An alpha 2-macroglobulin-like protein is the cue to gregarious settlement of the barnacle Balanus amphitrite

Many benthic marine invertebrates, like barnacles, have a planktonic larval stage whose primary purpose is dispersal. How these species colonize suitable substrata is fundamental to understanding their evolution, population biology, and wider community dynamics. Unlike larval dispersal, settlement occurs on a relatively small spatial scale and involves larval behavior in response to physical and chemical characteristics of the substratum. Biogenic chemical cues have been implicated in this process. Their identification, however, has proven challenging, no more so than for the chemical basis of barnacle gregariousness, which was first described >50 years ago. We now report that a biological cue to gregarious settlement, the settlement-inducing protein complex (SIPC), of the major fouling barnacle Balanus amphitrite is a previously undescribed glycoprotein. The SIPC shares a 30% sequence homology with the thioester-containing family of proteins that includes the alpha sub(2)-macroglobulins. The cDNA (5.2 kb) of the SIPC encodes a protein precursor comprising 1,547 aa with a 17-residue signal peptide region. A number of structural characteristics and the absence of a thioester bond in the SIPC suggest that this molecule is a previously undescribed protein that may have evolved by duplication from an ancestral alpha sub(2)-macroglobulin gene. Although the SIPC is regarded as an adult cue that is recognized by the cyprid at settlement, it is also expressed in the juvenile and in larvae, where it may function in larva-larva settlement interactions.

A review of some common Indo-Malayan and western Pacific species of Chthamalus barnacles (Crustacea: Cirripedia)

The type specimens of the common tropical intertidal barnacles Chthamalus malayensis and C. moro, were re-investigated and compared with other specimens of Chthamalus from the Indian Ocean, Indo-Malaya, northern Australia, Vietnam, China and the western Pacific, using ‘arthropodal’ as well as shell characters. Chthamalus malayensis occurs widely in Indo-Malayan and tropical Australian waters. It ranges westwards in the Indian Ocean to East Africa and northwards in the Pacific to Vietnam, China and the Ryukyu Islands. Chthamalus malayensis has the arthropodal characters attributed to it by Pope (1965); conical spines on cirrus 1 and serrate setae with basal guards on cirrus 2. Chthamalus moro is currently fully validated only for the Philippines, Indonesia, Taiwan, the Xisha (Paracel) Islands, the Ryukyu Islands, the Mariana Islands, the Caroline Islands, Fiji and Samoa. It is a small species of the ‘challengeri’ subgroup, lacking conical spines on cirrus 1 and bearing pectinate setae without basal guards on cirrus 2. It may be a ‘relict’ insular species. Chthamalus challengeri also lacks conical spines on cirrus 1 and has pectinate setae without basal guards on cirrus 2. Records of C. challengeri south of Japan are probably erroneous. However, there is an undescribed species of the ‘challengeri’ subgroup in the Indian Ocean, Indo-Malaya, Vietnam and southern China and yet more may occur in the western Pacific. The subgroups ‘malayensis’ and ‘challengeri’ require genetic investigation. Some comments are included on the arthropodal characters and geographical distributions of Chthamalus antennatus, C. dalli and C. stellatus