Evolution

Various predators feed on bryozoans, including sea spiders and sea slugs. As a result there has been selective pressure to evolve defences against these animals.

Defense

To defend against small predators, Cauloramphus disjunctus has evolved two features - avicularia and spines - that originate from extreme modifications of normal feeding zooids.

  • Avicularia in cheilostome bryozoans are zooids that have lost the ability to feed and instead use their jaw-like mandibles to deter would-be predators - the avicularia of Cauloramphus disjunctus are stalked, with jointed bases where they are attached around the edges of the feeding zooids
  • The feeding zooids of Cauloramphus disjunctus are encircled by about 20 overarching spines, which together form a protective grill over the vulnerable frontal surface of the zooid - each spine represents a zooid that has become highly specialized during the evolution of these bryozoans
Spines

Spines in primitive species of Cauloramphus were upright and spaced apart, but during the past 12 million years they changed in some species to overarch the feeding zooids, with spines from either side meeting in the middle to form the complete protection seen in Cauloramphus disjunctus.

Similar evolutionary pathways have been followed several times during the evolution of cheilostome bryozoans beginning almost 100 million years ago.

The basal attachments of the spines in Cauloramphus are almost identical to those of the avicularia. Scientists during the last century used this as evidence to suggest that the spines are themselves modified zooids, like the avicularia, and not just simple outgrowths of the feeding zooids.

Nested zooids

Despite these defensive structures, predators do sometimes succeed in eating individual zooids of Cauloramphus disjunctus. When this happens, new zooids can be budded into the space that is left. As this sequence of events is repeated, a nested set of zooids of ever decreasing size develops, reminiscent of a Russian Doll.