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Chelicerata
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- 1Phylum ChelicerataDerivation: Greek: chele = claw / talon; cerata = horns
The phylum Chelicerata consists of three rather disparate classes:
Class Merostoma
(Horseshoe crabs)
Class Arachnida
(Scorpions, spiders, mites, harvestmen, solpugids and several others)
Class Pycnogonida
(Sea spiders)
Note: The pycnogonids are probably unrelated to the other two classes.
The rich fossil record left by marine chelicerates this group had its origin in the ancient Cambrian or Precambrian seas (~605mya). Today most live on land, where they are second in number only to the insects. such as some mites, have secondarily invaded various aquatic habitats. On land, they are very adapted - 2Chelicerata Characteristic Features2 body divisions:
cephalothorax (prosoma)
abdomen (opisothosoma)
No antennae
cephalothorax segments:
Segment 1: No appendages
Segment 2: Chelicerae
Segment 3: Pedipalps
Segments 4-7: Walking legs (i.e. 4 pairs)
The chelicerae & pedipalps are specialized for roles in chelicerate groups, including
sensation, feeding, defence, locomotion, and copulation. - 3Chelicerata Characteristic FeaturesGas exchange by gill books, book lungs, or tracheae
Simple medial eyes and lateral compound eyes - 4Chelicerata Diagnostic and Special FeaturesBilaterally symmetrical
2) Body more than two cell layers thick
3) A through, straight gut, from the mid gut region of which issue from two to many pairs of digestive diverticula which secrete enzymes and intracellularly digest and absorb food - 5Chelicerata Diagnostic and Special Features4) Body divided into two regions
5) Appendages uniramous; all attached near to the ventral mid-line and, in some, extended by haemocoelic pressure; without antennae or mandibles.
6) Only the chelicerae form mouthparts, although coxal endites may crush or spoon food into mouth. - 6Chelicerata Diagnostic and Special Features7) Usually with direct median and indirect lateral ocelli on the prosoma.
8) Opisthosoma sometimes externally segmented
9) A prosomal excretory system of blind ending coxal glands, and/or an opisthosomal system of branched endodermal Malpighian tubules arising from the mid-gut and discharging mainly guanine.
10) A non-calcareous exoskeleton and sometimes a platelike mesodermal endoskeleton in the prosoma. - 7Chelicerata Diagnostic and Special Features11) Gaseous-exchange organs associated with the opisthosomal appendages;
12) Blood system involved in the circulation of respiratory gases and usually containing haemocyanin.
13) Nervous system with separate ganglia along the length of the body or, more usually, concentrated into a single prosomal mass.
14) External fertilisation in marine classes (although the two partners associate closely in pseudocopulation during mating) and internal fertilisation via copulation or spermatophores in the primarily terrestrial class. - 8Chelicerata Diagnostic and Special Features15) Juvenile stage small versions of the adult, usually hatching with the full complement of limbs.
16) Originally benthic marine, one class has colonised the land and fresh water highly successfully. - 9Class MEROSTOMA FossilsHORSESHOE CRABS
Fossil Record
Originate Late PreCambrian (~605mya)
then exhibited great diversification
By Permian (~290mya)
= dominant invertebrate group
included water scorpian 3m long - largest arthropod described - 10Class MEROSTOMA Present day4 species of ‘horseshoe crabs’
Virtually unchanged since
Jurassic era (~202mya)
'living fossils'
Limulus polyphemus = best known
Common in shallow coastal waters
Predator / scavenger
- clean sandy bottoms where crawl or burrow under surface and prey
Capable of withstanding wide
fluctuations in environmental
variables
- temp salinity - 11If horseshoe crabs have been successful at surviving why haven’t they diversified?Reached an evolutionary dead end
Lack of genetic diversity to evolve new forms?
But
Levels of heterozygosity in Limulus are similar to other animals - 12Horseshoe Crab ECOLOGICAL IMPORTANCEShorebirds
such as Ruddy turnstone + Red knot
rely on horseshoe crab eggs as fuel during their migration from wintering to summer breeding grounds.
