Echinoderm Reproduction & Larvae

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  • 0:00 What's an Echinoderm?
  • 0:34 Sexual Reproduction
  • 1:59 Asexual Reproduction
  • 3:19 Echinoderm Larva
  • 4:27 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

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. 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.

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