Tantulocaridans: Tantulocarida

Tantulocaridans (tan-too-loh-KAR-ee-dans) are very strange-looking animals that are external parasites on other deep-sea crustaceans. External parasites spend most of their lives attached to the bodies of their hosts. The larvae (LAR-vee), or young animal form, are very, very small, measuring from 0.00335 to 0.00709 inches (0.0085 to 0.0018 millimeters) in length. The head does not have any appendages, except for a pair of antennae that are present only during one larval stage. The mouthparts are platelike in appearance. The body trunk has six thoracic and two abdominal segments. There are six pairs of thoracic limbs that are used for swimming. The first five pairs are branched, or biramous (BY-ray-mus). The sixth pair is not branched, or uniramous (YU-neh-RAY-mus). The abdomen does not have any appendages underneath, but does have a pair on the very tip.

Adult females come in two distinct forms. Forms that do not mate to reproduce measure up to 0.08 inches (2 millimeters) long and have a small, narrow head with a small projection, or rostrum. They use their suckerlike mouthparts to attach themselves to the external skeletons (exoskeletons) of their hosts. The sacklike thorax is filled with eggs or developing larvae and does not have any limbs. There are no body openings. To escape the sacklike thorax, the larvae must break out through the adult female's body wall.

Females that mate to reproduce are smaller, less than 0.02 inches (0.5 millimeters) in length. Their bodies have two distinct body regions. The head and thorax are tightly joined, or fused, into a single body region called the cephalothorax (SEH-feh-lo-THOR-acks). The cephalothorax has a pair of antennae and a reproductive opening, but does not have any mouthparts. Small numbers of eggs are carried inside the cephalothorax. A five-segmented body follows the cephalothorax. The first two segments of the trunk each have a pair of biramous thoracic limbs that are used for grasping. The fifth and last segment has a pair of long, slender appendages. Adult males are similar in size and appearance to the adult females that mate to reproduce. They have six pairs of biramous limbs used for swimming.