Topographical variation

Two general processes affect the settlement of many balanomorph barnacles. Oceanic and tidal currents determine the distribution and abundance of larvae across temporal and spatial scales. Secondly the barnacles settle according to cue-driven settlement behaviour, largely centring on the detection and selection of the substratum (Hillset al.1998).

Significant variation in settlement each day was consistent between blocks for the 15 days over which the Latin squares were deployed, demonstrating significant temporal variation in the settlement of cyprids on the rock casts. A number of authors have demonstrated that these characteristic peaks in settlement are driven by transport of the larvae from the larval pool to the shore by oceanic and tidal features (Hawkins and Hartnoll 1982; Caffey 1985; Pineda 2000).

According toTable 4.1all four Topographies exhibited consistent differences independent of Block demonstrating that natural differences in topography alone result in consistent differences in settlement in the field, independent of rates of larval supply at broader scales between blocks. The recognition that topographic

heterogeneity is an important factor in determining settlement at small scales is well- documented in the literature, but many of these studies involve the use of artificially uniform topographic features (Crisp and Barnes, 1955; Hillset al., 1998). Very few experiments have investigated ecologically real, natural topographic heterogeneity (for exception see Wethey, 1986). The present study also demonstrates topographic variation in settlement is consistent over temporal and spatial scales and differential rates of supply at scales relevant in the field.

Figure 4.4shows that topography 3 had a significantly greater settlement density and topography 4 had a significant lower settlement density. Topography 1 and 2 demonstrated an intermediate settlement density and were not significantly different from each other. Topography 1 and 3 both demonstrated a deep ridge ~ 1-2 mm deep with a gently sloped wall. Larvae were observed settling in these ridges and on the lower side of these features on both these tiles. No obvious features were observed on topographic casts 2 and 4. Topography 3 demonstrated consistently greater densities however it is not immediately clear why there was greater settlement on this topography relative to topography 1 as the latter demonstrated a more

extensive ridge structure. Topography 4 may have exhibited particularly low

settlement due to the slightly convex nature of the tile. The patterns of settlement that were observed demonstrate the complex variability in settlement natural topographic detail alone can produce in settlement studies. For this reason, standardised artificial settlement substrate are paramount to controlling for this variation in studies

comparing rates of settlement between and within shores and even rock blocks.

4.4.2 Artificial settlement substrate selection

Yule and Walker (1984b) showed that abrading Perspex surfaces increased the forces required to detach a cyprid. When abraded Perspex was combined with a treatment of barnacle extract an even stronger bond strength by the cyprids resulted. Differences were also seen in the attachment forces of cyprids on light and dark Perspex panels. The darker panels demonstrated an increase in the force required to detach the cyprids. It is unlikely that the latter demonstrates an increase in the

opportunity to bond with the substrate and suggests the cyprids are able to control the degree of their attachment according to preferences.

Trials of artificial settlement substrates here in 2003 suggested settlement of barnacle cyprids on the old panel design was as a result of settlement against an edge in the deep vertical grooves. It was felt that to improve the settlement densities on the artificial settlement substrate, increasing the grooves per unit area would lead to an increased rate of settlement per unit area in the 2004 deployment. This led to the second (‘new’) design of panel that had a far greater number of grooves per unit area. Following the comparison tests of the two panel designs, however, it was apparent that there was a significant decrease in settlement on the new design. It is clear that this was due to a difference in the groove design. Due to the increase in the number of grooves it was not possible to cut as deeply as for the original design because it would have significantly weakened the acrylic panel. A loss of 1mm of groove height

appears to have rendered the new design significantly less desirable to cyprid

settlement. This may be due to the altered turbulence of flow across the surface of the new panel. Mullineaux and Butman (1991) demonstrated a difference in settlement of Balanus amphitritecyprids on plain polycarbonate panels of differing thickness and polycarbonate panels with a 5 mm plate attached vertically at the downstream end of the plate. This was as a result of boundary-layer flows affecting the rates of contact and subsequent exploratory behaviour of the cyprids. These factors may influence the settlement of larvae on the Perspex panels in the present study, with active increased substratum rejection because of a less favourable boundary layer condition on the new panels, or increased rates of passive contact on the deeper grooved tiles due to greater depth of the turbulent boundary layer. The variation in settlement observed on the natural topographic casts also demonstrated in this study highlight this effect.

