//
you're reading...

Ecology

The elephant in the oxygen minimum zone

Paper: Naito Y, Costa DP, Adachi T, et al. Oxygen minimum zone: An important oceanographic habitat for deep-diving northern elephant seals, Mirounga angustirostris. Ecol Evol. 2017;00:1–12. 

Introduction

Much like people and other animals that live on land, sea animals depend on oxygen to power their bodies. Their gills filter out the oxygen in the water much like our lungs filter out the oxygen in the air. Much like on land, too, there are places where there isn’t as much oxygen – places like the top of Mt. Everest, for example, have much less oxygen than the city of Boston, which is at sea level. The ocean’s equivalent of a low-oxygen area is called the oxygen minimum zone (Figure 1).

Figure 1: A depth map showing the oxygen minimum layer. The y-axis goes from the surface (top) down, and the line shows the concentration of oxygen in the water. Source: UCSD

Much like a person hiking up Mt. Everest, though, it takes a lot of effort for an animal to survive in the oxygen minimum zone. There isn’t as much oxygen for it to survive, so many animals there have to slow down their metabolism – similar to a bear going into hibernation. They become sluggish and can’t evade predators well, but the main advantage to being in the OMZ is that there are far fewer predators. Faced with a tradeoff between slowing down their metabolism and being around more predators, many animals choose the former.

That said, if predators are able to “brave” the low oxygen conditions of the OMZ, there are lethargic fish just waiting to be snapped up. However, we don’t know much about foraging in this zone because it’s hard to study! The authors of this paper are pioneering a relatively new method of observing this behavior in elephant seals that will hopefully shed some light on the darkness of the OMZ.

Elephant seals (Figure 2) are one of the animals that we know makes this pilgrimage to the OMZ – some of them are known to dive more than 800+ meters down (2400 ft) to find their prey. What we don’t know, though, is what their feeding rate is on these dives. Knowing the feeding rate is key to understanding the cost-benefit of these deep dives. The seal has to take on a tremendous energetic cost to dive that deep, so there must be a great benefit associated with foraging there.

Figure 2: Left: A male elephant seal, so called because the snout looks like an elephant’s trunk. Right: A human feeding an elephant seal to show scale. Source: Wikimedia Commons, EPA.

 

 

 

 

 

 

 

 

 

Methods

To investigate, the researchers attached many different (removable!) instruments to 14 elephant seals. Each seal carried a jaw motion recorder so that the researchers could determine feeding attacks and a satellite transmitter so they could track the seal’s movements. The satellite transmitter was also able to record temperature and depth. Additionally, two of the 14 seals also carried video recorders that started recording once the seal performed a feeding head strike motion – that way, the researchers could actually see what the seals were attacking.

Figure 3: An example of an elephant seal wearing a satellite tag. Source: Science Friday

Results and Importance

The researchers learned a lot about the foraging behavior of these seals from monitoring them. First of all, they learned that where seals were hunting depending on the time of day. They dove deeper during the daytime hours than during the nighttime hours, which corresponds well with the daily vertical migration of their prey (Figure 4).

Figure 4: Graph showing the dive of one seal over a period of 48 hours. The colored dots represent the number of Jaw-Motion Events (aka feeding strikes) that occurred at that time. The green box shows that this seal dove deep into the OMZ a few times during the daytime hours to forage. Source: Naito et al. 2017.

 

 

 

 

 

 

 

 

 

 

They also found that larger seals were more efficient feeders. It makes sense that the largest seals would need the most food in order to survive, but that doesn’t necessarily mean that those seals were attacking prey the most. In fact, as the body mass of the seal increased, the number of Jaw Motion Events (aka feeding strikes) decreased (Figure 5). That suggests that these larger seals are more efficient feeders – they may be feeding on larger prey so they get a larger “bang for their buck”. In contrast, smaller seals may be feeding more often on smaller prey items.

Figure 5: Larger seals are taking almost 1000 bites less per day than smaller seals! Source: Naito et al. 2017.

 

 

 

 

 

 

 

 

 

The video records from the deep diving seal revealed that the fish that were being attacked were not showing much of an escape response. Even when the seal was about to attack, the fish kept their dorsal side down, which is a sign of inactivity (Figure 6).

Figure 6: Video frames that show a fish not moving away before a seal attack. The bottom semi-circle is the seal’s snout. Source: Naito et al. 2017.

These fish are suppressing their metabolism to cope with the low oxygen in the OMZ, which is a good strategy to avoid predators in the upper zone of the ocean, but backfires when they come into contact with these deep divers. Clearly, being able to dive deeper into the OMZ to capture these sluggish fish is an advantage to these larger seals, and may be driving evolution in this species. As oxygen minimum zones expand due to climate change, seals and other animals that are able to tolerate these conditions will be rewarded with fuller bellies and the opportunity to pass on their genes.

Discussion

No comments yet.

Post a Comment

Instagram

  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 4 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com