Chemical settlement cues

Crisp and Meadows (1962; 1963) treated slate surfaces that were not readily settled by barnacles with an aqueous solution of whole adult barnacles. Cyprids were observed settling in numbers similar to those in areas where conspecifics were present, thus simulating the gregarious behaviour seen in the field. This study

for this behavioural aspect of larval settlement. The response was seen to greater degrees when newly-moulted individuals were used and there was a clear species- specific preference. Barnacle species have also been shown to respond to solutions of extracts from a number of related species, including other arthropods and some species of sponge. Even vertebrate (fish gut) extract offered a significant settlement preference; however these unrelated species were found in an area thought to favour recruitment success in barnacle so these species were not wholly unrelated (Crisp 1965). Soaked and unsoaked slates were dipped in a solution of the ‘arthropodin’ compound and settlement was shown to be preferential on the soaked slates. This result matched Knight-Jones’ finding that the response required the larvae to contact the substrate on which the substance had been absorbed. There was no evidence of chemotaxis in this study. This early work therefore laid the foundations for extensive investigation into the biochemical processes involved in gregarious responses in fouling organisms, particularly among cirripedes.

Extensive examination of the chemistry of the settlement-inducing compound was conducted by Gabbot and Larman (1987) following the early work on

‘arthropodin’ by Crisp, Knight-Jones and colleagues (Knight-Jones 1951; Knight- Jones 1953; Crisp and Meadows 1962). A brief review can be found in Clare and Matsumura (2000).

Some early evidence pointed towards responses in the swimming behaviour of cyprids when exposed to potential waterborne pheromones. It is suggested that a water-soluble factor may emanate from the adsorbed protein prior to cross-linking of cuticular proteins of newly metamorphosed juvenile barnacles. Partially purified seawater following conditioning with adult barnacles provided the first direct evidence of a waterborne pheromone. The response seen was, however, only of

temporary induction of adhesion to a surface. The compound has been isolated and described. The elicited response to a waterborne pheromone is less significant relative to the degree of response seen to the settlement-inducing protein complex described broadly as ‘arthropodin’ by early papers (Clare and Matsumura 2000).

According to Langmuir’s principle of independent surface action, the number of molecular layers required to produce a given field of force is very small, only one or two. Crisp (1974) argued that a cyprid will not be able to distinguish between a thin or thick film of extract. He demonstrates this with a comparative study of different monolayers of arthropodin.

The aims of this chapter are two-fold. First, to investigate the degree to which natural topographic variation alone effects settlement. Rugophilia is a long recognised phenomenon in settling larvae, particularlySemibalanus balanoideshowever no study to date has successfully investigated the patterns of settlement on isolated natural rock topography. A novel method for casting rock topographies in the field and then generating replicable tiles in the laboratory of uniform physical and chemical properties is demonstrated. Replicate tiles within blocks are examined with a latin square design for four topographic mimics to determine whether there is significant variation in settlement driven by natural topography alone, independent of differences in larval supply, shown to vary over very fine scales (Todd 2003).

Second, a series of artificial settlement substrates will be developed for use as part of a supply settlement array (Chapter 5) in a mesoscale study. These substrates must be affordable, repeatable and easy to deploy in the field. Crucially, they must provide a suitably desirable substrate so as to produce sufficient daily resolution to distinguish the characteristically high variation in larval settlement throughout the settlement season. For this reason they will be painted with adult extract, coloured

black and maintain a 3-dimensional component to their design. Machined Perspex and ceramic bathroom tiles will be investigated as suitable materials for artificial

settlement substrates. Two designs of machined Perspex and a pitted ceramic

bathroom tile will be examined for daily settlement per cm2in an attempt to produce the highest settlement per unit area and most suitable resolution in daily settlement variation. This will allow for the deployment of the tiles along a large stretch of coastline so as to distinguish variation in larval settlement daily and across metre and kilometre scales (Chapter 5).

In addition to these aims, settlement behaviour will be examined on the three dimensional surfaces of the ceramic tiles. The surface structure of the ceramic tiles provides three distinct areas within which settling larvae may explore and attach. Therefore preferences in settlement response can be investigated for larvae and observed over the season as a whole. Insufficient settlement was observed on the Perspex tiles for any behaviour to be investigated.

4.2 METHOD