No, I'm not "shellfish"... I'm a Decapod!

Carmen Cheung and Temara Brown
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© 2007 and


Shellfish, a word colloquially used to describe edible marine animals, has a controversial and conflicting English lexicographic definition that varies in various English speaking countries such as Canada, United States, New Zealand, Australia and United Kingdom.  Professor Andrew Pawley, at the Research School of Pacific and Asian Studies at the Australian National University noticed this inconsistency and conducted an informal survey based on 100 individuals from Australia, North America and New Zealand to put an end to this definition disparity (Pawley, 2004). At the end of the study, Professor Pawley redefined the word shellfish with consideration to the word variances and came up with the following definition (Pawley, 2004):

Shellfish n.

  1. Applied to a variable range of small edible invertebrates that live in water and that are eaten. (For most speakers not a category of fish.) The most common variant meanings are:
    1. Molluscs with shells, that live in water. (Usual in NZ, common in Australia and UK, rare in USA.)
    2. Molluscs and crustaceans. (Usual in USA, common in UK and Australia, rare in NZ, except among professionals in the food trade.)
    3. Edible bivalves only. (Occasional in Australia and USA.)
    4. Crustaceans only. (Occasional in Australia.)
  2. Food consisting of these animals.
  3. Extended by some to all members of the zoological classes listed under 1, whether eaten or not. (UK, USA, Australia, NZ.)

Contrary to the common North American description of the term shellfish, for the purpose of this investigation, we consider the order Decapoda to be strictly a non-shellfish.

Although decapods are more commonly known for their commercial purpose as seafood, there is much more to decapods than meets the stomach. This treehouse will cover a variety of information about decapods and will ultimately answer why they are one of the most unique and non-“shellfish” orders of marine metazoans out there.


The word Decapoda, created by the combination of the Greek words deka meaning ‘ten’ and pous meaning ‘foot’ is used to classify a variety of familiar marine animals such as shrimp, lobster, crayfish, hermit crabs and crabs (, 2004).

The classification of the order Decapoda and overall the class Crustacea have always been a difficult task due to their morphological diversity and the lack of a fossil record amongst its species (Martin and Davis, 2001). Classification of Crustacea is based on a variety of methods including the use of cladistics, molecular systematics, developmental genetics, sperm morphology, larval morphology and fossil record (Martin and Davis, 2001).

The groups contained in the order Decapoda are presented as follows :

Eukaryotes --> Metazoa --> Bilateria --> Arthropoda --> Crustacea --> Malacostraca --> Eumalacostraca --> Caridoida --> Decapoda

Major Groups in Decapoda (Salmon, 1983)

Natantia (shrimps and prawns):
Palinura (spiny lobsters):
Astacura (lobsters and crayfishes)
Brachyura (crabs):
Anomura (mole crabs, hermit crabs, and coconut crabs):


Decapoda have many shared traits. One trait, which is implied in their name, would be their ten legs. These consist of the last five pairs of the eight pairs of thoracic appendages. Decapods are also noted for their hard exoskeleton, segmentation, and jointed appendages, which are used for sensory reception, feeding, locomotion, and defense.

The body of a decapod is divided into several distinct morphological and functional regions called tagmata. The more basal decapods have less tagmatized body plans. As this morphology is thought to be similar to that of the last common mandibulate ancestor, understanding these underlying features and their development becomes pivotal for understanding the evolution of the Decapoda.

Gills are an interesting decapod feature with an extensive surface area that is used for gas exchange with the surrounding water. These gills are located on each appendage from the second maxilliped to the fifth pereiopod.

The auxiliary heart found in decapods is a notable feature. It is located at the anterior end of the dorsal median artery before the artery branches to supply the supraesophageal ganglion (the decapod brain), and the peripheral oculomotor and visual systems. Also referred to as the cor frontale, the auxiliary heart is controlled and integrated with several other systems.

The auxiliary heart is significant due to its adaptation. Decapods possess open venous systems, an inflexible carapace, and a highly variable volume of the stomach, which all influence the blood pressure. The auxiliary heart allows decapods to better maintain blood pressure levels.

The body of a decapod contains nineteen segments, which may contain a pair of appendages, and can be divided into two main divisions: the cephalothorax (head and thorax/pereon) and the pleon (abdomen). The following is a breakdown of these features (listed from anterior to posterior).


Pereon / Thorax

Pleon / Abdomen

Pleopods or swimmerets are used for swimming, brooding eggs (except for Natantia), catching food, sweeping food into the mouth, and sometimes bearing gills. In the males of some taxa, the first one or two pairs of pleopods are referred to as gonopods as they are specialised for fertilisation.

