Crustacean

Taxonomic Classification

Kingdom:	Animalia
Subkingdom:	Bilateria
Infrakingdom:	Protostomia
Superphylum:	Ecdysozoa
Phylum:	Arthropoda
Subphylum:	Crustacea
Source: Integrated Taxonomic Information System

Crustaceans belong to the phylum Arthropoda, but are more commonly referred to using their subphylum, Crustacea. Crustaceans are the sole members of Crustacea but represent an immensely diverse group that encompasses an approximate 50,000 known species which span an equally broad range of environments. Crustacea has a metropolitan distribution (worldwide) with members most commonly found in all aquatic environments like that of marine, brackish, and freshwater systems. Though members of Crustacea can be found roaming terrestrially, only two crustaceans are known to be fully terrestrial, living their entire lives on land. Land Crustacea are nutrient cycling detritivores while the rest, and vast majority of crustaceans, are scavengers, benthic bottom feeders, and general or obligate parasites.

Taxonomy

The Crustacea subphyla is a paraphyletic group which consists of all taxa that originally comprised all “traditional crustaceans”, and now classifies organisms within three superclasses. These three superclasses are the Multicrustacea, Oligostraca, and Allotriocarida, which together form the Pancrustacea monophyletic group with the addition of the insects of Hexapoda. Mulitcrustacea represents the largest proportion of crustaceans containing approximately 80% of all recorded crustaceans, like that of crabs, barnacles, and woodlice. Crustacea itself can is commonly subdivided into 10 subclasses, commonly referred to as “direct children” classes.

Children Classes

• Class Cephalocarida:

-Horseshoe shrimps which are among the oldest, most primitive members of Crustacea; all known species are bottom-dwelling, benthic organisms.

• Class Branchiopoda:

-Includes tadpole, fairy, and brine shrimps, all of whom are small, freshwater organisms which feed upon plankton and detritus. Make up an important base of the food chain.

• Class Ostracoda:

-Ostracods are microscopic, aquatic organisms which are among the most basal forms of Crustacea. They are commonly used for dating within the fossil record due to their abundancy and widespread distribution.

• Class Copepoda:

-Very small aquatic organisms which live in both fresh and saltwater. Copepods are commonly parasitic or have parasitic phases across their life cycle.

• Class Mystacocarida:

-Microscopic organisms which live in between the grains of sand of intertidal beaches. Form part of the meiobenthos, the trophic link between bacteria and larger fauna.

• Class Remipedia:

-Blind organisms commonly found in coastal aquifers; also the only known venomous crustaceans in the world.

• Class Tantulocarida:

-Ectoparasites which are highly specialized obligate feeders commonly found on isopods.

• Class Branchiura:

-Parasitic residing on or in most fish, back and forth debate placing the group within Crustacea and Annelida.

• Class Cirripedia:

-Barnacles which reside exclusively in shallow tidal waters. Are sessile (nonmobile) suspension feeders or highly specialized obligate parasites.

• Class Malacostraca:

-Approximately 70% of all crustaceans, including lobsters, crayfish, crabs, shrimps, isopods, and most other well-known aquatic crustaceans.

Crustacean Characteristics

Generalizations

The wide distribution and wider evolutionary track of crustaceans have created an extensively complex group of organisms which create numerous exceptions to any general rules and basic features used to generalize organisms. However, with that said, some generalizations surrounding the physical development and appearance of crustaceans can still be made and used to assist in taxonomic placement.

Firstly, Crustacea differ from their parent phylum Arthropoda due to the development of an additional pair of appendages known as antennules. Antennules develop on the first segment of the head, just ahead of the antennae which form on the second segment of the head. Crustacea themselves can then be delineated according to morphological composition based upon the number and makeup of body segments.

Body development

Each body segment will develop from a set of precursor cells called somite(s). Each somite can develop individually to allow for greater mobility or be fused together with any number of other somites to produce more complex body segments like that of the anterior (head) of the crustacean, which is created by the fusion of four somites and an unsegmented region called the acron. The posterior, or backend of the crustacean, develops via a similar mechanism by which somites fuse with another unsegmented region called the telson, to form a furca, which then itself undergoes further development into either a shorter, compact tail or the rare elongation event to create a tail equal to, or greater in length than the segmented body.

