While on holiday in Greece, we found some strange eggs in the sea. They were composed of a jelly-like substance with an embryo clearly visible (see photo, top right). Whatever laid them was obviously quite big because the eggs formed long strings similar to toad spawn. The eggs pulsated if they were taken out of water. Can anyone identify them?
• These are not actually eggs at all. The picture shows a gelatinous creature called a salp. These pelagic tunicates are often barrel-shaped and at least partially hollow. Believe it or not, they belong to the chordate phylum, the same as humans.
Advertisement
The questioner mentions that the “eggs” pulsated when lifted out of the water. This means they were holding a live salp. If you spotted one in the water – say when snorkelling – you would be able to see that the salp moves by contracting its main body walls and pumping out water. The opaque part in the photograph described as the “embryo” is in fact the salp’s digestive and primitive nervous system. Salps can be free individuals or form strings (see photo, bottom right) – some species even create huge mats or large hollow bodies many metres long which almost replicate their individual body shape on a giant scale.
Salps feed on plankton and are common in most waters, but more so in temperate and equatorial climes. They reproduce very quickly by budding off clones as a direct response to plankton blooms. Often, after plankton blooms have faded, dead or dying salps wash up on beaches or form tidal bands before sinking to the sea floor.
Dave Banks, Wellington, New Zealand
• Salps are pretty cool. Picture a transparent gelatinous barrel, open at both ends, with ring-like muscle bands and a primitive proto spinal cord (known as a notochord). They are a pelagic version of sea squirts, or tunicates, meaning that they spend their whole life high up in the water rather than stuck to the bottom. They move swiftly by jet propulsion.
The “long strings similar to toad spawn” that are described are the colonial part of the life cycle, where a nurse mother clones off many juveniles that remain stuck together while they grow.
They may look like jellyfish, but they are actually members of our phylum – a sort of a strange not-quite-vertebrate-not-quite-invertebrate link in the tree of life.
They are vegetarians, filtering plankton from the water, and are often highly bioluminescent (providing unlimited entertainment for bio-nerds like me). They are also the primary biological indicator that we use in Australia to predict and detect the presence of a type of dangerous jellyfish called an irukandji. So, these gelatinous vegetarian barrel cousins of humans also actually help us to keep our beaches safe.
Lisa-Ann Gershwin, State Marine Stinger Advisor, Brisbane, Queensland, Australia