Fish Larvae Dissection

While working on plankton with Dave, at the MBA labs, we were shown a collection of tiny larval fish, a mixture of pilchards and sprat, that had been previously captured and preserved in alcohol. To find out more about the lives of the fish in the sea, we need to understand the behaviour they show early in their lives. This includes what they eat. With a larger fish, this is as simple as catching it and recording the debris that falls out of their stomachs. On a larval fish, however, a dissection is not so easy, requiring a microscope, needles, steady hands and a lot of patience.

The guts are hanging out of this microscopic fish.

When the fish first go into the alcohol, they will often vomit up the contents of their bellies, leaving nothing behind for us to find except for lines of tubed intestine. Luckily, we couldn’t really spill the fish’s guts as we had never done it before, and it was harder than you might think. Dave himself has done thousands of such dissections and can now be considered a master. Looking under the microscope, I savagely ripped apart three tiny fish in search of the plankton that they were supposed to eat, finding it easier to take apart the smaller, squatter fish as their innards were visible from outside the fish, instead of stretched across the longer larvae. Eventually I managed to produce a small skeleton from one of the fish, the calcified remains of phytoplankton called cocoliths, the same type that form the chalk downs near my house, and the white Dover cliffs.

It was an interesting and challenging task, taking me several tries to yield even one measly plankton corpse, but with time and experience, people can find out what species these little fish consume, and tell us more about the miniature ecosystem that we can’t see.

Plymouth Plankton Study

The wonderful thing about studying plankton is that they are not a single species, but rather the collection of hundreds of creatures too small to move any great distance without the assistance of ocean currents. It’s like looking at a tiny ecosystem that exists in every cubic meter of the sea, comprising of plant-like phytoplankton, tiny hunting zooplankton and larvae of animals such as crabs, fish and bivalves. Every species is wildly different, with each tiny body under the microscope having a unique lifestyle.

Resident plankton expert of the MBA, Dave, took us out to observe the plankton life living in the Plymouth Harbour, just a stones throw away from the labs. Taking a fine, cone-shaped net attached to a high tech jam jar, he filtered a selection of plankton straight from the water. He only walked about 20m with the net in the water, and it wasn’t at all deep, but the wide variety of life we found under the microscope was astonishing. Even knowing that plankton were a diverse form of life, with around 5000 species in Plymouth alone, it was shocking to see the variety of each tiny lifeform.

High quality net, and an even higher quality jam jar.

Some species of plankton have extremely complex lifecycles, in particular the species of meroplankton, which are only planktonic for the first part of their lives, eventually turning into the larger sea species everybody commonly enjoys in their rock pools. The barnacle is an excellent example of this, living in two planktonic stages, a free-swimming nauplis which moults around 5 times before becoming a non-feeding cyprid which will use its oil energy reserves to burrow into an area of rock, ready to take root and become a sedentary adult. Looking down the microscope we saw such barnacle cyprids, comb jellyfish, which aren’t the same as true jellyfish, water fleas, algae and tiny fish.

It was extremely interesting to see what microscopic life was living in every cubic inch of water that covers the earth, a truly amazing prospect considering the size of our oceans. Countless billions of tiny organisms live around us, too small to see, and all too often out of mind.