By Julie Etra—

Salps are not jellyfish

Despite superficially looking a bit like jellyfish due to their jelly-like, translucent bodies, salps are members of the Tunicata, a group of animals also known as sea squirts or tunicates, appropriately named for their tunic-shaped bodies. They are of ancient evolutionary origin. For the biologically curious, one can find an enormous amount of information without much effort regarding these fascinating creatures, so I will just highlight their most interesting characteristics.

Oddly enough, they are more closely related to humans than to jellyfish. While they are pelagic organisms, meaning they live in open water, they can occasionally drift into coastal areas and be washed ashore, where they often break apart. Although they can be solitary, they often form chain-like structures. They also produce circular rings. I see them almost every year on the beaches of Huatulco, usually as part of a broken chain; I have not seen them in open water.

Physiology

Salps are classified in the phylum Chordata and are therefore related to all animals with backbones, including us, Homo sapiens. However, salps themselves do not have backbones. They move through pulsating contractions of bands of muscles, basically an extremely efficient jet propulsion system, sucking in and forcefully pushing out water.

They can reproduce both sexually and asexually. They typically live from a few days to a few months and can grow extremely rapidly, with some species reaching maturity in a remarkably short time.

Habitat and diet

Although widely distributed in equatorial, temperate and cold waters, salps can be particularly abundant in the Southern Ocean around Antarctica, where there are high concentrations of their preferred food, phytoplankton.

They are filter feeders, consuming phytoplankton and other small organisms and particles in a large variety of sizes. Occasionally, small fish seeking refuge from predators are known to swim inside salps.

Adding to their efficiency, when they find an abundant source of food, their populations can increase rapidly through reproduction, including the production of clones that form long chains. With dense phytoplankton blooms and resultant rapid growth, they can form dense, slimy aggregations.

Jellies

We do have numerous species of jellyfish in the bays, locally known as mal agua (bad water). These belong to a completely different group of organisms, with varying sizes, shapes, taxonomy and degrees of toxicity. Some species deliver venom through specialized stinging structures known as nematocysts, which they have in common with sea anemones. The harpoon-like projection occurs when the stinging cells are triggered by contact.

One of the species found along the coast is Stomolophus meleagris, popularly known as the cannonball jellyfish.

Contrary to what I thought I had been told, and to my personal but limited observations (we typically arrive in November and are gone by mid-April), jellyfish along the Oaxacan coast can be more common in warmer water, though their numbers also fluctuate with ocean currents and other environmental conditions.

So, the next time you come across a strange, gelatinous creature washed up on one of Huatulco’s beaches, take a closer look. What appears to be a jellyfish may actually be a salp, a rather remarkable animal with an important role in the ocean’s ecosystem.

Julie Etra is a vegetation restoration
specialist and botanist.