DIVERSITY OF LIFE

DESCRIPTION OF THE SUBPHYLUM CRUSTACEA (BRÜNNICH 1772)

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Crustacea (krus-TA-shuh) is formed from the Latin word crustaceus, which means having a crust or shell.  This is a reference to the hard exoskeleton of many members of this subphylum.  The name was coined by Brünnich (1772).

 

INTRODUCTION TO THE CRUSTACEA

The crustaceans are an old and diverse group of arthropods, mostly free-living and aquatic.  The vast majority of crustaceans live in marine environments where their diversity in form exceeds even the insects on land.  Crustaceans range in form from highly modified barnacles and unusual parasitic forms to crabs and lobsters.  Though quite diverse, they seem to be united by a set of characters that generally apply.  Most have a substantial exoskeleton, some of which are mineralized, especially in large taxa like lobsters.  Many go through distinctive larval stage called a nauplius.

Their size varies likewise from microscopic copepods to large animals like the coconut crab.  They are part of a larger natural group known as the Arthropods (Crustacea, Myriapoda, Chelicerata, Hexapoda) which in turn, is part of a larger clade that includes the Tardigrada, and Onychophora (Brusca and Brusca 2003, and Nielsen 2001).  

The traditional view of crustacean relationships within the arthropoda is summarized by Brusca and Brusca (2003) who suggest that the crustaceans are the stem group from which the other arthropods emerged.   Although the position of the crustaceans within the arthropods has been considered to be almost basal, or a sister group relationship with the chelicerates, molecular phylogenetics indicate that these might be the sister group to the hexapods (Lavrov et al. 2004, Mallatt et al. 2003, Giribet et al. 2004, Regier  et al. 2005, Wheeler et al. 2001, and Giribet et al. 2004).  

More detailed phylogenomic work (e.g. Reiger et al. 2008 and 2010, Reiger and Zwick 2011, von Reumont et al. 2012, and Oakley et al. 2013) points a paraphyletic phylogeny with Hexapoda emerging from within a crustacean clade.  Thus, hexapods and crustaceans are not sister groups but components of a large monophyletic clade called the Pancrustacea.  Indeed, Regier et al. (2004) suggest that the hexapods are terrestrial crustaceans. Figure 1 is based on the topology of Oakley et al. (2013) in which there are three nested clades, one of which includes the hexapods.

We will continue to describe the crustaceans as a separate group in a traditional way, but use the structure of the Pancrustacea.

 

 

MAJOR CLADES OF THE CRUSTACEA

Ma = Mandibulata

Pc = Pancrustacea

Ol = Oligostraca

Ac = Altocrustacea

Mu = Multicrustacea

Hn = Hexanauplia

Al = Allotriocarida

FIGURE 1. MAJOR CLADES OF THE CRUSTACEA.  This is a portion of the Figure 1 on the Arthropoda page and the clade designations should refer back to that cladogram.  The overall form of the cladogram was taken from Oakley et al. (2013).

 

 
Pancrustacea (Pc)

This includes all of the crustaceans and the hexapods.  The topology of the crustacea shows that they clearly are paraphyletic with three nested clades: Oligostraca, Multicrustacea, and Allotriocarida.


FIGURE 2
Oligocrustacea (Ol)

This is formed by Ostracoda, Mysticocarida, Branchiura, and Pentastomatida and is the sister group to all other pancrustaceans  (Oakley et al. 2013).

Ostracods (Seed Shrimp; Figure 2) are small and enclosed in a bivalve carapace that is hinged along the back.  The body looks is unsegmented from the outside.  They superficially resemble the cladocerans, but they swim with five to seven pairs of thoracic limbs and do not use the antennae. Thus, they swim in a smooth continuous motion rather than the jerky motion of the cladocera. The ostracod carapace is shed and reconstituted at each molt.

The mystacocarids are minute benthic marine animals.  Their bodies are elongate.  The head is divided into two regions.  They do not have compound eyes, but they do have two pair of ocelli.  On the other hand, their antennules, antennae and maxillae are large.  The appendages are reduced to four pair of simple legs (only one joint) on thoracic segments two through five, and the appendages of the first thoracic segment form maxillipeds.  The abdomen of five segments ends with a telson with a large, claw-like furca.  Characteristically, mystacocarids have pairs of dentate furrows laterally on the dorsal of the head, thorax and abdomen.  

