The Blue Velvet Nudibranch, Casella atromarginata, also known as the Head Sheild Sea Slug, features a beautiful blue and black body with electric blue highlights around its edges. Its head is in the shape of a hammer. These nudibranchs are excellent flatworm eaters and are recommended if you have a flatworm problem. 

Unfortunately Blue Velvet Nudibranchs have a relatively low life expectancy and therefore should only be cared for by expert aquarists. They should be drip acclimated and introduced into the tank with the lights off. They are reef safe, peaceful, active at night, and may sometime secrete trace amounts of poisonous toxins. Blue Velvet Nudibranchs are best kept in tanks with plenty of live rock, sand, and flatworms (if lacking supplemental feeding is necessary). They should also be kept away from aggressive predators that may see them as a meal. Overall, the Blue Velvet Nudibranch is an attractive and unique invertebrate species that thrives well in established reef tanks.