The tail fan is responsible for steering while swimming. Located at the posterior end of the pleon, the tail fan is composed of a pair of biramous uropods and the telson. The anus of the Decapod is located within the telson.

In crabs and other carcinised decapods, the pleon is folded under the cephalothorax

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The general anatomical features of a Decapod, using Palinura as a model.


Decapods are egg-laying invertebrates, mainly with seperate sexes. A change of sex during the lifetime of an individual is a regular occurence in some Dendrobranchiates. In Pandalus montagui, some individuals begin life as males but then transform into functional females after 13 months.

Males of crab and lobster species are usually larger than the females and possess much larger pincers. The males also possess clasping organs used to hold the female during mating. Appendages can also be modified to assist in the sperm transfer process. The sperm is often encased in a spermatophore, and the first and second abdominal appendages are modified to transfer these spermatophores.

Methods for brooding eggs vary among the decapods as follows:

Suborder: Dendrobranchiata

Suborder: Pleocyemata

The Larvae

The larva of decapods and of many other crustacea is the nauplius. It consists of an unsegmented body with only a head, telson, three pairs of appendages (antennules, antennae, and mandibles), and a simple nauplius eye. Once the body is more elongated and segmented, the nauplius is called a metanauplius. The nauplius larva makes an important evolutionary statement for the crustacea as they start to diverge from other arthropods (Encyclopaedia Britannica, 2005).

Behaviour and Mechanisms


Grooming is a common behaviour found in many crustaceans as a form of maintenance to rid their bodies of epizoic growth and particulate buildup (Bauer, 1981). In the Decapoda, this behaviour is extremely crucial and helps prevent the growth of foreign organisms on their body surfaces, loss of olfaction hairs, clogging of gills and embryo mortality. Grooming within the decapod crustaceans involves the use of chelipeds and the setal brushes at the tips of their walking legs.  The most frequently groomed areas of the body include the sensory and respiratory structures that consist of the antennules, antennae, and gills (Bauer, 1981).

Mechanisms of Grooming
Alternatives to Grooming  

Some decapod species use other mechanisms other than grooming to maintain their body surface cleanliness. Some alternative methods include:

Reproductive Behaviour

Attraction Cues

In most decapod species, the males and females live separately from each other and pair-up as adults only to mate (Salmon, 1982). Since males and females are separated by vast spatial distances they rely on a combination of chemical, visual and acoustic signals to attract a mate (Salmon, 1982). Some of these signals are further discussed below.

Chemical Signals. This type of attraction involves sex pheromones which are the most commonly used signal in aquatic  decapods (Salmon, 1982). The use of pheromones is most evident in decapod species where female molting and mating are closely linked temporally (Salmon, 1982).

Visual Signals. This type of attraction occurs most commonly in close-range courtship amongst aquatic species or long-range courtship amongst land species of decapods (Salmon, 1982). Visual signals may involve a "dance" or repetitive movement of a body part, such as "waving" (Salmon and Hyatt, 1983).

Acoustic Signals. This type of attraction occurs predominatly in ghost and fiddler crabs (Salmon and Hyatt, 1983). These acoustic "calls", that only occur during the mating season, are strictly produced by males to indirectly attract females (Salmon and Hyatt, 1983).

Sexual Selection

Decapods are polygynous and sexually dimorphic (Salmon and Hyatt, 1983). Males tend to be physically bigger and this allows competitive behaviour between males for access to females (Salmon and Hyatt, 1983). Females on the otherhand are selective in choosing their mates and base their decisions on the male's ability to acquire resources and the male's ability to dominate other males (Salmon and Hyatt, 1983).


Most decapods are scavengers, feeding mostly on detritus, but it is also known that if small live fish come too close, decapods with chelae may thrust their claws after them.


It is well known that some animals such as the Monkeys  use tools, e.g., sticks to get something out of reach. However, it is only well known in the scientific community that some decapods will use tools as well! There are at least two crabs that carry living sea anemones to be used as weapons. Lybia tessellata and Lybia leptochelis are two species that seem to be most unhappy without at least one sea anemone. The sea anemone Triactis producta seems to always be the anemone of choice as it is bestowed with nettle-cells. Any small animal that comes into contact with these cells would likely soon perish. The claws of Lybia are especially adapted for grasping the sea anemone's fleshy body. If live fish, amphipods, or any other of the like come too close, the crab thrusts out the sea anemone. The victim, if struck, will fall to the sea floor where it will be picked up by the crab. Hermit crabs have also been seen keeping a sea anemone on their shell for protection.


