Quite a few people know that a Portugese Man o’ War jellyfish, Physalia, is actually a colony of specialised polyps fused
together, but we didn’t realise that each element is called a "person" by marine biologists. This and many other
fascinating aspects of marine biology were divulged by an old friend of Coastwise, Tegwyn Harris, who is a retired Exeter
University biology lecturer, in a talk on plankton.
I certainly didn’t know that jellyfish are macro-plankton, but given that the label "plankton" is simply derived from the
Greek for "wanderer" this does seems reasonable. The Portugese Man o’ War is not quite completely at the mercy of sea
currents, as it, in common with some other jellyfish such as the Velalla, is able to exert some control over its direction. The float which keeps it at or near the surface can catch the wind, and is inflated with a surprising mix of gases –
nitrogen and carbon dioxide seem reasonable, but there’s carbon monoxide, argon and neon as well ! The gas production
allows for extra weight of fish that the siphonophore (the generic name for jellyfish) has caught with its poison tentacles
to eat.
Some jellyfish are preyed on by sea slugs such as the Glaucus Atlanticus which actually uses the stinging cells from the
jellyfish for its own use and defence – and to think that humans had the monopoly of recycling !
At the other end of the size range, marine gastropod species such as Lucopleura have sophisticated mechanisms for
selectively feeding on nanoplankton, with a fallback method for when their feeding screens become blocked. At this size
range, a scanning electron microscope is necessary to see the plankton and their structure, but biologists in Victorian
times deduced the existence of nanoplankton by realising that very small animals – too small for them to see – had to pass
through the feeding screens.
Tegwyn also talked through some of the commoner microplankton that Coastwise members have been seeing through their
microscopes, such as the Serratium Tripos and the fascinating Massilaria groups, which adjusts their surface area
reflecting the local salinity by each element sliding relative to its partners.
He’s definitely booked his return ticket to Barnstaple !