As many of you may know already, crustaceans are classified within the phylum, Arthopoda. They share this classification with 3 other groups, which includes hexapods, myriapods, and chelicerates. In general, hexapods refer to insects; myriapods refer to millipedes and centipedes, and chelicerates refer to horseshoe crabs and arachnids.
Qualities of Arthropods
Though arthropods are the most diverse group of animals with regards to number of species, they still retain some common characteristics that are listed up on the screen. These include the presence of rigid exoskeletons that provide the animal with support for walking and some protection against predators, as well as segmented bodies, jointed limbs, and muscle attachment inside the exoskeleton that allow the animal to perform complex movements.
As a taxon, crustaceans are a diverse group of approximately 52,000 species featuring familiar animals such as crabs, shrimp, lobsters, krill, and barnacles, some of which are depicted here. Like other arthropods, crustaceans have segmented bodies, jointed limbs, and an exoskeleton, which they must molt in order to grow. Yet, on the other hand, they are distinguishable from other arthropods in three main ways. These include a nauplius larval stage, biramous appendages, and a cephalon. Unlike other arthropods, most crustaceans go through a series of larval stages, the first being the nauplius larva, in which only a few limbs are present, near the front of the body. Other limbs do not show up until later in development. Secondly, crustaceans exhibit limbs or appendages that are split in two, usually as two segmented branches, one internal (known as an endopod) and one external (known as an exopod); hence, two-part or biramous. Lastly, crustaceans have a unique five-segmented head (known as a cephalon), followed by a long trunk typically regionalized into a thorax and abdomen.
And now I’m going to isolate two particular crustacean groups. The first is copepods. Copepods are a group of small crustaceans that are found in the sea as well as nearly every freshwater habitat. Some species are planktonic (drifting in sea waters), some are benthic (living on the ocean floor), and some are continental that live in other wet terrestrial places, such as swamps, bogs and springs. However, one type holds a particular interest for humans. The marine benthic copepod, Robertsonia propinqua, is currently being studied as a bioindicator of sediment-associated contaminants. In the lab, scientists at Lincoln University in New Zealand are determining the effect that particular contaminants such as atrazine and zinc sulfate have on its life cycle by injecting them into the copepod and observing the results.
The second crustacean group I would like to focus on is barnacles. Barnacles are a group of arthropods that are exclusively marine and tend to live in shallow, tidal waters. They are sessile (non-motile) filter feeders that obtain food by straining and suspending food particles from the water. At first glance, it might be hard to believe that barnacles are classified as arthropods. Though segmentation is usually indistinct, their bodies do possess unequal divisions of a head, thorax, and abdomen. Adult barnacles have few appendages on the head, with only a single pair of antennae. They also have six pairs of thoracic limbs, referred to as “cirri”, which are long, feathery appendages that are used to filter feed. Barnacles were originally thought be mollusks because of their apparent possession of a shell, but they are actually crustaceans with their nearest relatives being shrimp and lobsters. The barnacles depicted on the slide, Chamaesipho tasmanica, are known as honeycomb barnacles because they form dense covers of hundreds or even thousands of barnacles over rock surfaces in tidal waters.