No. 23–Echinoderms, Part I

The fact that estuarine life slows down in the winter months provides me the opportunity each year to reflect upon the hectic season past, assess what has been learned, consider possible applications for the acquired knowledge and also to catch up on my reading and internet studies. Lately I’ve been spending some time with my favorite tome on invertebrate zoology* and site it a source for this column. The fascinating phylum Echinodermata representing some 6,000 species has my attention at the moment.

Species of echinoderm, translated “spiny skin”, most frequently found on local beaches are the sand dollar (keyhole urchin), sea star and sea cucumber. Less frequently found are the dome shaped tests, skeletal remains, along with the spines of sea urchins which wash ashore to the puzzlement of many who find them. Sea biscuits also reside off shore but their tests rarely make the beach because of their weight and shape. A few species of brittle stars may be found in our high salinity estuary and fossils of the “common sand dollar” (no holes) may be found at The Sands Beach in Port Royal.

The most recognizable distinguishing factor shared by all echinoderms is their pentamerous radial symmetry. That means their bodies are divided into five roughly equal sections around a central core.

The burgundy colored, when alive, keyhole urchin wears a thin layer of skin and mussel with a felt like covering of tiny spines and irregular tub feet on the upper, surface with the bottom, surface covered by tube feed used in locomotion and feeding. The animal’s movement over and through the sand is powered by a water- vascular system with water drawn in through the flower shaped vents called petalliods easily observed on the aboral surface. Aside from powering the animal over and through the sand, oral tube feet also convey food to a central oral opening which contains a set of five beak like teeth. Close observation of the underside of a test will reveal faint channels along which tiny bits of organic matter are transported. The specialized tube feet associated with the aboral petalliods serve as gills of a sort for the exchange of oxygen and waste gasses. **

There are male and female sand dollars but the gender of an individual may not be determined by simple observation. Reproduction involves the release of sperm and eggs with external fertilization. If you observe the aboral surface of a test you will find a star at the center of the petalliods. There are tiny holes at four of the five points of the star from which the sperm and eggs emerge during spawning. Many years ago by sheer luck I happened to be passing a tank at the old North Street Aquarium when a spawning event began. I was enthralled as milky white strands arose from the stars of three of the four resident sand dollars then slowly dispersing and clouding the water. That was one of the moments of this calling that I cherish to this day.

One thing I have not found covered in the literature to date is the sand dollar’s innate ability to heal itself after being damaged. I have collected numerous tests over time that show healed fractures and epidermal regeneration on edges where chunks are missing. I have also observed live specimens in the field that have suffered such injuries yet appeared no worse for the wear. I will make inquiries to satisfy my own curiosity on this and share what I learn in a future column.

Please keep in mind that live sand dollars and all other living creatures found on our beaches are protected by law and their removal could result in a heavy fine.

I will be spending more time with the sources listed below and others in the preparation of next month’s column as we further explore the realm of spiny skinned wonders.

* Invertebrate Zoology – A Functional Evolutionary Approach – Ruppert Fox & Barnes – Seventh Edition

**Invertebrate Anatomy OnLine – Mellita quinquiesperforata – Richard Fox Lander University