Echinoderms
Adult echinoderms are a familiar group even if they are rarely seen, with the most associated member of this group being a starfish. Numerous tube feet under the body is a characteristic feature of echinoderms with a "spikey" appearance on the top surface often with skeletal plates. Echinoderms are susceptible to dehydration and so will live low down on the shore or in rock pools. Species of the rocky and sandy seashore have a larval form living in the plankton. Most are long lived in the plankton and develop increasing complexity as they age, usually with more arms and spines to maintain a large surface area to reduce sinking in the water column. All are spectacular and show a bilateral symmetry before maturing to an adult when they take on the more primitive radial symmetry.
Skeletal plates along the edge of juvenile Luidia ciliaris
Tube feet on juvenile Luidia ciliaris
Brittlestars
The brittlestars, so called as they will easily "break" when handled, casting off arms, are probably the most abundant of the echinoderms living on the shore. Tiny juveniles can be especially abundant in rock pool crevices. Away from the shore carpets of adults may cover the seabed. It is therefore the most likely echinoderm larval form found in the plankton, the young ophiopluteus stage. As they age in the plankton the main arms extend longer and longer and at the point of maturation the central body metamorphoses into a juvenile brittlestar.
Ophiopluteus of a brittlestar and taken with DIC lighting. This shows up the beautiful calcite rods that form a type of skeleton.
Ophiopluteus undergoing metamorphosis with the new juvenile in the centre
Sea Urchins
The proximity of the sandy beach at Dale means that the pluteus larva of the Sea Potato Echinocardium cordatum, where the adults are common at low tide in the sand, is a fairly common example of the echinoderms to be found in the plankton samples during the summer (June -August). These will be young individuals and over time as they grow and age they tend to be moved by the currents and tide to off-shore locations. Those specimens will have grown multiple arms and extended skeletal plates.
Early pluteus of the Sea Potato, taken with phase contrast lighting
Starfish
Starfish can pass through several larval types depending on the species. Commencing with the bipinnaria it ends up as a complex brachiolaria. The Bipinnaria larva in a starfish is around 200 microns and typically found in spring although they are never common in the shore plankton. Later stages tend to form off-shore but on occasion will come in on a high spring tide. These are the brachiolaria larvae and grow to around centimetre but what happens here depends on the species. For Luidia sarsi the juvenile starfish grows on the brachiolaria, attached by an "umbilical" cord and eventually sheds the larva. In L. ciliaris the larva is absorbed into the starfish.
Early bipinnaria larva of starfish
Early bipinnaria larva
Luidia sarsi. Late brachiolaria larval stage with the new starfish growing attached to the larval body
The late brachiolaria larva being absorbed by L. ciliaris
Late form of a bipinnaria larva
Sea Cucumbers - Holothuria
The auricularia larva of the sea cucumbers (holothurians) is large and complex. Nearly 1mm long it is difficult to see under the microscope as it is so transparent. All the edges shimmer as they are edged in cilia. The stomach is the large organ in the centre and near is a trapped Sabellaria larva. Below is a closeup photo of the ossicles. This is the only specimen found in samples at Dale but this may be due to the short time they spend in the plankton and only a few species have planktonic larvae. Generally they are difficult to find.
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This was photographed in 12 sections with DIC light, each a composite of about 20 images. Then the 12 composites were stitched to create one image. To increase contrast the photo was reversed to make a negative.
Ossicles of the auricularia x400