Opportunism and the resilience of barnacles (Cirripedia: Thoracica) to environmental change

Cirripede‐like organisms have their origins in the Palaeozoic, but until the Cainozoic, were represented primarily by pedunculated forms, such as the Scalpelliformes. Acorn barnacles (Balanomorpha) are first recorded after the Cretaceous–Tertiary extinction event. During the late Palaeogene, rapid radiation of cirripedes resulted in sufficient diversification for them to occupy most marine environments. That they survived both the Palaeocene–Eocene Thermal Maximum and the Pleistocene glaciation is testament to their ability to rapidly adapt to opportunities. The wide habitat distribution of balanomorphs in particular is unparalleled; they are known from the upper littoral (Chthamalus) to depths of 3600 m (Tetrachaelasma) and within this attached to rock, wood and miscellaneous flotsam, plus in symbiosis or commensalism with larger marine organisms. Darwin's (1854) view of the Tertiary as the age of barnacles is reflected in this diversity, distribution and biomass. All cirripedes are, nonetheless, at risk, from rapid habitat change, competition, pollution and, especially in light of their sessile habit, from predation. This paper assesses the viability of a number of cirripedes and concludes that the Lepadiformes, Scalpelliformes and Balanomorpha are the most resilient, and will most quickly adapt to occupy new niches when opportunities arise.