These baby Brittle Starfish will not exceed 1.5" in growth. I've seen them come in various colors, black & striped, brown, tan, and white. If you look closely in your refugium you'll see them in the rockwork, the substrate and even the macro algae. They can be quite small, and can inhabit the display tank as well. They eat detritus and are excellent sand sifters. Reef-safe.
The Harlequin Starfish is a beautiful and unusual creature. I placed one in my tank several months ago, and it has done very well ever since. What I found ironic was when the Harlequin Starfish and the Harlequin Shrimp gathered together. The shrimp is known to eat starfish (preferably Linkia, Chocolate Stars, and similar), but in this case there was no risk at all. Reef-safe.
One tiny creature that is often asked about is the Peanut Worm. It can extend out several inches as it seeks food late at night. Once it detects any light at all, it will quickly retract within itself (like laundered socks turned inside out) until it is deep within its protective nook. Once fully out of sight, the hole it bored out will be perfectly round. I've seen many of these over the years, but have never captured a good picture. In this image, most of it has already withdrawn into the rockwork.
These large fluffy feather duster-looking heads belong to the Coco Worm. When you buy one, you'll get a thick, hard, calcified tube that the worm resides in, and this crown is what you can look forward to in your tank. Coco worms are filter feeders, so dosing phytoplankton would be beneficial. They move to and fro in gentle flow, and add nice color to the tank. Reef-safe
These are Christmas Tree Worms - Spirobranchus giganteus, living in a colony of Porites. They are symbiotic, in that once the coral dies, the worm does too. Christmas tree worms come in many colors, and you may obtain a colony with four, five, or six different colored trees. When frightened, the worms will quickly retract into the coral, pulling their crown out of sight. The crown is used to trap plankton-sized food from the water for its meal. Reef-safe
Below, more Christmas Tree Worms living in some pocillopora.
A friend of mine discovered this strange earthworm-looking creature in his sandbed, devoid of the typical bristles we see. It is a capitellid worm, normally found in oxygen-deprived areas. It eats detritus and waste we don't want in our tanks. For more discsussion and pictures, here's the discussion thread. Reef-safe.
Photo by Agu Lukk
Blue Linkia starfish are very pretty, but difficult to acclimate successfully. Some people suggest not to touch it with your hands, don't allow any part of it to contact air, and acclimate it slowly for 6 to 8 hours before placing it in your tank. If you try one, I wish you success! Reef-safe.
Typically only purchased as a 'hitchhiker', the Striped Baby Brittle Starfish is a nice surprise if found in your reef tank or refugium. They will not grow as large as you might hope. The center will get as big as an eraser head, and with tentacles between 2" to 3" (from tip to tip). These are good detrivores (detritus consumers), but will also capture solid foods when available. Look closely in your rockwork and you may see a few arms extended, or in your macro algae. Reef-safe
Above, you can see the arms of Baby Brittle Starfish. These come in many colors and sizes, but usually they are less than 3" from tip to tip. My reef is packed with these little guys who keep it clean of detritus. They do try to catch floating foods during feeding time, as I can see their little arms coming out of every crevice. For a sense of scale, the coral above is a closed up Star Polyp colony. They do not grow up to be enormous like other Brittle Stars. Reef-safe.
This interesting critter is found in most reef tanks near the rockwork or in the substrate of the refugium. The worm (possibly a Chaetopterid or Spionid) secretes a mucous to build a small tube with sand grains (two tubes visible in this image), and extends two 'palps' for feeding. The worm itself is very small (1/10" to 1/4") but its tentacles can be up to 10 times longer than its body. Chaetopterid is pronounced "key-top-terid." Reef-safe