Up to 1 million migrants descend on Delaware Bay in May-early June and may increase their weight ~80% during this time on a diet of horseshoe crab eggs
The conservation of the birds is therefore highly dependant on the preservation of the horseshoe crab population. - 13Horseshoe Crab BIOMEDICAL IMPORTANCETheir blood contains amoebocytes that encapsulate (surround) invading bacteria
-isolating them from the body
Amoebocytes stimulated by endotoxins
on surface membrane of Gram-negative bacteria
Used as pharmaceutical test for presence of bacterial toxins
= Limulus amoebocyte lysate test (LAL-test)
Blood from ~ 250,000 crabs collected annually to prepare test kits - 14Horseshoe Crab BIOMEDICAL IMPORTANCEBlood from ~ 250,000 crabs collected annually to prepare test kits
Blood sells for ~ $13,200 per litre
Limulus blood is blue
because it contains oxygen-transporting
hemocyanin that contains
copper - 15Class ARACHNIDADerivation: Greek: arachni = a spider
The class Arachnida comprises of a number of groups, the most important of which are:
Order Scorpiones (scorpions).
Order Araneae (spiders).
Order Opiliones (harvestmen).
Acari. (an assemblage of seven orders containing the mites and ticks). - 16ArchnidaToday the arachnids are almost entirely land-based creatures and over 98% of living chelicerate species.
diversity of forms but arachnids exhibit many features in common.
The unsegmented cephalothorax is usually covered dorsally with a solid carapace.
The appendages common to all arachnids arise from the cephalothorax and consist of
a pair of chelicerae, a pair of pedipalps, and four pairs of legs. The chelicerae are used in feeding but the pedipalps serve a variety of functions and are often highly modified. - 17Order SCORPIONESAmong oldest terrestrial arthropods
Fossils in Silurian period - aquatic
terrestrial appears in upper carboniferous
Characteristic Features
Chelicerae: short (grinding food)
Pedipalps: large claw-like
(grasping food)
Sting on posterior last segment - 18Scorpion EcologyPredatory (inverts / small vertebrates - small lizards and mammals)
- totally carnivorous
Stand + guard by burrow
Or
Run into prey while patrolling territory at night
Grasp prey with pedipalps + use sting
-but if small prey will just eat alive
Mostly secretive + nocturnal
Terrestrial but a few species arboreal / inter-tidal - 19Scorpion VenomsSting is on the posterior end of the last segment
a bulbous base and a sharp curved barn that injects the venom.
Kills smaller invert; painful but not usually fatal in humans
Neurotoxic effects vary; could be useful in drug design.
Used in traditional Chinese medicine
the asian scorpian - Buthus martensi = no effect mice but anti-epilepsy in rats - 20Scorpion VenomsHadrurus aztecus = contains novel peptide with antimicrobial properties
(presence of antibiotics in venoms may protect against pathogenic microbes during prey consumption?) - 21Order ARANEASPIDERS
range from less than 0.5 mm in length to large tropical tarantulas have bodies up to 9 cm long; leg span can be much greater.
They are the most familiar of all chelicerates
are one of the most abundant groups of land animals
They have exploited nearly every terrestrial habitat, and many freshwater and intertidal areas as well. - 22Order Aranea Characteristic FeaturesChelicerae:
fang + basal piece (deliver venom + macerate food)
Pedipalps:
female = short finger-like, sensory
male = enlarged, knob-like, copulation
Spinnerets: spin silk formed by abdominal silk glands - 23Order Aranea ECOLOGYPredatory (inverts / small vertebrates)
Stand + guard by burrow
Or
Stand and guard web
Or
Run into prey while patrolling territory
Grasp prey with chelicerae and inject venom
Diurnal + nocturnal
Terrestrial, arboreal, inter-tidal, some aquatic - 24Spider Ecology BALLOONINGBallooning, sometimes called kiting, spiders, and some other small invertebrates, can move through the air by releasing gossamer threads to catch the wind.
Carried by where ever air currents take
Mostly used by spiderlings to disperse but large sometimes do - 25Spider Ecology BALLOONINGThe spider climbs to a high point
stands on its toes and points its abdomen to the sky
releasing fine silk threads from its spinnerettes until lift off occurs
Journeys achieved vary from a few metres to hundreds of kilometres. Even atmospheric data have collected samples of balloons at five kilometres above the earth and ships in mid-ocean have reported spider landings.
Mortality is high. - 26Spider VenomThe chelicerae of spiders are unique among arachnids, they have poison glands that open near the tip of the fang.
