The Phylum Echinodermata is distinguished by organismal characteristics involving spiny-skin, radial symmetry, skin covered endoskeleton and calcareous ossicles. Given that all classes of this phylum are exclusively marine dwellers, they also have evolutionarily developed a specialized water-vascular system. This is inclusive of a plethora of canals, that are designed to serve as hydraulic systems for functions such as the extension of limbs, movement, gas exchange and feeding . Another fascinating aspect of Echinoderms is that, similar to many cephalopods, most exhibit a single, or many suction cups! Due to the structural makeup of these organisms, largely in part to their calcareous plates, they dry and preserve well as both specimens and fossils. So, luckily for you reader, there are many pictures and diagrams to go along with this post, enjoy!
Below is featured a dried sea star. Sea stars is a broad ecological term that refers to the entire Class Asteroidea. Sea stars typically exhibit five arms, which they rely upon for movement; these animals also utilize their limbs and the various forms of suction cups on them for the capture and drawing in of prey. Their diet includes but is not limited to: gastropods, bivalves and many annelid worms. Interestingly enough there are examples of Asteroidea in nature that externally protrude their stomach, to digest organisms they have captured and rest on top of. The aboral (superior) surface of the specimen shows the spiky-skin described above, jutting out from calcareous plated dermis. The small white structure, slightly off center to the left is the sieve plate or madreporite. This structure is the exterior opening to the water-vascular system of the sea star.
The oral (inferior) surface shows many interesting distinctions. Among them, ambulacral grooves lining the arms. Lining the ambulacral groove are several dozen podia, which are also described as tube feet and are the primary mechanism for movement and predation. In the inferior view, posted below, these are all of the tiny black apparati that appear sunken into the organism (this is due to the fact that this specimen is long dead). As one might speculate, these tube feet are the mechanism for prey entrapment and bodily movement, as briefly mentioned above.
Echinoidea is the class of Echinoderms that is inclusive of sea urchins, sand dollars and many variants in between. The spines observed on these groups of organisms are actually mobile which serve to enhance protection, feeding and aid in movement. Echinoidea are encased in an endoskeleton commonly called a test. The UCMP Berkley page on Echinoderm morphology tells us, that much like other endoskeleton exhibiting marine dwellers, the test is comprised of Calcium Carbonate . Other morphological characteristics to note are that sea urchins exhibit longer spines than other members of the class such as heart urchins and sand dollars. Where the latter are very short relative to the organism’s size .
Sea cucumbers are a unique divergent class from the rest that Echinodermata encompasses. Unlike their counterparts, their ‘skeletal’ structure is greatly reduced resulting in a soft body that is mailable and with many captivating traits. Without any arms, class Holothuroidea relies on the use of small tentacles that surround the mouth. They prey on miniscule food items that are afloat in the oceans or rest on the benthic level of the ocean floor .
Some unique aspects of sea cucumbers lie in their rather appalling defense mechanisms. Methods of prey deterrent that are far beyond our own physical limitations are executed by Holothuroidea with ease. If touched or squeezed they will rapidly shrink in size and project water from several pores all over their body. It is quite the shocking sight to witness. This, in addition to the fact that they are able to expel and regrow their internal organs, to preserve individuals of the population while predators feed on the expendable bits, is nothing short of astonishing. More on internal anatomy may be seen in the diagram below.
Not available in live or preserved specimens were the classes Crinoidea (Feather Star) and Ophiuroidea (Brittle star/basket star). Crinoidea are also inclusive of sea lilies. The defining characteristic of this class is that they anchor themselves to a substrate through the use of cirri. These cirri are dually attached to a long stalk which keeps them in place, as most of the species comprising this phylum are sessile. There is an exception to this general rule in the case of Analcidometra armata, commonly called the swimming crinoid. This is a rare group of crinoids that may be observed across the Caribbean; they use their ten arms to gently manipulate water as a medium in order to become mobile . These Crinoids are particularly threatened and if seen they should not be touched!
Ophiuroidea are morphologically slenderer, with the dispersal of their body being distally stretched from the central disc. In the case of the basket stars, the structure of their limbs is highly branched, resembling capillary systems or an entire conglomeration of roots. The complexity of these limbs may be attributed to a volume vs. surface area evolutionary pressure. The ability to be able to reach out and capture large zooplankton prey without excess expenditure of energy is favorable to these organisms. The brittle star on the other hand exhibits almost-serpentine like movement of the arms, giving them the nickname serpent stars. The diet of these organisms is near limitless, as they will consume essentially anything that they can come in contact with.
A special note in the story of the brittle star comes from recent reports that these organisms may actually share a symbiotic relationship with corals. An observational study in 2010 showed deep sea corals in the Gulf of Mexico covered in a substance called floc, which is speculated to be an odd combination of trace amounts of oil, dispersants and excess mucus from stressed or sickly corals. Brittle stars were observed brushing off this substance which would have likely otherwise lead to the death of some very old coral reef colonies . The extent and mechanism of this relationship is still not fully understood. We might however begin to speculate that amongst the chaos currently afflicting the world, there is a natural order and balance that is forever in oscillation with the abstract forces of entropy. Animals may very well have a better sense of purpose and serenity than humans!
- Brusca, Gary J., Brusca, Richard C. 2003, Invertebrates 2nd ed. ISBN 0-87893-097-3
- De Klujiver, M., Gijswijt, G., De Leon, R., & Da Cunda, I. (2019). Swimming crinoid (Analcidometra armata). Developed by ETI BioInformatics
- Hall, D. (2018, May 10). A Brittle Star May Be a Coral’s Best Friend.
- Hyman, L. H. 1955. The Invertebrates. Volume IV: Echinodermata. McGraw-Hill, New York.
- Peachey, Donna & Gordon, The Biocam Museum of Life Series. Kelowna, B.C. Canada VIY 7N8 Box 417 PBC, 1999