Locomotion of crustaceans is very important in a variety of activities including for feeding, courtship displays, grooming and walking (Evoy and Ayers, 1982). The movement of crustacean limbs involves a network of muscles, neurons and appendages used to coordinate the movement of appendages that make locomotion possible (Evoy and Ayers, 1982).

Muscles within crustaceans are involved in different types of movements where the muscles used can be classified as being synergistic or antagonistic (Evoy and Ayers, 1982). Synergistic muscles involve a single type of movement, such as walking forward (Evoy and Ayers, 1982). While anatagonistic muscles involve a variety of movements such as sideways walking, swimming, or courtship displays (Evoy and Ayers, 1982). The jointed appendages supported by muscles are highly specialized to various types of movements (Lochhead, 1961). Also, Decapod appendages are arranged in a fashion where their legs hang to allow maximum stability for their unique morphology (Lochhead, 1961). This appendage arrangement allows legs located on the posterior region to be highly specialized in pushing, while legs in the anterior region are specialized in pulling (Lochhead, 1961). Legs that are intermediate can be used for either pushing or pulling (Lochhead, 1961).

Types of Locomotion

Swimming. There are two forms of swimming associated with decapod crustaceans (Evoy and Ayers, 1982). One that involves rhythmic extension and flexing of muscles contained in the abdomen to allow jet propulsion through the water (Evoy and Ayers, 1982). The other form of swimming involves rhythmic sculling movements of the appendages (Evoy and Ayers, 1982). The mechanisms involved in swimming are expanded further below (Lochhead, 1961):

Walking Forward. The forward walking motion for decapods involves the coordinated pushing and pulling of paired legs moving at a rhythmic out-of-phase pattern (Lochhead, 1961).

Walking Sideways. This type of locomotion is commonly found among crustaceans that have fewer than six pairs of legs (Lochhead, 1962). This special physiological adaptation allows individuals to exhibit less instances of appendages overlapping during walking (Lochhead, 1961). Sideways walking also involves the coordinated flexing and extending of legs at either sides of the body (Lochhead, 1962).

Cell Division and Cleavage Patterns

Cell cleavaging is an important and crucial step in the development of any animal especially during embryogenesis. This ubiquitous process is different in many metazoan groups and ultimately can be used to classify various groups of metazoans.

During embryo development, the decapod crustaceans undergo protostomic development where the mouth is the first developmental opening in the blastopore. The second opening then develops into the anus of the embryo. This protostomic development is also commonly found in other arthropods and in other groups such as annelids and mollusks.

Most malacostracan crustaceans undergo superficial cleavage, however the Decapoda undergo complete holoblastic cleavage. Due to the spiral holoblastic cleaving of decapod cells, they characteristically have larger eggs than other crustacea, alongside with a greater yolk mass, intralecithal cleavage, blastoderm formation and epimorphic later development (Anderson, 1973). This characteristic spiral cleaving is different from the spiral cleaving noticed in annelids (Anderson, 1973).


Dispersion and vicariance events are known to be responsible for the creation of faunal provinces or regions with similar groups of organisms. Physical barriers, such as ocean currents and land masses can seperate organisms allowing them to evolve away from each other. Fauna found in close proximities have been noted as being more similar (Feldman and McLay, 1993)

Different vicariance effects influencing the Decapoda

Decapoda of the North Pacific region can be assigned to five major origins. These are:

Domestication of Decapoda

Decapods are becoming more and more popular as pets in aquaria. More stores are now carrying them as popularity increases.


Decapod Records

Largest Decapod: : Macrocheira kaempferi, the giant spider crab. Individuals can reach a diameter of 12-14 inches (30.5-35.6cm), with a claw span of 8-9 feet (2.4-2.7m). There is a report of a crab weighing 14 pounds and a claw span of 12 feet (3.65m).

Smallest Crab: The pea crabs of the Pinnotheridae are approximately 0.25 inches (0.635cm) across the shell.

Heaviest Decapod: Homarus americanus, Atlantic Lobster. Records exist of individuals weighing over twenty pounds. The largest record, however, belongs to a lobster weighing 42 pounds and 7 ounces. This lobster was caught in 1934 and bestowed with the nickname "Mike."


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Learning Information

About This Page
This treehouse was created as partial fulfillment for credit in the course ORIGINS 2FF3, Origins and Evolution of Organisms, offered by Jon Stone, Associate Director of the Origins Institute at McMaster University.

McMaster University

Temara Brown
McMaster University

Correspondence regarding this page should be directed to Carmen Cheung at and Temara Brown at

All Rights Reserved.

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