Ophiuroidea Modern Forms
Ophiuroids have changed very little over a 500 million period. The same body shapes and forms are still seen today,although there is greater diversification at a lower taxonomic level (at genus and species level). There are over 250 genera of ophiuroids described with more than 2000 species. Ophiuroidea are very abundant in modern oceans in a range of environments. They span the seas from the poles to the tropics at various depths. They are found in huge numbers at bathyal (continental shelf) and abyssal (deepest seas up to 2000m) depths, but are also found in inter-tidal areas. They are generally confined to normal saline conditions but a group of very adventurous brittle stars are now quite content living in brackish estuarine waters, this is unique among the echinoderms .
They have various feeding strategies which has allowed divergence of the species:
Ophiuroids have developed many defences against predation:
- Filter feeding - the long spiny arms are
outstretched into the water column to collect debris from the water,
this may be living zoo plankton or detritus floating in the water. They
use their flexible arms to pass the food back to the mouth. Basket
stars are especially adapted for this and use their branched arms as
nets to catch large amounts of food.
- Deposit feeders - they feed either by
selectively eating detritus that they find on the sea floor and rocks
or they non-selectively take in large amounts of sediment and remove
the nutrients they want and spit back out the rest.
- Scavengers - they will eat on plant or animal material that comes their way.
- Predators - some species will feed on small
bivalves but due to the lack of suction in their tube feet they do not
have the same success as starfish. Some species in the deeper parts of
the ocean have been known to predate squid and fish. They can
stand up on their tips of their arms to trap their prey. They can
attempt to eat more than their stomach can hold and may puncture their
- Symbiotic relationships - some basket stars wrap
themselves around corals and effectively provide them with a cleaning
service by filter feeding and detritus around the coral.
- They can drop their arms to escape when grabbed or on order to confuse their attacker.
- Due to their rowing and lashing movement of their arms they can move much faster than regular sea stars to escape predation.
- The size of the central disc allows them to fit into very small spaces to hide between rocks.
- Although they have no eyes they have developed sensory organs that detect changes in light intensity, if a predator casts a shadow they can move quickly to get out of the way.
- They also camouflage, dull colours will
blend in with the ocean floor, green colours blend in with algae and
brighter colours bland in with highly coloured corals.
Author: Lorna O'Brien
Last updated: 19/11/2006
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All images have be reproduced with the permission of the Southeastern Regional Taxonomic Center/South Carolina Department of Natural Resources
produced by students
on the MSc
Palaeobiology programme in the Department
of Earth Sciences at the University of Bristol for academic