Five hundred million years to mobility: directed locomotion and its ecological function in a turtle barnacle

Movement is a fundamental characteristic of life, yet some invertebrate taxa, such as barnacles, permanently affix to a substratum as adults. Adult barnacles became 'sessile' over 500 Ma; however, we confirm that the epizoic sea turtle barnacle, Chelonibia testudinaria, has evolved the capacity for self-directed locomotion as adults. We also assess how these movements are affected by water currents and the distance between conspecifics. Finally, we microscopically examine the barnacle cement. Chelonibia testudinaria moved distances up to 78.6 mm yr-1 on loggerhead and green sea turtle hosts. Movements on live hosts and on acrylic panels occasionally involved abrupt course alterations of up to 90°. Our findings showed that barnacles tended to move directly against water flow and independent of nearby conspecifics. This suggests that these movements are not passively driven by external forces and instead are behaviourally directed. In addition, it indicates that these movements function primarily to facilitate feeding, not reproduction. While the mechanism enabling movement remained elusive, we observed that trails of cement bore signs of multi-layered, episodic secretion. We speculate that proximal causes of movement involve one or a combination of rapid shell growth, cement secretion coordinated with basal membrane lifting, and directed contraction of basal perimeter muscles.

The evolutionary diversity of barnacles, with an updated classification of fossil and living forms

We present a comprehensive revision and synthesis of the higher-level classification of the barnacles (Crustacea: Thecostraca) to the genus level and including both extant and fossils forms. We provide estimates of the number of species in each group. Our classification scheme has been updated based on insights from recent phylogenetic studies and attempts to adjust the higher-level classifications to represent evolutionary lineages better, while documenting the evolutionary diversity of the barnacles. Except where specifically noted, recognized taxa down to family are argued to be monophyletic from molecular analysis and/or morphological data. Our resulting classification divides the Thecostraca into the subclasses Facetotecta, Ascothoracida and Cirripedia. The whole class now contains 14 orders, 65 families and 367 genera. We estimate that barnacles consist of 2116 species. The taxonomy is accompanied by a discussion of major morphological events in barnacle evolution and justifications for the various rearrangements we propose.

Microsatellite DNA markers applicable to paternity inference in the androdioecious gooseneck barnacle Octolasmis warwickii (Lepadiformes: Poecilasmatidae)

The gooseneck barnacle Octolasmis warwickii has a rare sexual system called androdioecy, in which hermaphrodites and dwarf males co-occur. It has been hypothesized that dwarf males can coexist with conspecific hermaphrodites when dwarf males are capable of leaving more offspring than hermaphrodites via male reproduction. This hypothesis of reproductive superiority of dwarf males can be validated by comparing the reproductive success between dwarf males and hermaphrodites through DNA marker-based parentage testing. In the present study, we developed microsatellite DNA markers for O. warwickii, and evaluated the power of these markers to infer parentage based on simulation analysis. Using next generation sequencing, we obtained 344 microsatellite sequences suitable for designing primer sets for amplification in polymerase chain reaction (PCR). Of these, we examined the PCR amplification efficiency of 54 primer sets, of which 11 passed our primer screening in a population sample (n = 35). The developed markers exhibited moderate to high levels of polymorphisms, and met Hardy-Weinberg equilibrium with little evidence of significant allelic association to each other. Our simulated paternity inference suggested that the combinational use of the markers allows a high resolution of parentage (success rate of > 99.9%) if all candidate fathers are available.

Hermaphrodites and parasitism: size-specific female reproduction drives infection by an ephemeral parasitic castrator

Sex can influence patterns of parasitism because males and females can differ in encounter with, and susceptibility to, parasites. We investigate an isopod parasite (Hemioniscus balani) that consumes ovarian fluid, blocking female function of its barnacle host, a simultaneous hermaphrodite. As a hermaphrodite, sex is fluid, and individuals may allocate energy differentially to male versus female reproduction. We predicted the relationship between barnacle size and female reproductive function influences the distribution of parasites within barnacle populations. We surveyed 12 populations spanning ~400 km of coastline of southern California and found intermediate-sized barnacles where most likely to be actively functioning as females. While it is unclear why larger individuals are less likely to be actively reproducing as females, we suggest this reduced likelihood is driven by increased investment in male reproductive effort at larger sizes. The female function-size relationship was mirrored by the relationship between size and parasitism. We suggest parasitism by Hemioniscus balani imposes a cost to female function, reinforcing the lack of investment in female function by the largest individuals. Within the subset of suitable (=female) hosts, infection probability increased with size. Hence, the distribution of female function, combined with selection for larger hosts, primarily dictated patterns of infection.