of targeting the tuna, such as the use of Fish Aggregation Devices (FADs) – that is, manmade floating devices that attract small fish that in turn attract the tuna. This just goes to show how important the public’s voice can be; with enough people voicing concern, change can be implemented.
OK, first things first: what on earth is commensalism?! Well, it is a symbiotic relationship where one species provides protection for another less mobile or more vulnerable species. This protection can come in the basic sense (you’re less likely to get eaten if you’re associated with a larger animal) but also in more indirect ways, like for example protection by obtaining food. Here, we’ll go through three examples of commensalism relationships with marine mammals.
1. Whale Lice
Yeees, I know, lice are gross! But these are not true lice: whale lice are actually a type of shrimp (family Cyamidae). They’re also not specifically “whale” lice: they are found on all types of cetacean (whales, dolphins and porpoises). So how are whale lice commensalists? Well, they will eat algae and dead skin cells on the cetacean’s body, and are also afforded a place to live. They are often found around openings (like the eyes or the blowhole), and are particularly prevalent around skin lesions or wounds where there is more bacterial and fungal growth, and also more dead skin cells to snack on! They will also colonize barnacles that are found on larger whales (more on those in a minute).
The other really cool thing about whale lice is that they are often specific to a species, or even sex, of cetacean! That’s right: certain species of whale lice preferentially live on specific whale species, and some of these even show preference for female or male whales. For example, whale lice species found on male sperm whales were different than those found on female sperm whales. Who knew?!
Barnacles are typically only found on larger, slower moving whales – in fact, masses of barnacles on whales can weigh up to 1,000lbs! But how does a stationary barnacle find and attach to a moving whale? Well, the current theory is that when barnacles are in their free-swimming larval stage, they encounter whales feeding at the surface and attach to them (probably by somehow latching on to the whale’s’ skin). They are then able to start creating and building their little shell-homes. Once attached to the whale, the barnacles can stay on for years and years, gaining a great home and plenty of access to food moving through the water column. Just like whale lice, barnacles also show a preference for whale species. So the type of barnacles you might find on a gray whale, for example, would likely be different than the species you would find on a humpback whale.
This is a classic relationship, although one typically thought of between remoras and sharks. Remoras are suckerfish that can grow up to about a foot in length. They attach to the bellies and back of whales and dolphins, often strategically placed around sheltered areas (such as behind the dorsal fin), or near food sources (e.g. blowholes). So what do the remoras eat? Again, dead skin cells are the primary delicacy! But they will also eat parasites and leftover food fragments, so although can be a bit annoying for their kind hosts they actually to ensure the parasite-loads of the whale or dolphin are kept to a minimum.
And so we come full-circle: the whale lice live on the whales, or maybe on the barnacles that live on the whales, and then the remoras come along and help clean up any excess whale lice and other parasites. And they all get a safe, secure home and lots of food while being either completely unobtrusive to their host, or sometimes even doing them a bit of a favor in keeping their skin in tip-top condition.