Range: Oceans worldwide

Habitat: Reefs, rocky shores, muddy or sandy bottoms and seagrass beds

Size: Average 1 inch (2.5 cm) in diameter

Diet: Plants and animals

Threats: Fish and invertebrates

Lifespan: Unknown

Transcript:

Today on Animal Fact Files we’re discussing pencil urchins. Although they have the common name “urchin” and they live in the sea, these invertebrates aren't true sea urchins. They're separated from typical urchins because they have a more primitive form. In contrast to the multiple spines per plate on typical urchins, pencil urchins have only a single spine. It's worth noting, however, that there are true urchins which have the name “pencil urchin” due to their long, thin spines, such as the red slate urchin, however these “pencil urchins” are not the same pencil urchins described in this video. 

For further clarification, pencil urchins are different from typical urchins because their test, their outer shell, is less intricate and easier to see because they have fewer spines; their Aristotle's lantern, the organ they use for eating, is shaped in a way that they can't eat as much as typical urchins; they grow slower; and their tube feet are smaller, so they can't move around as easily. Pencil urchins also lack skin. 

The name “pencil urchin” comes from their spines’ resemblance to thick, wooden pencils. Although the pencil urchin isn't using its spines to write a novel, they do come in handy for wedging the invertebrate into hard rock and coral formations. Since their tube feet aren't as strong as those in typical urchins, pencil urchins can't as easily cling to surfaces, so they use their sturdy spines to keep them secure throughout the day. Pencil urchins live in reef habitats, rocky pools, and other hard bottomed places where they can find a hole to wedge themselves in. They also inhabit muddy and sandy bottom areas with little wave action where they don't have to worry so much about being tossed around by the ocean's current. 

There are nearly 500 described pencil urchins species. They're found worldwide and range from the shoreline to depths of 4,400 feet (1,350m) or more in some extreme cases. In size, pencil urchins average an inch (2.5 cm) in diameter, but that's not including their long spines which can more than double that length. Their spines are typically blunt at the end and smooth along the shaft, but some are serrated and some have oddly shaped ends, such as a fan or funnel. They're often tan but can range into subtle pinks and other warm colors. What's more, a pencil urchin’s spines can act as habitat for other animals! Tube worms, barnacles, and other small invertebrates use pencil urchin spines as homes.

Pencil urchins are omnivores: they eat plants and small animals. In turn, they’re eaten by other invertebrates such as sea stars as well as fish including triggerfish. Their eggs and larvae are even more at risk of predation as they simply float through the water column at the whims of the tides. Filter feeding animals can easily catch these young pencil urchins before they even have a chance to settle to the seafloor. Pencil urchins have sexual reproduction. Males and females release their sperm and eggs into the water simultaneously. There’s evidence that pencil urchin spawning occurs during the warmest parts of the year and may be affected by the cycles of the moon, which also impacts tidal patterns. The eggs are large with big yolks because the larvae don’t eat, so they need nourishment until they develop further. From initial fertilization, it takes about four days for a pencil urchin to settle to the seafloor.

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