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Phylum Arthropoda



Mantis shrimp, from Tomiyama et. al., 1970.
The largest and most diverse group of animals known, and by many criteria the most successful, arthropods can be found in a bewildering array of body forms, lifestyles, and habitats. All Arthropods have jointed appendages and a hard exoskeleton. In the marine environment the dominant arthropod group is the crustaceans (compared to the terrestrial environment which is dominated by insects ). There are also many other groups of arthropods that can be abundant in certain environments.
CLASSES OF ARTHROPODS

SUBPHYLUM TRILOBITAMORPHA (Trilobites and their relatives, wholly extinct)

SUBPHYLUM CHELICERIFORMES
Class Chelicerata
  • Subclass Merostomata (horseshoe crabs)
  • Subclass Arachnida (spiders, scorpions, mites, ticks)



Horseshoe crab , Limulus polyphemus , class Arachnida , subclass Merostomata , from Pratt, 1923.
The Chelicerates are an ancient group, originating from at least early Cambrian times, and are well represented in the fossil record. They include the extinct Eurypterids , or water scorpions , which reached sizes of more than 2 meters in length. The most familiar living chelicerates are the spiders, scorpions , ticks, and mites (subclass Arachnida ). The only primarily marine group is the subclass Merostomata (horseshoe crabs).
CLASS PYCNOGONIDA (the sea spiders)


Sea spider, class Pycnogonida , from Pratt, 1923.
Despite their common name, sea spiders are not true spiders. This is an unusual group that has been known for a long time, but it has been difficult to place relative to other arthropod groups. Pycnogonids are currently thought to probably represent an early divergence from the evolutionary line leading to the other Chelicerates . There are approximately 1,000 species of Pycnogonids , all of them found in the marine environment. They are distributed worldwide, including polar regions. Although commonly found in the intertidal they are also found at depths up to 7,000 meters. Most pycnogonids are about 1 cm. or less in size, but some deep sea forms reach up to 70 cm. across between leg tips. They have a unique "proboscis " which they use to pierce and suck liquid from seaweeds and various animals such as hydroids , seaweeds, bryozoans , ascideans , and other invertebrates.
SUBPHYLUM UNIRAMIA
  • CLASS INSECTA (insects)
  • CLASS MYRIAPODA (Centipedes, millipedes, pauropods, symphylans)
The Uniramia are primarily terrestrial, although many species have secondarily invaded freshwaters, - and a few have reinvaded the marine environment. However, the group is thought to have originally evolved in the marine environment as benthic organisms. About one million species of the class Insecta (Hexapoda ) have been described, and estimates are that up to 50 million more remain to be discovered. The uniramia is an extremely diverse and ecologically important group, but because it is not abundant in marine environments will not be treated further in this overview.
SUBPHYLUM CRUSTACEA


Blue crab, Callinectes, sapidus.
There are about 30,000 species of Crustacea . Most are marine, but many species live in freshwaters and a few species are terrestrial or amphibious . They include some of the most familiar forms, such as crabs, shrimps , and lobsters . They also include some of the most abundant and ecologically important forms, such as the planktonic copepods , and the mysids and euphausids (krill ) which are important food items for so many other marine organisms (such as fish).
CLASS REMIPEDIA (remipedes )

The Remipedia is a small group of crustaceans that were recently discovered in a cavern on Grand Bahama Island in the Caribbean . These unusual organisms, which superficially resemble centipedes , possess an interesting combination of primitive and advanced characteristics. Not much is yet known about their biology.


CLASS CEPHALOCARIDA


Hutchinsoniella sp., class Cephalocarida , from Pratt, 1923.
Only about nine species of living Cephalocarids have been described. They are small benthic marine animals, less than 4 mm long, that retain many primitive features. Cephalocarids feed on organic detritus in mud or sand sediments. They range from the intertidal zone to waters as much as 1500 meters deep.
CLASS BRANCHIOPODA


Podon sp., a planktonic marine cladoceran , class Branchiopoda . from Bianco , 1904.
The Branchiopods consists primarily of freshwater forms, although there are a few genera of marine Cladocera found in the plankton . The brine shrimp , closely related to the freshwater fairy shrimp , has reinvaded high salinity habitats in inland areas, but does not normally inhabit marine waters (except in some isolated areas where fish predators are absent).
CLASS MAXILLOPODA


Calanoid Copepod, Class Maxillopoda, subclass Copepoda. From Bianco, 1904.
The Maxillopoda is a large and diverse group of Crustaceans , and contains many familiar forms, as well as many other less familiar but ecologically important groups. The subclasses of this large group are:

  • Subclass Ostracoda (seed or mussell shrimps)
  • Subclass Mystacocarida (Mystacocarids)
  • Subclass Copepoda (Copepods)
  • Subclass Branchiura (fish lice)
  • Subclass Cirrepedia (barnacles and relatives)
  • Subclass Tantulocarida (tantulocarids)


Although its difficult to generalize about such a diverse group, most Maxillopods are relatively small organisms, with the barnacles having the largest individuals. Most are marine, although there are many freshwater species of copepods and ostracods . There are about 18,000 described species, most of these in the subclasses Ostracoda and Copepoda .
CLASS MALACOSTRACA



Spiny lobster, Panuliris sp., class Malacostraca . From Tomiyama et. al., 1970.
The Malacostraca is also a large and diverse group of Crustaceans , and contains the most familiar forms, especially on dinner tables. This group is treated further in the section on Malacostraca .

The subclasses of Malacostracans are:

  • Subclass Phyllocarida (leptostracans)
  • Subclass Eumalacostraca (shrimps, crabs, lobsters, amphipods, isopods, mysids, euphausids, and others)



REFERENCES :

Barnes, 1980
Bianco, 1904.
Brusca & Brusca, 1990
Barnes, Calow, and Olive, 1993
Meglitsch & Schram, 1991
Pratt, 1923.
Tomiyama et. al., 1970
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