The parasitic oligocrustacea:

Branchiura are the fish lice (Figure 3).  They are flattened ectoparasites of fish in which the carapace covers head and most of body.  They have 3 free thoracic segments and a head with paired moveable compound eyes, and a median ocellus.  The first maxillae form sucker and the antennules have a large terminal hook.  The abdomen has two lobes and is unsegmented.  Attachment to host is periodic.  The biramous thoracic limbs are used for locomotion.

Pentastome literally means five mouths, which is a reference to the five lobes, often with hooks (Figure 4) at the anterior end of the animal.  The pentastomes are parasites of vertebrate carnivores and usually have an intermediate host for their larvae and three larval stages.  The first larval stage develops in the egg after it is ingested by the intermediate host.  That is followed by the second stage, which resembles a tardigrade and burrows through the gut of the host where it seeks out organs like the liver, encapsulates and forms the third larval stage (Figure 5). The adult develops when the encapsulated larva is eaten by a host animal. As adults, they seem to prefer the lungs of vertebrates.  As adults, they show a pronounced sexual dimorphism (Figure 6).  Although they superficially resemble acanthocephalans, the pentastomes have an obvious segmentation (Figure 7).

The pentastomes are parasites of vertebrate carnivores and usually have an intermediate host for their larvae.  They appear to be highly modified members of the panarthropoda.  Indeed, Margulis and Schwartz (1998) and Brusca and Brusca (2003) place them together with the crustaceans.  Nielsen (2001) is more careful and suggests a "proarthropod" position mainly because they are so modified and synapomorphies are difficult to identify.  


FIGURE 3


FIGURE 4


FIGURE 5


FIGURE 6


FIGURE 7

Altocrustacea (Ac)

This is the sister group to the Oligocrustacea, and contains the most speciose taxa including the hexapods.

 

Multicrustacea (Mu)

The taxa in this clade consistently emerge in the same clade, but relationships within the clade are inconsistent.  Figure 1 follows the analysis of Oakley et al. (2013) in which the Malacostraca is the basal group.

The Malacostraca range in size from small to very large, often with heavily calcified exoskeletons.  The organization of the body by segmention is: 5 cephalic, 8 thoracic, and 6 (sometimes 7) abdominal segments.  The eyes are stalked and compound.  The carapace covers the thorax, but never covers the abdomen.  The first 1-3 pairs of thoracic appendages form maxillipeds, five or more posterior pairs form the walking or swimming legs (pereiopods).  Usually, the legs terminate in claws.  The abdomen has appendages (pleopods) that are modified for swimming and reproduction (sometimes for respiration).  The last pair (uropods) is broad and forms a tail fan with telson. Often, they brood their eggs.

The Phyllapoda

The phyllocarids are the only malacostracan group with seven abdominal segments.  All thoracic appendages are phyllopodous.  The carapace covers the thorax and is laterally compressed.  They are distinctive in that the head has a moveable rostrum.  The antennules are biramous and antennae are uniramous.

The Eumalacostraca

The Eumalacostraca are the large crustaceans and include crabs, shrimps, lobsters, and crayfish (Figure 8).  They have a standard malacostracan body plan but the relative importance of the cephalothorax (that portion covered by the carapace and the abdomen vary considerably).  Sometimes, the thoracic segments are fused with the head.  Antennules and antennae are bi- or uniramous, and the antennae often are associated with an exopod, a diminutive leg-like appendage.  The carapace is well-developed, and the thoracic epipods function as gills, usually tucked inside the carapace. 

Hexanauplia (Hn)

Taxa in this group are very different as adults.  Copepods tend to be highly mobile microscopic plankton or benthic animals.  Barnacles, on the other hand are sedentary, many with armored plates such that they were considered to be mollusks until Darwin's careful study and resulting treatise on them.  As different as the adults are, though, the larvae are remarkably similar.  The nauplius larva stage goes through six molts, thus the name of the clade which means six nauplii (reviewed by Oakley et al. 2013 and von Reumont et al. 2012). 

