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 Fish Survey

Having spent a day surveying oysters on my MBA work experience week, my group and I went out to the Plymouth coast to look at some of the more charismatic denizens of the ocean. Where the estuary mouth meets the south coast, there is a remarkable selection of both freshwater and saltwater aquatic life living in the shallow bracken beaches. These areas are devoid of larger predators that would happily eat smaller marine life, and so make fantastic nursery grounds for young fish and crustaceans hiding in the sand. Taking our push and sweep nets in hand, we waded into the crystal waters to stir up fish life living under the surface.

We used two kinds of net to scour the estuary bed, large push nets which stirred up the sediment and captured the creatures that were revealed and smaller, finer sweep nets which could be used the capture smaller fish in clumps of seaweed or small rock formations. Spreading out and searching, we found a wide selection of flatfish in the knee deep water, as well as a wide selection of crabs, shrimp, sand eels, and, much to our delight, a large pipefish from among the short greenery. The flatfish were identified as a variety of young flounder and plaice, which look extremely similar apart from the small sharp spikes that can be felt on the sides of the flounder, setting it apart from the smooth plaice. We placed the animals into little trays while we collected, much like when I went pond thrashing with Matthew Smith, then chased the slippery customers about with our hands, trying to hold the still for measurements. The lesser pipefish, with it’s winding serpentine body and long nose was an immediate favourite – our catch of the day.

A lesser pipefish, held captive in a temporary tank, awaiting measurement.

We then moved onto the ocean itself, stretching out for mile upon mile of open blue. Ordinary nets would be of little use here, we’d be up to our necks in water before we saw anything, and so we brought out a huge weighted net. Three of the group brought the net out into the ocean, while the rest of us kept the net on the sea floor and near the shore, creating a semicircle of water, from which no fish could hope to escape from. Dragging the net in, we scrambled to seize the myriad of flatfish, such as flounder, turbot, plaice and brill, sandsmelts, baby sand eels and gobi.

It was a brilliant time out on the beach, and I learnt a huge amount about the fish species living around Plymouth. I feel sorry for all of the people on the beach building sandcastles and taking in the sun, instead of having such a good time with the marine life all around them.