Introduction to Crustaceans
The Latin root, crustaceus, “having a crust or shell” really doesn’t entirely narrow it down to crustaceans. They belong to the phylum Arthropoda, as do insects, arachnids, and many other groups; all arthropods have hard exoskeletons or shells, segmented bodies, and jointed limbs. Crustaceans are usually distinguishable from the other arthropods in several important ways, mainly:
- Biramous appendages. Most crustaceans have appendages or limbs that are split into two, usually segmented, branches. Both branches originate on the same proximal segment.
- Larvae. Early in development, 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 on the body; crustaceans add their more posterior limbs as they grow and develop further. The nauplius larva is unique to Crustacea.
- Eyes. The early larval stages of crustaceans have a single, simple, median eye composed of three similar, closely opposed parts. This larval eye, or “naupliar eye,” often disappears later in development, but on some crustaceans (e.g., the branchiopod Triops) it is retained even after the adult compound eyes have developed. In all copepod crustaceans, this larval eye is retained throughout their development as the only eye, although the three similar parts may separate and each become associated with their own cuticular lens. In other crustaceans that retain the larval eye into adulthood, up to seven optical units may develop.
- Labrum. Crustaceans have a lobe-like structure called the labrum anterior to the mouth that partially encloses it.
- Head. Crustaceans are distinguished by a five-segmented head (cephalon), followed by a long trunk typically regionalized into a thorax and abdomen.
- “Baby teeth.” Most crustaceans in their early larval stages chew their food with a unique structure called a naupliar arthrite, which is on the second antenna. This chewing tool is lost later in development, and chewing is taken over by the mandibular gnathobase.