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deep sea

How do skates of the deep survive the crushing pressure?

Background Definitions

Skates: The often-forgotten cousins of sharks and rays, sometimes called flat sharks, particularly on #flatsharkfridays.

Skates, just like sharks and rays, belong to a group of fishes termed elasmobranchs that lack real calcified bone and instead are composed of cartilage.  Skates are often mistaken as rays given their flattened body plan, but from a biological standpoint they are very different groups of fish. First, rays give birth to live offspring after months to years of gestation, while skates lay egg capsules that develop for part or all of their time on the ocean floor. Furthermore, ray species are known for their venomous barbs at the base of the tail used for protection while skates instead tend to have prickly thorns on their back to ward off predators (Figure 1). Besides a few other differences such as body and fin shape, reproduction and absence/presence of a barb are the easiest defining factors.

Figure 1: Noticeable differences between a ray and skate.  (Image source: Seacoast Science Center, NH)

Osmoregulation: A wonderful process that your kidneys perform for you 24/7 that you’ve probably never heard of.

Osmoregulation is simply the process by which bacteria, plants, and animals alike balance water and electrolytes (salts like sodium, potassium, chlorine, etc) within the organism. Without these balances, an organism can die from too much or too little osmotic pressure in its cells and tissues. In humans for example, our kidneys are great at determining how much water to retain or excrete. Fish also have kidneys that function similarly, however because fish live in water, they must also account for the issue of constant water intake through the gills, mouth, and skin as well as tons of salt if living in a marine environment.

Furthermore, sharks, rays and skates have developed particularly interesting mechanisms to combat the environment in which they live. Typically, organisms that produce urea as a waste product will get rid of it through urination, however these fish when living in salty water will actively retain urea in a series of steps to maintain a proper water and salt balance (Figure 2). But urea degrades tissues (hence why most organisms aim to get rid of it) so these fish also possess a molecule called trimethylamine N-oxide (TMAO) which protects the body from the retained urea. TMAO has been hypothesized to have a second function, particularly in deep water fish and crustaceans, where the substance helps to stabilize proteins that can be negatively impacted by the crushing pressure of living at the bottom of the deep ocean. This hypothesis, called the piezolyte hypothesis is where the work of Yancey et al. 2018 comes in.

Figure 2: General inputs and outputs of elasmobranch osmoregulation. (Image source: Chris_huh via Wikimedia Commons)

The Study

In Yancey et al. 2018, the authors wanted to assess if the ratio of urea to TMAO as well as other osmolytes changed with depth in the Arctic skate (Amblyraja hyperborea), a cold-water species with a depth range from 300-2,500 meters. To give some context, the pressure at 2,500 meters depth is approximately 3,700 pounds per square inch! For the study, specimens were caught at depths ranging from 500-1515 meters in the Canadian Beaufort Strait. Water conditions across the depths were relatively constant for temperature, salinity, and dissolved oxygen concentration, isolating depth as the likely factor for any observed differences. In the lab, scientists isolated the white muscle tissue from caught individuals and tested for levels of TMAO, urea, and other osmolytes also used to balance water in the body (e.g. creatine, amino acids). The results showed that indeed, urea decreased with depth while TMAO increased (Figure 3). Additionally, the group also found that TMAO replaced the more minor osmolytes, further strengthening support for the piezolyte hypothesis. This was the first definitive study to show that within a skate species TMAO does increase with depth, lending support to the idea that this substance helps these fish cope with the high pressure of deep water.

FIgure 3: (A) The changes in urea and TMAO as depth increases. (B) The change in the ratio of urea to TMAO as depth increases (Adapted from Yancey et al. 2018)

It is still not completely understood why elasmobranchs use this urea/TMAO combination for osmoregulation, particularly for species that do not experience high pressures (i.e. shallow water dwellers) that impact protein structure and function. Perhaps it is the cheaper option energetically for these fishes. Furthermore, the scientists in this study cannot be sure if the individuals sampled had been living in the deep for a long period and had therefore adapted to this pressure. We cannot definitively say at this time if a skate that moved from shallow to deep water could quickly change their urea and TMAO amounts accordingly. More work is sure to come in this area but for now we have some cool evidence that these fish have a unique answer to the everyday problems of living in the deep.

 

 

 

 

Reference:

Yancey PH, Speers-Roesch B, Atchinson S, Reist JD, Majewski AR, Treberg JR (2018). Osmolyte Adjustments as a Pressure Adaptation in Deep-Sea Chondrichthyan Fishes: An Intraspecific Test in Arctic Skates (Amblyraja hyperborea) along a Depth Gradient. Physiological and Biochemical Zoology, 91(2): 788-796.

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