Starfish are the most unique marine wildlife species that comes in various shapes, sizes and colors. They look iconic under water as well as classic when come floating on beaches. You can find them in tropical regions and deep sea, even in the icy polar areas also. Although easily recognizable, there are around 2000 species of these Echinoderms and got plenty of surprise to explore within them. So Let’s start,
1. Starfish are not actually fish. They don’t share any fish anatomy like gills, scales or fins. Instead they are echinoderms, meaning invertebrate with spikes covering the body. They share more similarities with sea Urchins, sea cucumbers and sea lilies. So, scientists prefer to call them Sea stars.
There are many sizes available, like the Sunflower sea stars (Pycnopodia helianthoides) of southeast pacific are 3.3 feet in diameter and weigh at a hefty 3.3kgs, whereas the Parvulastra parvivipara is smallest in all sea stars, being only 0.4 inches in diameter and found in South Australia.
They vary in colors and shapes also. Like bright red or orange and some species are blue, Grey or brown. The pink sea star (Pisaster brevispinus) is bright bubblegum pink and appears very attractive. Some species do have more than 5 arms, like the sun sea star has 40 arms while the Cushion Starfish have round blob like shape.
Starfish do have radially symmetrical body, all their body parts are arranged surrounding the central part. Hence, they don’t have any particular left or right side. The upper side is covered up by calcium carbonate scales to provide protection and some species also developed spikes on their body.
They don’t have brain and blood in their body. Their vascular system use sea water to pump in, providing the needed nutrients and allow all organs to function properly. It eliminates the need of a complex blood circulation system hence, saving space inside body. Not having brain doesn’t make them fool, they have a central nervous system which senses all body parameters even pain.
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6. Sea stars have eyes. Yes, close look reveals red dots at the end of their arms which are simple eyes. Actually each one is made of 80–200 simple ocelli. These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. These eyes don’t provide detailed pictures but able to sense light and dark. These are enough to help them navigate and keep aware of any danger.
But, surprisingly number of eyes can be varied depending on the number of arms of the species. As commonly sea stars having 5 arms have five eyes, whereas the sunflower sea stars have 40 arms and do have 40 eyes.
Most interesting is their eating habit. Starfish are one of the most fierce predator in the ocean, and they can eat preys larger than their mouth in an unusual method. Their mouth is located underside of the body so, they grab the prey with their arms and pull out the stomach outside the body. The stomach releases digestive enzymes into the prey to break down the tissues, then pull back the stomach in and eat the prey slowly. Yes, they digest their food outside their body.
Their diets include clams, oysters, arthropods, small fish and molluscs. Some are not pure carnivores, and often feed on algae or organic detritus.
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They have unique reproduction system. Males and females are hard to differentiate as they both look similar but they do release sperm and eggs in the water, the sperm fertilizes the gametes and produces swimming larvae which eventually settle on the ocean floor, growing into adult sea stars.
9. But, external fertilization is not the only way of reproduction. As they have all the needed organs in their arms, If a sea star loses a arm, the arm will regrow and develop as an fully identical sea star within a year. The donor sea star will regenerate it’s lost arm in a year also. It is very interesting that sea stars have not one but, two unique methods of reproduction.
They don’t swim, instead they walk on the ocean floor. They have hundreds of tiny feet under their arms, which get stretched by pumping in sea water, the water vascular system passes sea water through a sieve plate. It is a sort of trap door called a madreporite, often visible as a light-colored spot on the top of the starfish. Through madreporite, seawater moves into the tube feet, causing the arms to extend. Muscles within the tube feet are used to retract the limb. These feet help the Sea stars to move quicker than you might think.
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11. Due to their survival and unique reproduction techniques, sea stars can live up-to 35-40 years.
They can be seen on the shore or attached to rocks and pilings during low tide. But, they can’t stay without water for long period, specifically Salt water. Although, sea stars can survive for short time periods but, larger species can live up-to 28 hours out of water.
The Bat starfish also known as bat sea star, webbed star, sea bat, or broad-disk star got its name due to the similar look with bat wings where the arms have webbing in between. Normally, the bat starfish have 5-9 arms and the color ranges from orange, purple and green along the West Coast of North America, from Baja to Alaska.
There are many species of starfish that are deadly to humans, such as the crown of thorns, the sun star, the spiny sun star and the leather star. Specially the crown of thorns sea star, it has no mechanism for injecting the toxin, but the spikes are able to perforate tissue. It immediately causes sharp, stinging pain that can last for several hours, persistent bleeding due to the haemolytic effect of saponins, nausea and tissue swelling that may persist for a week or more. The spines are brittle that may break off and become embedded in the tissue, which are need to remove surgically.
Star fish are eaten in Japan, China and Micronesia. Other factors like habitat loss and pollution are drastically reducing the starfish populations. As a result, today they are considered as threatened species.