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Biology 122L – Invertebrate zoology lab Crustacean diversity lab guide Author: Allison J. Gong Figure source: Brusca and Brusca, 2003. Invertebrates, second edition. Sinauer Associates, Inc.
Introduction
As you have probably realized, the crustaceans are an enormously successful group. They are the dominant arthropods in aquatic habitats, and occur terrestrially as well; the ubiquitous "pillbugs" that many of us played with as children are, in fact, crustaceans. The diversity of crustaceans, manifest as morphological and ecological heterogeneity, makes it difficult to tease out the evolutionary relationships of various crustacean groups, and the taxonomy of crustaceans is by no means cut and dry. In today's lab you will observe living specimens from some of the more conspicuous crustacean taxa. Please do not forget that these animals represent merely a tiny fraction of crustacean diversity.
General crustacean morphology
Crustacean bodies consist of at least two tagma, the head and thorax (= pereon), and many also include a third tagma, the abdomen (= pleon). Appendages associated with the head include 2 pairs of antennae and various feeding structures (1 pair of mandibles and 2 pairs of maxillae), for a total of 5 pairs of appendages. Thoracic appendages, or pereopods, are generally used for locomotion, although in some crustaceans the first few pairs may be turned forward and used for feeding. The abdominal appendages, or pleopods, are often termed "swimmerets," giving you some idea of their function. Female crustaceans also brood fertilized eggs and embryos on their pleopods. This generalized morphology is best exemplified by the malacostracans, so we will begin our observations with this group. Note that your textbook covers many taxa that I have not mentioned in lecture; please skim through this information, but don't lose any sleep trying to memorize everything about them.
Class Malacostraca
Roughly half (~23,000) of the known crustacean species, and most of the easily recognized ones, are malacostracans. This class contains ecologically and commercially animals such as crabs, lobsters, shrimps, and krill, distributed in several orders and suborders. As you observe representatives of this class, keep in mind the generalized malacostracan body plan (remember the "formula" of 8 thoracic + 6 abdominal segments?) and try to discern how it has been modified in various taxa.
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Order DECAPODA. In decapod malacostracans, the first 3 thoracic segments are fused with the 5 head segments in a tagma called a cephalothorax. The cephalothorax is covered by an unsegmented fold of the exoskeleton called the carapace; dorsally, the carapace is fused to all of the thoracic segments. The carapace encloses a branchial chamber into which the gills hang. The thoracic appendages fall into two distinct functional groups: the first 3 pairs (1st, 2nd, and 3rd maxillipeds) are turned forward and used for feeding, and the last 5 pairs are used for locomotion. These five pairs of walking legs give the group the name Decapoda. The first pair of thoracic walking legs is often enlarged and modified into a cheliped, or claw. The major taxa within the Decapoda are the Anomura, hermit crabs and others, and the Brachyura, the so-called true crabs. These taxa can be considered suborders or infraorders, depending on the source you consult, demonstrating the difficulty of modern crustacean taxonomy. Your textbook (Ruppert et al, 2004) presents the Anomura as a transitional stage between ancestral swimming (= natant) crustaceans and the walking (= reptant) forms. Can you see characteristics of both natant and reptant forms in hermit crabs? Anomurans can be distinguished from brachyurans by counting the number of thoracic walking legs: although they are still considered decapods, anomuran crabs have only 4 pairs of walking legs (the 5th pair is vestigial). Hermits comprise most of the anomurans, and the locally abundant mole crabs, Emerita analoga and Blepharipoda occidentalis, are also anomurans. See (C)-(H) of Figure 16.11 below. Hermit crabs inhabit the abandoned shells of gastropods, and their abdomen has been modified to curl around the columella of the shell. As the crab grows, it must periodically find a larger snail shell; as shells can be of limited supply, hermit crabs often compete pugnaciously for ideal homes. Mole crabs live burrowed in sand on wave-swept beaches, with the head facing seaward. They use their enlarged 2nd antennae to filter food particles from receding waves. They are a favorite food source for seabirds such as sanderlings and sandpipers, and you can often find their discarded empty carapaces littering sandy beaches as the birds forage.