This variable somite fusion leads to the immense morphological disparity in body form and appendage development seen across the subphylum. While diverse, there is commonly consistency seen across any one class: for instance, Malacostraca, which contains about 70% of crustaceans, follows a body plan of head, trunk, thorax, and abdomen. Though different in appearance and or use, the carapace, a hard protective structure behind the head, is also a widespread morphological feature. Appendage appearance and use tends to allow for specialization, often creating or heightening functions associated with feeding, movement, reproduction, or just about any other task. Appendage and body form disparity is highlighted when comparing the 12 ft leg span of the Japanese Spider Crab to that of an approximately 0.009-inch parasitic copepod, which ironically can be found in most Japanese Spider Crabs.

Life Histories

The development of crustaceans follows the now common theme in which diversity creates an increasingly variable means of development and subsequent natural history.

Reproduction

In terms of reproductions, Crustacea can display both sexual and asexual reproduction, a few species are hermaphroditic, others can change sex across their lifespan, and when unable to find a mate, parthenogenesis is commonly used.

Juvenile Development

Visual representation of the crustacean life cycle

In all known forms, the crustacean life cycle begins with the egg, in which the immature crustacean will either emerge as a scaled-down version of an adult or as a larva which will develop further. Crustaceans' developmental path is thus classified as being either anamorphic (larvae-based) or epimorphic (smaller adult hatchling); the dichotomous nature of hatchling development is highly conserved across taxa and is often used for classification. Additionally, like other organisms classified under Arthropoda, crustaceans must molt and shed their exoskeletons to further grow in body size, and in the case of larval organisms, to add appendages.

Anamorphic Development

Anamorphic development involves a series of larval stages which utilize the process of molting to further morphological development of segments and appendages which aren’t initially present. Anamorphic development is typically comprised of three larval stages referred to as the nauplius, zoea, and megalopa / mysis stages.

• ‘’’Nauplius:’’’

The nauplius (plural nauplii) stage details a larva which has recently emerged from its egg and is categorized as a “floating” head with a single eye and typically three pairs of appendages which it uses for mobility. Some anamorphic crustaceans will “skip” this larval stage and instead emerge from the egg at further into development.

• ’’’Zoea:’’’

During the zoea stage, the larva is acknowledged by the presence of both a cephalon (head) and thorax (a body segment). By the end of this stage, it will add abdominal segments and may also have a pair of compound eyes. Zoea swim using biramous (branched), thoracic appendages.

• ’’’Megalopa or Mysis:’’’

By the megalopa (typically for crabs / lobsters) or mysis (usually more “basic” crustaceans) stage, the crustacean will have added the segments and appropriate appendages of all three body regions (cephalon [head], thorax, and abdomen), as well as, for aquatic species, the development of at least one pair of swimmerets (appendages used for forward motion). At this point, the individual will look like a proportionally accurate, but smaller version of an adult that is sexually immature.

Epimorphic Development

Epimorphic crustaceans instead develop within their eggs and emerge at the same respective stage of development as anamorphic organisms do following the completion of their larval stages. Therefore, epimorphic development is defined by the absence of any larval stages, instead opting to follow a similar series of development stages within the safety of their egg to then emerge as a fully segmented, appendage-baring, downsized version of its adult counterpart.

Soil Crustaceans

Crustacea utilize a wide range of habitats in conjunction with their wide range of morphological forms. Environmental conditions range anywhere from the pressure of the deep ocean, freshwater systems, high salt concentration brine lakes, and even the high arctic where a dormant overwintering stage follows reproduction during the short summers. Many crustaceans have adapted to terrestrial life in order to hunt, reproduce, or even avoid predation. Notably, the only known fully terrestrial members of crustacea are the sow bug and pill bug of the order Isopoda and class Malacostraca.

A: A diagram depicting the different physical characteristics of terrestrial isopods

Sow bugs and pill bugs are commonly grouped together and referred to as woodlice (singular woodlouse). These fully terrestrial isopods reside in moist soil and are commonly found under log, stones, and any other ground debris. Woodlice are primarily detritivores which feed upon dead or decaying plant matter and are critical to nutrient recycling and the subsequent soil health of a habitat. Though very similar at first glance, the two isopods can be distinguished due the sow bug’s lighter colorway and more articulated (jointed) appendages. Additionally, the pillbug, also referred to as a roly poly, has the ability roll into a tight protective ball, an ability the sow bug lacks.

References

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