Arthropoda- Malacostraca Flashcards
How are the class Malacostraca described?
Includes over half known crustacean species
Includes most of the larger forms
Trunk of 14 segments plus the telson
First 8 segments from thorax (may be covered by carapace)
Last 6 segments from the abdomen
All segments bear appendages
What are the segments on the class Malacostraca modified for?
Anterior thoracic appendages modified for feeding (maxillipeds), remainder for crawling or attachment (pereopods)
Anterior abdominal appendages (pleopods) modified for swimming, burrowing, ventilating, carrying eggs or for gas exchange
Last pair abdominal appendages (uropods) flattened to form tail fan (together with telson)
What sub classes exist in the class Malacostraca?
Sub-class Pyllocarida (small group)
Sub-class Hoplocarida (Mantis shrimp)
Sub-class Eumalacostraca (includes most of the species in the class)
What super orders exist in the sub-class Eumalacostraca?
Super order Eucarida (decapods and euphausids)
Super order Peracarida (amphipods, isopods)
How are the order Decapoda described?
Largest order of the crustaceans (10,000 spp)- mostly marine
Shrimp, crayfish, lobsters and crabs
First 3 pairs thoracic appendages modified as maxillipeds
Last 5 pairs thoracic appendages modified as legs (hence decapoda)
Anterior legs may be chelate
Legs usually not biramous
How does the Decapod stomach work?
It is a gastric mill- it is a lined cuticle that forms 3 teeth and associated muscles allow it to break up food
In the posterior part of the stomach there is a filtration system- fine particles can pass to the hepatopancreas (digestive organ) larger fragments pass into the intestine to be formed into faecal pellets
How are the gills arranged in the Decopoda?
There are various gill arrangements but they typically occur near the base of the appendages and are enclosed in a gill chamber below the carapace
A ‘gill bailer’ (part of the second maxilla) generates a water current through the gill chamber
What are the various mechanisms to prevent the gills from clogging with detritus in the decapoda?
Setae form filters where water enters the chamber, appendages modified to scrape detritus off gills
What are the shrimp like decapods?
3 distinct decapod groups have a shrimp like body plan- largest group is the caridea
Like most decapods the majority are predators/scavengers
Some shrimp (and many other decapods) are commercially exploited
What do the shrimps include?
The shrimps include the only true pelagic decapods
These often show vertical migration
They are often partially transparent
Mesopelagic species may be luminescent
What are most shrimps?
Most shrimps are benthic
Many are epibenthic or crevice dwellers- some are shallow burrowers
What are some examples of the decopods?
Snapping shrimp- large cheliped makes a loud click or pop- possibly a warning signal
Cleaning shrimp- have an association with reef fish- clean parasites off the surface of the fish
What are the larger groups in the decapods?
Lobsters and crayfish- similar body structure to shrimp but more robust and more strongly developed chelipeds
How are the lobsters described?
Crevice dwellers- asymmetric chelipeds (occurs in many decapods)- one adapted for crushing the other for cutting- commonly occurs as an adaptation to feeding on molluscs
What are the Nephrops?
Order decapoda
Burrows in soft sediment- commercially fished and marketed as scampi
Can some lobsters lack the chelipeds?
Some lobster-like decapods lack the large chelipeds e.g. crawfish
Some freshwater crayfish build burrows in damp ground and can be abundant
Can shrimps burrow?
The burrowing shrimps are a group that is intermediate in form between shrimp and lobster-like decapods- some build very extensive burrows
Many filter feeders
What are the Brachyura?
Order decapoda
The crabs
Includes more species than any other group in the decapods (>4500)
Body is relatively short and the abdomen is reduced and folded under the ventral surface
What appendages are important in the crabs?
The uropods are absent in most species
Pleopods are present in females and function in brooding eggs, in males only the anterior 2 pleopods are present and function in transferring the spematophore during copulation
How do most crabs move?
By crawling but most can swim to some extent
Some have a flattened paddle-like 5th leg as an adaptation for swimming
What is an example of a crab species?
Graspus- are intertidal tropical crabs- remarkably fast moving and adept at clinging to the rock despite crashing waves
Where do crabs reside?
Some crabs are terrestrial but most need to return to the sea to reproduce
One group has severed the link to the sea (direct development rather than larvae) and has colonised freshwater environments hundreds of miles inland
What group includes the largest known crustaceans?
Order decapoda
Spider crabs
Spider crabs use other organisms such as algae for camouflage
What lifestyle do some crabs?
Some crabs have a commensal lifestyle
What are fiddler crabs?
Order decopoda
live in burrows on sandy beaches- they emerge at low tide and rapidly silt organic matter from sand with their mouthparts spitting out balls of sand
Large cheliped of male used in courtship display to females
What are the Anomura?
Includes the Paguroidea (hermit crabs)
They mostly inhabit gastropod shells but a few use other things (like bamboo fragments)
Abdomen is asymmetric with pleopods reduced or absent on one side
Uropods are adapted to grip the columella of the shell and the last 2 pairs of legs are modified to grip the inside of the aperture
What are some examples of commensal association in the decopoda?
