Echinoderms are not random; instead they are

Echinoderms are a strictly marine group of invertebrates that include starfish, sea urchins, and sand dollars. Learn about both the asexual and sexual reproduction of different echinoderms.

What’s an Echinoderm?

Have you ever sat around wondering why Bob Barker never closed out an episode of The Price is Right by reminding you to get the world’s starfish and sea urchins spayed or neutered? Well, there is good reason for that.

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Even though the vast majority of the world’s echinoderms reproduce sexually, their methods aren’t the same as cats and dogs (and humans for that matter), so their reproductive success rates are far lower. So while you may never need to worry about an overpopulation of sand dollars on the beaches of the world, it is still important to develop an understanding of the echinoderm reproductive system.

Sexual Reproduction

There are five main groups of echinoderms:

  • Starfish,
  • Brittle stars
  • Echinoids
  • Sea cucumbers
  • Sea lilies

Most echinoderms are gonochoric, or individually sexed. Humans, cats, and dogs are all gonochoric, having one definite sex. Because both males and females are present, sexual reproduction is common.

It takes a few years, however, for echinoderms to become sexually mature and able to reproduce.Unlike in most mammals, which require physical contact for the exchange of reproductive materials and undergo internal fertilization, echinoderm species typically release their sperm and eggs right out into the water to make their own way. Millions of individual sperm and eggs may be released at one time.Fertilization takes place when sperm and egg encounter each other while floating freely in the water. These instances of sperm and egg release are not random; instead they are synchronized, usually in alignment with the lunar cycle or seasonal changes in water temperature or salinity.The success rate for these spawning events isn’t very high, when you factor in both the odds of a sperm and egg encountering one another in the vast ocean and also other animals feeding on the spawning materials. Echinoderm spawning events often set off feeding frenzies in the fish and coral communities.

After spawning, a few species of echinoderms will care for their fertilized eggs, most commonly the cold water starfishes.

Asexual Reproduction

The most common form of asexual reproduction in echinoderms is a process known as fragmentation. This is when an animal’s body is divided into two or more parts, and both become individual animals. It’s a lot like how a split earthworm can form two separate worms.

This is common in many of the starfish species and a few types of sea cucumbers.Starfish are well known for regenerating lost limbs, so it might not be surprising that they can reproduce themselves successfully without compromising the health of either (eventual) individual. But, just like the earthworm, they can’t just break off anywhere and start a new organism. Part of their central disk (the fleshy part of the starfish where their five limbs meet) must remain attached to each individual after division. A lone starfish leg is not going to be capable of surviving and regenerating into a whole starfish; a fragment of the central disk must remain attached for survival.It’s not only starfish that are capable of regenerating lost body parts.

Ever had the painful experience of stepping on an urchin? While you’re off cringing in pain, the urchin is starting the process of growing back the spines that you broke off. Brittle stars and sea lilies are also able to regenerate body parts. And sea cucumbers can even regenerate lost internal organs that they might have expelled during an attempted escape or attack.

Echinoderm Larva

The majority of echinoderms undergo a process known as indirect development, which means they grow and develop independent of their parents. This means that they go from the egg to larva to juvenile to adult stages all on their own.After fertilization occurs, the echinoderm larva begins to develop either into a free-floating planktonic form or a non-swimming form that is relegated to the ocean floor like its parents. Some of the planktonic varieties, which don’t so much swim as they drift in the ocean currents, will feed on their own yolk, whereas some that have less yolk will begin to get their nutrients from organisms in the water column much sooner.In the few species that experience direct development, the female parent is often responsible for the care of the young. The main reason for direct development in echinoderms is purely environmental – in locations where the conditions are extreme and cold, parental care ups the juvenile survival rate. In deep sea or polar regions, starfish, sea cucumbers, and sea urchins will protect their young by attaching them to the outside of their own bodies until they are developed enough to fend for themselves.

Lesson Summary

Members of the phylum Echinodermata are non-swimming, marine invertebrates that live on the ocean floor. There are five main classes of echinoderms:

  1. Starfish
  2. Brittle stars
  3. Echinoids (which include sea urchins and sand dollars)
  4. Sea cucumbers
  5. Sea lilies

The majority of echinoderms reproduce sexually, by releasing sperm and eggs into the water to be fertilized. These spawning events are usually synchronized to the lunar cycles and seasonal changes in water temperature and salinity.

Once fertilized, they can develop either through direct development or indirect development. Indirect development, in which the fertilized eggs develop from egg to larva to juvenile without any rearing from parents, is most common. Most species of echinoderms are also capable of asexual reproduction, primarily for the purpose of regenerating lost body parts.

In more extreme polar and deep sea conditions, some species will care for their young through the direct development process to better their chances of survival.Echinoderms are marine invertebrates made up of five classes: starfish, brittle stars, echinoids, sea cucumbers, and sea lilies. Most echinoderms reproduce sexually and develop through indirect development, although some reproduce asexually and develop through direct development.

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