Blue Planet II – Week 5: Green Seas

This episode introduces us to the source of 95% of the energy in our oceans through the process of photosynthesis, here sunlight provides energy to plants which then acts as a food source for the rest of the food web. These areas high in concentrations of plant life are abundant in life and are also ridiculously competitive against different organisms, observed through Garibaldi fish that act as farmers protecting their fields of short-turf seaweed against the relentless of sea urchins. The invasion of sea urchins is so intense that large expanses of the kelp forest have been lost over the years, known as “urchin barrens”. This was also partly due to the large-scale hunting of sea otters, natural predators of sea urchins, who need to eat 1/3rd of their body weight each day to meet their energy upkeep in the cool Pacific waters off the North American coastline. However, in the past few decades sea otter populations have rebounded (mainly due to their protection against hunting), allowing for the recovery of many kelp forest expanses.

In warmer waters, off the coast of western Australia, we experience a different type of green sea, dominated by shallow waters and sea grass. These patches of sea grass are apparently 35 times greater at absorbing carbon dioxide in comparison to an equivalent area in the Amazon rainforest. Such patches are excellent feeding grounds for green turtles, providing a readily-available snack for the creatures. The green turtles aren’t the only consumers here though, tiger sharks up to 5m in length stalk the turtles, picking off the weak and preventing the turtles to graze in different sections of the sea grass meadows.

Once a year the same patch of sea grass is invaded by an invasion of peculiar creatures from the deep, spider crabs. These alien-looking crabs ascend from the cold, deep waters on the first full moon of winter, piling onto each other in numbers of hundreds of thousands. Initially, I thought that this was for a mass-spawning, however we are soon addressed with a much creepier display. To grow, spider crabs need to shed their other shell. This process exposes a new soft body which takes days to harden sufficiently, therefore these crabs accumulate in such large swarms for protection. Throughout their time in the shallows they seek protection in numbers against the giant 4-metre long stingrays which pick out the softer crabs whose limbs have not hardened enough to quickly escape. Once all of the crabs have moulted they once again descend into the deep waters for another year, leaving behind a graveyard of discarded shells.

Off the coast of South Africa are extensive kelp forests, we observe how a local man leads a team of experts around the areas, exhibiting the differing mannerisms that are displayed by the common octopus. These cunning organisms stalk their prey using their ability to blend in with the surrounding kelp forests and hidden crevices. The octopus shares its territory with many other predators, including up to 100 different species of shark. Its main predator, the pyjama shark, uses its small size and incredible strength to seek out any octopi seeking refuge. However, one octopus managed to place its tentacles in the shark’s gills, preventing it from being able to breathe. This clever tactic was something that has not been filmed before. Another escape tactic in the octopus’ arsenal was the ability to cover itself in an armour made from shells, confusing the shark for long enough to make a crafty getaway behind a cloud of ink.

The final scene involves a feeding frenzy in Monterey Bay, California, an algal bloom provides a feast for plankton-feeding fish. Billions of anchovies are drawn to this, in turn causing thousands of dolphins, sea lions and humpback whales to hone in on the super-concentrated source of food.

Next episode we explore the communities which reside along the coasts of our oceans, including the ugly but colossal elephant seals.


James Deed

Image: BBC