In an earlier post, I mentioned how the spermaceti organ and junk may help sperm whales when they dive deep into the ocean. The main reason for their impressive dives is their diet of giant squids that live around the depths of 1,000m. Bring mammals, sperm whales would need to breathe and not be able to hang around at 1,000m, yet other critters can survive (and even thrive) at that depth and beyond.
What is the deep sea?
Before we can discuss the animals that live in these parts of the oceans, we need to understand what is meant by “the deep sea”.
The ocean can be divided into 5 zones, similar to how we have different layers of the rainforest. Beyond the continental shelf, the ocean depth starts to go beyond 200m and hardly any sunlight penetrates beyond this depth. Along with this loss of light, temperature also drops with depth, and the surrounding pressure increases. At 4,000m, the water temperature is near to freezing and sunlight is non-existent. It is not surprising that this bathypelagic zone is also known as the midnight zone. Sure sounds like a forbidding place to me. I can’t even imagine how life may be possible in the zones beyond that (abyssopelagic and hadal zones).
Life in the Midnight Zone
Life in the midnight zone can be challenging. Unlike the zones above, there is a total absence of sunlight, and any light source in this zone would have to be produced by creatures living here. The lack of sunlight also means that photosynthesis cannot take place. Most of the critters living here would have to be scavengers or carnivores.
The water pressure at these depths (1,000-4,000m) would be 100-400 times that of sea level. This higher pressure means that swim bladders (usually filled with air) would not be very useful, and the fish here would make use of fats (lipids) in their body to maintain neutral buoyancy.
Given the conditions of the dark depths, it is not surprising that quite a few of these fish and critters look like they were spawned on some alien world or in some nightmare. Their bizarre features are actually ways in which they have adapted so that they can survive and reproduce.
The Deep Sea Anglerfish
The one deep sea fish that has caught my attention since I was child is the deep sea anglerfish. Call me weird but the deep sea angler has always looked intriguing to me, or what the Japanese might describe as kimo-kawaii or “creepy cute”.
So what’s so interesting about this fish? Firstly, the lure on its head is glows, aiding it in catching prey, pretty much like the human anglers that it got its name from. The glow comes from the bioluminescent bacteria that live in it. And guess what! Only the female anglerfish have this specially modified dorsal spine. (Must come in handy if they fancy any reading ;p)
The male anglerfish are much smaller, and their main life purpose is to find a female and live off her. Literally. The males have evolved to be parasitic, and when they come across a female, he will latch on with his teeth. Gradually, the male is physically absorbed by the female, until only his useful bits (the gonads) are left…. What’s more, the female anglerfish can repeat this process numerous times, building up her personal sperm bank over her lifespan! For those of you who just can’t visualise this process, The Oatmeal has a lovely comic strip summarising it.
And that’s one way for the deep sea denizens to be rolling in the deep…..
(1) Bathypelagic zone, The Encyclopedia of Earth: http://www.eoearth.org/view/article/150442/
(2) The Deep Sea, Smithsonian National Museum of Natural History http://ocean.si.edu/deep-sea