crustaceans form large group usually treated includes familiar animals crabs shrimp thanks recent molecular studies accepted group comprises animals clade words crustaceans closely related insects certain crustaceans described species range size japanese spider crab leg span mass like crustaceans grow distinguished groups insects possession parted limbs forms stage crustaceans free living aquatic animals terrestrial parasitic fish tongue worms group extensive fossil record reaching includes living fossils existed apparently unchanged period million tons crustaceans produced fishery farming human consumption majority shrimp widely animals greatest planet form vital food chain scientific study crustaceans known alternatively scientist works structure body composed segments grouped regions head abdomen head fused form covered single large body protected hard animal grow shell divided dorsal ventral lateral parts fused body segment bear pair segments head include pairs antennae segments bear legs specialised walking legs feeding legs abdomen bears ends bears flanked form tail fan number variety different crustaceans partly responsible group success typically meaning divided parts includes second pair antennae usually exception class generally unclear condition derived state evolved crustaceans second branch limb lost groups instance possessed main body cavity open system blood pumped heart located near oxygen carrying pigment canal consists straight tube like mill grinding food pair digestive absorb food structure goes spiral format structures function located near antennae brain exists form close antennae collection major found gut second pair specialised male sperm transfer terrestrial crustaceans christmas island red crab mate return sea release eggs lay eggs land albeit damp conditions females retain eggs hatch free swimming larvae ecology majority crustaceans aquatic living marine freshwater environments groups adapted life land terrestrial crabs terrestrial crabs marine crustaceans oceans insects land majority crustaceans moving independently taxonomic units parasitic live attached hosts including sea fish whale tongue worms referred adult live life attached substrate independently able withstand rapid changes switch hosts marine non marine species layer important food chain animal communities crustaceans significant invasive species chinese crab asian shore crab life cycle mating system majority crustaceans separate sexes reproduce sexually small number including change sex course life widespread crustaceans viable eggs produced female needing male occurs certain higher crustaceans eggs groups crustaceans eggs simply released water column developed number mechanisms holding eggs ready hatch carry eggs attached form brood limbs female carry eggs external attach rows rocks objects carry eggs limbs carry eggs special thin walled attached long strings larvae crustaceans exhibit number forms earliest characteristic pairs emerging young animal head single eye groups stages including given believed separate species follows stage post larvae swim opposed use use abdominal swimming spikes assist small organisms maintaining directional swimming accelerated development stage cases stage followed stage stage depending group involved classification dates earliest works describe animals including later authors including included crustaceans earliest valid work use included group comprises described species thought total number majority species remain crustaceans small morphology varies greatly includes largest world japanese spider crab leg span metres smallest long despite diversity form crustaceans united special form known exact relationships taxa completely settled april studies based morphology led hypothesis insects allies sister groups recent studies dna sequences suggest larger clade classification crustaceans variable system martin davis largely earlier works treated treated classes classes usually recognised fossil record crustaceans rich extensive fossil record begins animals major groups crustaceans appear fossil record end including tongue worms debate animals assigned truly start classes appear later fossil record described fossil appear early crustaceans rare fossil crustaceans abundant period onwards fossils known contain important groups extinct extant members shrimp extant extinct extinct extant known true shrimp appear shrimp crabs appear great radiation crustaceans occurred cretaceous particularly crabs driven radiation main predators bony fish true appear cretaceous consumption humans crustaceans consumed humans nearly tons produced vast majority output crustaceans crabs shrimp weight crustaceans caught consumption shrimp nearly produced asia china producing nearly half world total non crustaceans widely consumed tons caught despite having greatest planet
Crustacean