With Cnidarians such as Calliactis, Adamsia and Hydractinia
What else does the Anomura include?
Includes the Galatheoidea (squat lobsters)
These are mostly benthic crevice dwellers (e.g Munida) but also include the lobster krill which are pelagic
What are the porcelain crabs?
Order decopoda
The body shape is usually distinctive but the porcelain crabs also belong in this group although they resemble true crabs
Porcelain crabs can be very abundant- the last pair of legs is very reduced and the abdomen is sometimes used for escape swimming
What are the Hippoidea?
Order decapoda
The anomura also include the Hippoidea (mole crabs)
These are burrowers on sandy beaches and use their 2nd antennae as nets to filter food from the water
How are the order Amphipoda described?
6,000 spp- mostly marine- body generally laterally compressed
Sessile compound eye, no carapace, both pairs of antennae well developed
1st thoracic appendages are maxillipeds, next 7 pairs are legs, the first 2 of these are typically gripping ganthopods and remainder are walking legs
How are the abdominal appendages arranged in the order Amphipoda?
The first 3 pairs of abdominal appendages are pleopods- function in swimming and in circulating water to ventilate the gills which are on the inside of the bases of the thoracic limbs
The last 3 abodminal segments are modified as uropods- function in jumping, burrowing or swimming
What are the largest group of the amphipods?
The gammarids
They are basically benthic- most can swim on occasion using pleopods and/or uropods
Many are burrowers- they may just pack sediments into the burrow wall but many secrete a cement from the thoracic appendages and build tubes
Most tubes are fixed in the sediment or on a hard surface but some remain mobile and drag the tube with them
How do the order Amphipoda feed?
Most are detritus feeders or scavengers- a few are are herbivorous, amphipod predators are unusual but do occur
Some are filter feeders and various appendages are modified as filters
What lifestyles exist in the order amphipoda?
Parasitism is unsual- a few species are ectoparasites of fish
Some species are commensal on organisms such as sponges, sea urchins and whales
How big are the order amphipoda usually?
Most species are 5-15mm in length but there are vermiform interstitial species <1mm in size
Where do the order amphipoda reside?
Most species are marine but they can also be very abundant in fresh water
Species of Talitridae are found at the top of the shore and in the Southern hemisphere some have colonised soil well above the shore
They are still reliant on gills and have low tolerance to desiccation so need a moist environment and tend to feed at night
Talitrus has been shown to navigate by the sun when displaced on a beach
What are the hyperiidean amphipods?
They are pelagic amphipods, spending all or part of their life cycle commensally with jellyfish (or jellyfish-like) animals
Their swollen head and thorax may be related to the commensal lifestyle or may be to enhance buoyancy
They generally have large eyes and are predatory
What are the caprellid amphipods?
Skeleton shrimp are adapted for climbing
The body is long and slender and the abdomen is extremely reduced
They have grasping claws on the legs to grip the hydroids or algae that they live upon
Some are predatory and others graze on microalgae or detritus
How are the order isopoda described?
4,000 spp- mostly marine but many fresh water and terrestrial species
The most successful of the crustaceans in colonising terrestrial environments
How are the order Isopoda structured?
Body generally dorso-ventrally compressed, no carapace, sessile compound eyes, 1st antennae normally small
Thorax with 1 pair maxillipeds and 7 pairs of legs- often structurally similar and adapted for crawling- in some species the anterior legs are modified as gnathopods
Abdominal pleopods modified as gills- in some species there is a protective cover over the gills formed by the 1st pair of pleopods or by the uropods
Some abdominal segments may be fused with the telson
How do the Isopoda feed?
Most are scavengers and omnivores and deposit feeding is common
There are some herbivores and some carnivores
There are many parasitic species- larvae are ectoparasites of fish and have specially adapted large mandibles
How do the isopoda move?
Most are crawlers- Ligia can move very rapidly over rock surfaces
Many are burrowers and some build
Limnoria bore tunnels into wood- they feed on the wood and can be a serious pest of wooden structures in the marine environment
How do the isopods swim?
Most can also swim using the pleopods
In some groups the first 3 pairs pleopods are adapted for swimming and only the posterior pairs are used for gas exchange
How are the order Stomatopoda described?
Mantis shrimp (-300 spp)- all marine
Highly specialised predators
First 5 pairs appendages are maxillipeds. The second of these is large and raptorial. Remaining 3 pairs are walking legs
Large second thoracic appendages adapted for spearing or smashing prey
Live in burrows and often territorial
What are the thoracic appendages adapted for in the order Stomatopoda?
Large second thoracic appendages adapted for spearing or smashing prey
How are the order Euphausiacea described?
Krill (85 spp)- all marine
Thoracic appendages biramous and used for filter feeding
Thoracic appendages not modified as maxillipeds
Free swimming and planktonic- pleopods used for swimming
How is the order Mysidacea described?
Opossum shrimps (780 spp) mostly marine- often free swimming in swarms
1st (and sometimes 2nd) thoracic appendages are maxillipeds, remainder adapted for swimming, pleopods often reduced
Carapace not fully fused to posterior throacic segments
Statocysts in uropods can be used to distinguish group
