🐋🐠 Following on in the steps of Fossil Friday and Wildlife Wednesday is Marine Monday, where I’ll share marine related finds from along our beautiful coastline. Hidden mainly from view, below the surface of the sea, in the rock pools, under the waves, there is much “going on” underneath.

Today’s post features the Blue Blubber Jelly or Jelly Blubber (Catostylus mosaicus)

A jelly encounter

The sea jelly is made up of 95% water with only 5% being a solid matter, which is why they are lovely to watch in the ocean but then appear blob-like when washed up upon the shore. Jellies are not fish, and therefore no longer referred to as the good old jelly fish, but are sea jellies (or Cnidarians).

They don’t have a backbone (being invertebrate animals), and the general anatomy of jelly’s includes the bell, the tentacles, and the oral arms. Jellies are composed of three layers: an outer layer (epidermis), a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer (the gastrodermis). They have limited control over movement, but can use their muscles (the hydrostatic skeleton) to accomplish movement through a pumping action, contraction-pulsations, of the bell-like body, to propel themselves through the water. (Some species of jellies actively swim most of the time while others are passive much of the time.)

Their habitat is the water column. Drifting at the mercy of the currents they often accumulate in large numbers in sheltered bays and estuaries. (The formation of these ‘blooms’ is a complex process that depends on ocean currents, nutrients, temperature and ambient oxygen concentrations.)

Snorkelling along and finding oneself in a bloom (or in a “smack” which is the term for a group of jellies) is a memorable, exhilarating and gentle experience.

The Blue Blubber jelly is the most commonly seen jelly in Victoria and I’ve encountered good numbers in both Geelong and St Leonards.

An elementary nervous system, a “nerve net”, allows sea jellies to smell, detect light, and respond to other stimuli. The simple digestive cavity of a sea jelly acts as both its stomach and intestine. The nutrients are absorbed using the gastrodermal lining of the gastrovascular cavity. Like all members of the phylum Cnidaria (which anemones and corals belong to), the body parts of a sea jelly radiate from a central axis, and it is this radial symmetry that allows jellies to detect and respond to food or danger from any direction. Pretty cool. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion.

To identify as a Blue Blubber, they will have a noticeable cross (the gastrovascular cavity) visible through the surface of the bell. The bell is rounded, up to 35 cm across, they have eight textured arms (the texture like cauliflower), and each arm has three ‘wings’. They have stinging cells along the arms which help catch prey (being carnivores). Their body, is fleshy, sturdy and fairly opaque. There colour can vary slightly from a creamy white to a bluey-white as produced by the jellies own pigment. (Some sea jellies have symbiotic relationships with algal plant cells that are kept inside their bodies, which create the colour hues. For those jellies, the algae photosynthesise, converting sunlight into energy that can be used by the sea jelly.)

They are either male or female (dioecious). In most jelly species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn. To reproduce, both genders release either sperm or eggs into the surrounding water, where the unprotected eggs are fertilized and mature into new organisms.

The lifespan of sea jellies typically ranges from only a few hours up to 6 x months, with most of the large coastal jelly’s, like this one, living for about 2 to 6 months.

There is an interesting theory associated with the noticeable increase in the numbers of jellies in general around the world. Increased nutrients in the water, ascribed to agricultural run off, climate change, pollution, habitat destruction, trash, chemical waste, and overfishing are suggested for seeing increased numbers of jellies. This is due to; ecosystems in which there are high levels of nutrients, provide nourishment for the small organisms, on which jellyfish feed. In waters where there is eutrophication, low oxygen levels often result, favouring jellies as they thrive in less oxygen-rich water than fish can tolerate.

Plastic bags immediately look like sea jellies when in the sea (and in any water). Sea turtles are their natural predator, and whilst the leatherback sea turtle relies on jellies as their main meal, many marine animals including penguins, birds, and crabs, include jellies in their diets.

Large adult sea jellies are often accompanied by small fish which hide amongst their tentacles for protection which is super cool to see.

The Blue Blubber sea jelly lives in Vic (notably in areas such as Port Philip Bay) NSW and QLD. I photographed and videod these examples at St Leonard’s.

The mini photos are parts of various sea jellies as they appear when washed ashore.

Happy sea jelly day to you 😀

-Rebecca Hosking