Round 2 - Arthropoda - Thecostraca
(Sources - 1, 2, 3, 4)
Thecostraca is is a class of crustaceans, many of which have planktonic larvae which become sessile or parasitic as adults. The most well-known group are the Barnacles (subclass Cirripedia), but Thecostraca also includes the parasitic Ascothoracida, and the mysterious Facetotecta.
Facetotecta, comprising only the genus Hansenocaris, are known only from their larvae (image 3) and adults have yet to be recognized, though some scientists believe they may actually be larval tantulocaridans.
Ascothoracidans are parasites of echinoderms and cnidarians. Most genera are meso and endoparasitic (living inside the host) while some are ectoparasitic (living on the outside of the host). They are similar in anatomy to copepods, with six pairs of legs, an abdomen with four segments, a telson, and a bivalved carapace. They feed on their host via piercing and sucking mouthparts, and some more advanced species also absorb nutrients through the carapace. They are sexually dimorphic, in many cases so much so that the smaller males will live inside the larger female’s mantle cavity.
Barnacles (subclass Cirripedia) are more well-known than other Thecostracans. Adult barnacles are sessile filter feeders, except for the infraclass Rhizocephala, which are parasites of other crustaceans. Barnacles attach themselves to a surface as adults, be that a rock, the shell of a mollusc, a ship, or a large animal such as a whale. They come in two common forms: acorn barnacles which grow their shells directly on a surface (image 4) and goose barnacles which attach themselves via a stalk (image 1). Barnacles have a carapace made of six calcareous plates, with a lid made of four more plates. They attach themselves to the substrate by means of a cement gland at the base of their antennae. Eight pairs of thoracic limbs, called cirri, extend from the carapace to filter plankton from the water and bring it towards the mouth. The hairs on these limbs are very sensitive to touch, and help the barnacle sense the world around them. They also have three simple eyes (ocelli) which can sense changes in light, allowing them to close their plates quickly if a shadow is detected.
Thecostracans have nauplius larvae, characterised by a head with antennules, antennae, mandables, and a single eye, three pairs of limbs, a carapace, and a telson. Barnacle larvae are brooded by the parent until their first moult, after which they are released to swim freely using setae.
The oldest known thecostracan fossil is dated from the Middle Cambrian. Traces of the parasitic forms have been dated from the Cretaceous.
How do you feel about Thecostraca? (Remember to vote for your favorite animal within this group, not how you feel about the group as a whole!)
One or more of my favorite animals is in Thecostraca
I love one or more of these animals
I like one or more of these animals
I am neutral about all of these animals
I dislike all of these animals
I hate all of these animals
Propaganda under the cut:
