A new year and time for some new blog posts. And let’s start the year with something that is happening to the beached of the coast of North-Holland. The mass strandings of Sea stars, also known as star fishes. Mass strandings happen more often than you think, and mostly its due to environmental circumstances.
What kind of creatures are sea stars?
They are commonly known as starfishes, however they aren’t fishes at all. Sea stars are echinoderms, which are invertebrates, whom are characterized with a hard, spiny skin. Sea stars are closely related to sea urchins and sea cucumbers. On average sea stars can live up to 35 years, and can be found in all the world’s oceans. There are around 2000 different species of sea star. Most of these species have 5 arms, but there are species with more arms. Such as the sunflower sea star which can have up to 40 arms.
Sea star have some unique abilities, such as regeneration and the way they eat their prey. Sea stars have most of their vital organs in their arms. By having their vital organs in their arms they can regenerate an arm if they lose an arm. Some species can even grow an entirely new body out of the lost arm. For this to happen a part of the central disk needs to be part of the lost arm. Sea stars truly have an unique way of eating their prey, as they consume their prey outside of their bodies. They do this by putting their sack-like stomach through their mouth and envelop their prey with the stomach to digest, and then withdraws the stomach into the body. Sea star are carnivorous and feed mostly on molluscs like clams, mussels and oysters.
Mass stranding.
Early January I was walking along the beach with a friend and parts of the beach were filled with heaps of sea stars. All were common sea stars (Asterias rubens). Although, if you look closely to the photo in the left corner, you might see one brittle star (brittle stars have very thin snake like arms and are closely related to sea stars). I am used to seeing the beach being covered with shells, but couldn't remember seeing this many sea stars. Mass strandings on beaches happen more often than you think. Most of the time it is creatures like clam species which can be found in masses on the beach or coast. In the end of 2021 parts of the Dutch beaches were covered with Electra pilosa or hairy sea-mat, and around summer-time in 2021, it were jellyfish that littered the beaches.
We know that mass strandings happen, however what is the reason for a mass stranding?
Well, there are several possible reason for a massive stranding of sea stars or any other sea creatures, as mentioned before.
Most of the time mass strandings happen due to a combination of various environmental conditions. Sea stars, live on the seafloor and hold on to hard substrates like mussel beds or rocks. But with either a storm or a strong current it is possible that the sea stars get loose from the sea floor and swept away with the currents. During the winter the temperature of the sea also has an effect, as in colder waters sea stars can get paralysed by the cold and therefore cannot hold on to the sea floor as tightly as they normally would. In general most strandings for any sea creature happen when the wind blows from land towards the sea, (offshore wind). This offshore wind blows the upper layer of the sea away from the land, this then causes for the undercurrent to flow towards the land. During the high tide this under current and everything in the current washes up on shore. This is why in case of the Netherlands that you will find more interesting things, like clams, on the beach when the wind comes from the east. When the wind comes from the sea to the land (onshore wind), you'll be able to find objects that float, on the beach such as seaweeds and the cuttlebone of the cuttlefish (Sepia officinalis). The mass strandings are quite a natural phenomenon caused by either wind, currents or a combination of the two.
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