Copepods (Figure 9) are small, free-living or parasitic crustaceans that can be benthic or planktonic, marine or freshwater.  The small body has a head fused to one or more thoracic segments and eight free segments plus a telson (with further fusion in some), and the abdomen ends in a caudal furca.  The carapace covers the head and thorax and gives the anterior end a rounded appearance.  From there, the body tapers to a smaller thorax and a thin abdomen.  They have paired eyes and a well-developed nauplius eye.  The antennules are uniramous and the appendages of the first thoracic segment form maxillipeds.  They have four to five other pairs of thoracic appendages with which they can swim, but planktonic taxa use their antennae to move quickly to avoid fish and capture prey.  Parasitic species are highly modified.

The Barnacles (also called Thecocostraca and Cirripedia; Figure 10) mostly are free-living sessile marine animals that are attached to a substrate by cement from the antennules.  The carapace covers the body and limbs, often with discrete calcareous plates.  They have a reduced abdomen and some body segmentation with 6 pairs of biramous thoracic limbs (cirri) used to comb or filter water.  Adults are very reduced and they lack antennae or compound eyes.  Mostly the taxa are hermaphroditic with cross-fertilization by elongate penis. In some cases, the sexes are separate and the male is reduced to an organ in the female.  The parasitic forms are even more highly modified than the free-living barnacles.

The tantulocarids are parasites of other crustaceans in which the adults are reduced to a sac with a reduced abdomen, almost having nothing else but sex organs.  The larvae, however, are free-swimming with flattened cephalic shield, a thorax of six segments, and an appendage-free abdomen of up to seven segments.  They are highly modified and relationships are difficult to establish with other taxa; however, typically, they are associated with the copepods and barnacles.


FIGURE 8


FIGURE 9


FIGURE 10

Allotriocarida (Al)

This is a mixture of taxa that have been joined by molecular phylogenetics (von Reumont et al. 2012, Oakley et al. 2013).  The major interest in this clade is the location of Hexapoda and its sister group.  For a description of the Hexapoda, click on the link below.

The Branchiopoda, a group made up mainly of Cladocera (Figure 11), are usually small, with a short body (number of segments varies).  The antennule is small and unsegmented.  The thoracic legs are leaf-like with epipodal gills.  The maxillae are small, and the second maxillae usually are absent.  They have no abdominal segments except for a caudal furca.  They have a characteristic nauplius larva with numerous segments and further appendages added at each molt.  The carapace, when present, forms bivalved shell.  Compound eyes occur in combination with simple eyes. The females females usually reproduce parthenogenically, and produce males only at the end of the season for the formation of sexual resistant eggs. Cladocerans are  found mostly in freshwater as components of the zooplankton.  In general, they are filter-feeders, but some like Leptodora are raptorial predators of other cladocera.

The cephalocarids (Figure 12) are benthic marine animals that have a head covered by a cephalic shield, which forms a horseshoe-shape.  The thorax has eight segments followed by an abdomen of eleven segments, which terminates in a telson.  The antennule is uniramous, and the anterior-most seven thoracic limbs are triramous, each with a large, flattened pseudepipodite.  The abdomen has no appendages except for a caudal furca with long bristles.  Cephalocarids have no carapace or eyes.  They are hermaphroditic, with separate paired testes and ovaries discharging through common ducts.  

The Remipedia (Figure 13) are considered the sister group to the hexapods, the most speciose group of animals.  At present, this group is made of twelve cave-dwelling species with an interesting mix of primitive and advanced characters.  The body is composed of a head covered by a cephalic shield and a very long 32-segment trunk, each segment with a pair of biramous legs.  The body is not covered by a carapace, and the last segment is fused.  the body terminates in a telson.  The mouth has an unusual poison-injecting fang.

HEXAPODA


FIGURE 11

 
FIGURE 12


FIGURE 13

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By Jack R. Holt and Carlos A. Iudica.  Last revised: 02/05/2015