Topographic variation can produce unpredictable variation in settlement as a result of passive and active processes.

The deployment of the ceramic tiles lead to a marked increase in the rate of settlement of cyprids compared to the new panel design. The proportion of cyprids on the tiles compared to the new panel design was close to 6:1 in favour of the tiles. Comparisons of the two Perspex panel designs showed settlement was 4 times higher on the old design. This demonstrates the tiles perform better in terms of absolute settlement than either of the acrylic panel designs and the ceramic tiles can be

expected to perform 1.5 times better than the best Perspex panel design. It is assumed that this is due to a combination of deep grooves and the unique texture of the tiles. Acrylic, even when sanded and milled is a relatively smooth surface and appears to show low wettability (pers obs.). The porous, rock like nature of the tiles seems to be a more preferable surface for the attachment of the cyprids.

Besides Tentsmuir, where particularly high values of settlement were

observed, the new panel design did not have sufficient settlement to discriminate daily variation in cyprid settlement (Figure 4.6a-d). The ceramic tiles demonstrated

sufficient settlement for temporal variation to be distinguished. This is an important factor in selecting a settlement substrate for use in mesoscale analysis of temporal and spatial variation in settlement of barnacle cyprids within and between sites.

4.4.3 Settlement behaviour.

At relatively low settlement densities the cyprids settled against the edges of the pits. rugophilic settlement behaviour is well recognised in the literature and this drives the initial phase in the settlement preference cascade seen here. Once the pit edges are sufficiently full with cyprids that newly arrived cyprids cannot find any space against the wall during its exploratory phase, the cyprid individual will be forced to settle on less desirable surfaces; in the middle of the pits and finally, outside

the pits on the surface of the tile.Plate 4.8demonstrates typical patterns of settlement on the ceramic tiles for days of moderate and high settlement.

Plate 4.8Observed settlement patterns ofSemibalanus balanoidescyprids on the blackened, ceramic tiles painted with crude adult extract. At moderate densities cyprids settled against the pit walls (left). At high densities they filled the pits. Some cyprids would settle outside the pits on the high relief areas (“top”) of the tile (right).

Figure 4.8demonstrates that the cyprids occupy the middle of the pits

essentially only once the pit edges are saturated. Some settlement is observed outside of the pits on the high relief parts of the tile but this is clearly less preferable to the cyprids than in the pits themselves, whether against the pit walls or in the middle of the pits. From these data it is apparent that cyprids demonstrate a preference cascade when settling in increasing densities onto the pitted ceramic tiles.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 ₂ 2 ₂ 2 2 2 2 3 3 3 3 3 3 3 3

Figure 4.9Schematic representation of a cascading settlement response inSemibalanus balanoidescyprids on pitted ceramic tiles.

Prime settlement areas become more densely occupied and areas on the tiles that are less preferable to settlement begin to fill up. Pre-emption of space by other cyprids limits suitable settlement of larvae. Space pre-emption by other species or

adults is a common density-dependent process affecting settlement (Morgan 2001). Larvae that cannot settle are forced to delay metamorphosis whilst searching for more suitable substrate.

Settlement ‘choice’ is crucial to adult survival but presumably also larval survival because cyprids are lecithotrophic. As a result, extended delay of

metamorphosis can lead to settlement failure due to depletion of the lipid stores used for energy by the cyprid during this phase (Pechenik 1990). A cyprid may, therefore, become increasingly or suddenly “desperate” to settle (Marshall and Keough 2003). This may alter the shape of, or even overpower, a cascading settlement preference, most likely at high settlement densities or late in the season (Gribbenet al.2006).

Chapter 5 –Larval Supply and Settlement