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The true crabs, or brachyurans, are highly adapted to a benthic walking lifestyle. They are dorso- ventrally flattened and compressed along the anterior-posterior axis, resulting in a body that is often wider than it is long. The abdomen is reduced and recurved under the cephalothorax, and cannot be seen unless you hold the crab upside down. Female crabs use their pleopods to hold onto embryos and early stage larvae, and have a much broader abdomen than males. This sexual dimorphism makes it easy to sex crabs. See (N) of Figure 16.10 below. In male brachyurans, the pleopods are reduced to two anterior pairs used for copulation. Brachyurans can scuttle sideways very quickly, but walk forward only slowly. The first pair of thoracic walking legs is modified into a large cheliped and is not usually used in locomotion. Chelipeds are used in feeding and defense, and may be quite large. The heavy claws and overall robust nature of the brachyuran body contribute to the success of these crabs in the benthos; with a few exceptions, adult brachyurans do not swim.
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Superorder PERACARIDA. The peracarids "pouch shrimps" are usually smaller than decapods and includes many of the more familiar shrimplike animals. The major peracarid characteristic is a ventral marsupium, or broodpouch on the thorax of females. The female carries her fertilized eggs in the marsupium until they hatch as post-larvae. Although there are many orders of peracarids, we will concentrate on two of the most easily available orders, the Isopoda and the Amphipoda. Animals in these orders can be distinguished visually by the general form of the body: amphipods are laterally flattened and isopods are dorso-ventrally flattened. Amphipods also have some morphological and functional differentiation among appendages, while isopods' legs are generally all the same. Looking more closely at amphipods, you will see that they have sessile eyes, opposed to the stalked eyes of crabs and shrimps. The gammarid amphipods (Suborder Gammaridea) tend to have characteristically curved bodies and a waxy exoskeleton that makes the animals stick to the surface tension of water. The differentiation of body segments and appendages is more apparent in the skeleton shrimps, or caprellid amphipods (Suborder Caprellidea). These are very abundant in harbors, and any handful of algae collected from a floating dock will contain hundreds of them. They are highly adapted to clinging to algae and other encrusting organisms and can quickly move away from grasping forceps with an inchworm-like motion. It's not always easy to catch up to these guys. Isopods are widely distributed in the sea, and are also found in freshwater habitats. Incidentally, isopods are the most successful terrestrial crustaceans, found in forest duff, domestic gardens, and inside homes. Remember those "pillbugs" I mentioned earlier? Those are isopods! Compared to amphipods, isopods have generally undifferentiated bodies. There are 7 pairs of walking legs, more or less the same. Some isopods, terrestrial forms in particular, use their walking legs for walking. The marine forms tend to use their legs for clinging to algae.
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Class Cirripedia
Cirripedes are the sessile, filter-feeding crustaceans more commonly known as barnacles. They are highly modified for their unusual lifestyle, inhabiting a calcareous shell consisting of several plates, that is permanently glued to a surface. They extend their thoracic appendages through an aperture in the shell and rake them through water, grasping suspended particles. The vast majority of barnacles are free-living, although there are about 260 species of rhizocephalans that are parasitic on other crustaceans.
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Living permanently attached to a rock presents some reproductive difficulties. Being arthropods, barnacles have non-swimming sperm and must copulate in order to get eggs and sperm together. Barnacles have two adaptations that enable them to overcome the constraints of their sessile lifestyle. First, they are simultaneous hermaphrodites, Page 11 of 11 functioning as both male and female at any given moment. Second, they have a disproportionately long penis that enables them to introduce sperm into the mantle cavity of neighbors. Copulatory success is also improved by living in clusters, and natural populations of barnacles are usually densely clumped. It is not difficult to imagine the selective pressures on penis size in barnacles, is it?