Flatworms and Parasites
Flatworms are not true worms but a more primitive form of life, a step up from the sea anemones. Typically carnivorous the free-living forms evert a stomach to cover and digest prey. Common in freshwater they are abundant but not fully understood in the marine environment. Many are very small and have not even been given names. Most marine flatworms are parasitic forms with complex life cycles. Those commonly found in the plankton are the larval forms called cercaria.
This is a very specialised topic and although I have tried to ID both flatworms and cercaria it is very difficult.
This is likely to be a late juvenile Rhabditophoran flatworm. Note the surface is covered in beating cilia, like a halo, a feature of larval forms. Approximately 400 microns in length it was caught in mid-November 2023 sample.
Above, probably a Leptoplanid species, 3mm in length.
Below, a possible larval form about 200 microns.
Parasitic Cercaria larval types
Marine flukes are the adult form typically found in sea birds. It is thought that at least 95% of gulls have flukes producing eggs that pass out in the faeces to be consumed, most likely, by molluscs such as periwinkles. Larvae live in this second host before leaving as a cercaria larva swimming through the plankton looking for a third host to parasitise. When these are consumed by gulls the cycle is complete, back as an adult. At least a few cercaria are found in most samples although autumn is when the density is highest.
The third host can be fish but the small cercaria of Opechona have been found on a wide range of organisms including Obelia medusa and Arrow Worms. Opechona is one of many species that have bristles and have been placed in a group called Trichocercous Cercariae (from The Trematoda by Ben Dawes)
A large cercaria about 400 microns
Trichocercous Cercariae possibly of the species Opechona
Many cercaria are covered in long hairs that keep the larva buoyant. They are very active wrigglers and fast swimmers (tricky to photograph).
Parasitic Crustaceans
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A Bopyrid isopod (Cryptoniscus larva) that is ectoparasitic on copepods
Hyperia galba, an amphipod that parasitises jellyfish
The stage 3 zuphea larva of the isopod Gnathia approximately 3 mm in length. In a complex life cycle this larva is an ectoparasite on fish, sucking blood. Click here for more info in the crustacea section.
Caligus is a parasitic copepod on the outside of fish. The front section, cephalothorax, has adapted to become a sucker with sides curved in to create a seal. Their attachment to the host is only temporary and that is why they occur in the plankton as they move between fish. Where most copepods have 5 or 6 nauplii stages these only have 2. 12 species are known in UK waters and they are quite large and visible at 5mm in length or more.
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More on copepods here