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