A strange, round, white and hard organism has washed up on the beach. Looking carefully, you may notice a pattern on top of it, like a carved starfish⭐. What is this peculiar organism?
It is actually the skeleton of 𝘗𝘦𝘳𝘰𝘯𝘦𝘭𝘭𝘢 𝘭𝘦𝘴𝘶𝘦𝘶𝘳𝘪 (L. Agassiz, 1841), also known as the pink sand dollar. They are found on the shallow banks of the Indo-Pacific ocean coasts, including Hong Kong, and measure 13 to 15 cm in diameter. Sand dollars are actually a flat type of sea urchin, and thus belong to the Echinoidea class; a group that we also discussed here (https://bit.ly/3P2DIL7).
To find live specimens, one needs to dive 4 to 8 meters deep into the sea and start looking around on the sediment surface. While the lifeless specimen initially found on the beach was white, those here are pink, and their body surfaces are full of tiny spines. The starfish-like structures⭐ on top are called petaloids. This is where small structures called tube feet are located, allowing gaseous exchange for breathing.
If we observe carefully the side of the pink sand dollar facing the sediment surface, we can see how they are using their spines and tube feet to slowly select and move sediments towards their mouth👄, located at the center of their oral surface. If it may look like they are eating sediments, they are in fact carefully extracting the diatoms (a group of algae) and other phytoplankton species, as well as the bacteria living there.
As it keeps extracting its food, the pink sand dollar needs to move to access new patches of food. This is achieved thanks to their spines which are used for both feeding and locomotion🏃🏻. They can also burrow themselves under the surface sediments once again using their spines to stir up and turn over the sediments. This is important for the entire ecosystem, as it helps rework the sediments and improve conditions for benthic microalgae production by cycling nutrients like nitrogen. Therefore, sand dollars are actually ecologically significant since they are acting as sediment bioturbators. This positive action is actually increased during summer time as they move faster to eat more in order to fulfill their energy requirements for reproduction.
𝘗𝘦𝘳𝘰𝘯𝘦𝘭𝘭𝘢 species have an annual reproductive cycle, with spawning occuring in summer. During this specific period, sperms and eggs are released into the water at the same time. Fertilized eggs will then develop into larvae that only have 2 to 4 arms💪🏻. They don’t have a mouth and feed on food reserves contained in the egg yolk. The larvae will then undergo metamorphosis within 3 to 4 days to become juveniles, with a miniature version of an adult's external features, such as mouth opening, spines, tube feet and skeleton. After about 2 years, 𝘗𝘦𝘳𝘰𝘯𝘦𝘭𝘭𝘢 juveniles will attain sexual maturity, and the cycle will go on. Finally, 4 to 5 years into the future, you may find them as a distinctive specimen, lying on the shore with their purpose of life fulfilled.
Visit the HKBM to observe these beautiful sea creatures in person!
Text: Arthur Lai
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