Marine Creatures of the Northern Gulf of Mexico – Meroplankton

What are meroplankton? How do they differ from regular plankton?

In the plankton world there are those that are plankton (drifters) their whole lives – holoplankton – and those who are plankton for only part of their lives – meroplankton.  Most know the meroplankton as larva – the early stages of large creatures like fish, crabs, and shrimp.  When you pull a plankton net through the waters of the Gulf of Mexico you will collect a lot of meroplankton – and yes… they are food for the plankton feeders just as the other forms of plankton are.  Like most of the holoplankton, most are microscopic and swim through the water using cilia or flagella.  Like copepods, they are multicellular and are considered true animals.  Here are a few that you could find in a plankton sample…

Planula larva

These are the larva of jellyfish, sea anemones, and corals.  They are ciliated cells that move through the water column until they metamorphose into the adult forms.  Most will settle out on hard substrate on the seafloor and develop into a flower-like structure called a polyp.  Some grow into adult polyps – like sea anemones and corals – while others will go through a second stage and become swimming medusa – the jellyfish.

Trochophore larva

This is another ciliated larval form that is the first stage of some mollusk, annelid worms, and nemertean worms.

Veliger larva

This is a ciliated larval form of several mollusk.  Those that go through the veliger stage begin as trochophores.  Some go through the trochophore stage while in the egg, others hatch and go through the trochophore before metamorphosizing into the veliger.  The veliger stage will develop the characteristic mollusk shell, and many will develop a foot which can be used in locomotion on the seafloor searching for suitable habitat.

Nauplius larva

This is the first stage of a crustacean.  It is very hard to tell which crustacean the nauplius will become but they do resemble crustaceans with segmented body parts and an exoskeleton.  Most crustaceans will molt into the zoea stage, and some then into a megalops stage, before becoming the adult.

Bipinnaria larva

Another ciliated larval form that will eventually become an echinoderm – starfish, sea urchin, sand dollar.

Ichthyoplankton

Most of the fishes in the northern Gulf of Mexico begin life as meroplankton as well – these are called “ichthyoplankton” and are quite abundant in a plankton sample.  Even large fish, such billfish and swordfish, begin life at this stage.

The first nine post in this series have been about microscopic creatures found in the northern Gulf.  It goes without saying that there are literally thousands of other forms of plankton we did not mention.  It is also important to mention how important these creatures are to the health of the Gulf and why they were as much of a concern during the oil spill as were dolphins, sea turtles, and sea birds.  As the plankton go… so goes the Gulf.  We will now turn our attention to the larger creatures – ones you do not need a microscope to see.