Kills smaller invert;
most species fangs cannot penetrate skin and even if they can the bite is painful but not usually fatal in humans
Spider venom may be useful source pharmaceutically active compounds
Black widow spider (Latrodectus): neurotoxic venom = pain + respiratory paralysis
Recluse spider (Loxosceles): haemolytic venom = tissue necrosis (despite toxic venom, this species is mostly a detritivore). - 27Spider SILKSpinnerets seen in Devonian fossils (~409mya)
Several types of silk produced: some species can produce up to 7 kinds of silk protein
Produced as proteinaceous liquid whose molecular configuration is altered by being drawn out and this makes it harden
Not all species produce webs
Probably originally used as in reproduction (e.g. protective wrapping for eggs)
Now also used: prey capture, drag-line, dispersal, production of nest / retreat - 28Spider SILKspider silk is about five times as strong as steel
if it could be produced in sufficient quantity
could be used biodegradable sutures for surgery, replacement tendons, and industrial fibres for use in everything from fashion to fishing lines. - 29Order OPILIONES Characteristic FeaturesHarvestmen / Daddy long legs
Chelicerae: short, pincer-like
(chewing /handling food)
Pedipalps: long, leg-like
(grasping food)
Only arachnids (except a few mites)
in which male uses penis for direct copulation
- spiders males collect sperm from their genitalia using their pedipalps and introduce the sperm into the female’s genitalia. - 30Order OPILIONES Characteristic FeaturesLegs often very long and slender
(species’ leg length related to density of vegetation)
Why legs are so long not known
Self-amputation
= anti-predator response?
but leg does not grow back - 31Order Opiliones ECOLOGYPredatory but scavenging is more important than in other arachnids
The prey or food is seized by the pedipalps and passed the chelicerae, which hold and crush it.
Widespread distribution, sub-arctic – tropics
Leaf litter, grassland, arboreal, caves - 32The AcariDiverse arachnids containing mites and ticks
(Some authors place them in a single order, others divide them into 7 orders!)
General Notes
Extremely varied assemblage (~30,000 species described but probably ~1million or more)
Probably polyphyletic: evolved from various arachnid groups - 33Acari Characteristic FeaturesMostly small (<1mm)
Chelicerae: pincer like / stylet like
Pedipalps: varied structure, often leg-like, sometimes pincer-like, sometimes vestigial.
Apparent lack body divisions: abdominal segmentation lost and abdomen fused with prosoma.
Entire body often covered with a single sclerotinized shield. - 34The Acari ECOLOGYAll habitats exploited:
Terrestrial
Aquatic
Desert
Arctic
Parasites of invertebrates and vertebrates - 35The Acari ECOLOGYFeeding Behaviour
Feed on fluids but all types of food exploited:
Predators
Herbivores
Detritivores
Parasites - 36Acari IMPORTANCEMajor components of some ecosystems (e.g. soil)
Parasites of wild animals
Parasites of domestic animals
Parasites of humans
Vectors of diseases
Plant pests
Biological control of crop pests - 37Importance of mites as vectors of Lyme’s diseaseLyme disease is an infectious disease caused by bacteria of the Borrelia type
Lyme disease is classified as a zoonosis, as it is transmitted to humans from a natural reservoir among small mammals and birds by ticks that feed on both sets of hosts. Hard-bodied ticks of the genus Ixodes are the main vectors of Lyme disease. - 38Class PYCNOGONIDASEA SPIDERS
1000 described marine animals name owing to their somewhat spider-like appearance as they crawl slowly about on their long legs
(pyk = thick; gonida = knees)
Characteristic Features
Size: 1-40mm long
Chelicerae: often reduced in size,
may be absent
Pedipalps: present highly modified
Proboscis: adapted for sucking
4 pairs long legs - 39Pycnogonids Characteristic featuresProsoma: narrow
Opisothosoma: reduced / absent
Digestive organs / reproductive
organs extend into legs
Ova ripen in legs of females
Eggs carried by males on modified
appendages on 1st pair walking legs - 40Taxonomic position of PycnogonidaPreviously associated with many different invertebrate groups
Molecular evidence = definitely belong among chelicerates - 41Pycnogonida ECOLOGYCommon but little studied
All marine
Mostly carnivorous / detritivores
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- (Invertebrates) Animal Diversity and Evolution Fichas sobre Chelicerata, creado por Sarah Bell el 12/01/2018.
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- Sarah Bell
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