Topographic diversity and positional effects

In an attempt to address the complexity of accurately measuring and replicating realistic textured surfaces Hillset al.(1998) developed a method of replicating surface texture whilst maintaining the consistency of other confounding variables such as surface chemistry or colour. They used polyvinylsiloxane dental impression material to cast four textured surfaces. The surface textures were achieved by sieving four different sizes of sand particles onto pieces of flat wood covered in glue and leaving them to set. The casts were then used to manufacture replicate panels from green polyester resin that would contrast well with the brown of the cyprids during image analysis (Hillset al.1998). A similar method is used here to examine the effect of natural rock topography removed of adult barnacles on the settlement of Semibalanus balanoidesbarnacle cyprids.

Multiple layers of 5mm thick Perspex were clamped together and cut into 13x13cm squares. Two 1.5cm borders were drawn on the squares so that the second border was 3 cm in from the edge. The squares were then cut diagonally into

triangles. The outside border was then cut away, followed by the inside border. 1cm thick square frames were constructed from pairs of L-shaped Perspex borders by abutting the diagonal corners of each pair and then layering on top of a second pair. Each square’s abutting corners were arranged in opposition to the layer beneath. Using Araldite glue and clamping the layers as they set ensured a strong bond and a sold square frame 1cm thick.

Two sets of squares were produced, one having a external square edge of 10 cm and an internal square edge of 7 cm. The second larger square had an internal square edge of 10 cm and an external edge of 13cm. The larger squares were made

from four layers of Perspex, the smaller only two, which meant that when fitted back together, the internal square sat recessed by 1cm.

Before the smaller squares were assembled 1mm grooves were cut across one side of the L-shape half way along. When glued together into a square shape two lengths of 1mm microfilament nylon were laid in opposing grooves to form a cross in the centre of the square. The grooves ensured there was no gap between the layers caused by the nylon wire.

Four roughly 11x11cm areas of natural rock were scraped clean of adult barnacles, washed and dried. All four areas came from a single large boulder at Fife Ness (Plate 4.2). The smaller frames were placed onto the clean clearances and sealed to the rock by pushing water-soluble play-dough putty around the edges (Plate 4.3). Polyvinylsiloxane dental impression material (Type 3; low consistency – light bodied. Sds Kerr UK Ltd., Product # 28418) was applied to the rock surface inside the sealed frames to a depth that would cover the monofilament cross (Plate 4.4). Once the impression material had hardened the play-dough could be removed from the edge of the frames and the frames carefully lifted off the rock. The dental impression material was held within the frames by the monofilament and friction along the sides of the frames. This result was a negative copy of the specific rock topography (Plate 4.5).

In the laboratory the small frames containing the four casts of natural rock topographies were placed inside the large square frames after the inside edges of the large frames had been coated with J-wax. More J-wax was applied to the cast surface and the small frames. Unstained filled polyester resin (FC702PA Trylon;

www.trylon.co.uk) mixed with liquid catalyst hardener (product # 1010171; Trylon)

was poured into the large moulds over the top of the cast and the small frames and left to set overnight in a fume cupboard (Plate 4.6). The casts were then turned out

producing a positive cast of the specific rock topography. The resin casts were cut down into 5x5cm squares (Plate 4.7).

Each small frame containing the polyvinylsiloxane was carefully

disassembled, removing the monofilament from the natural rock casts (negative) whilst making sure they were kept intact. An 8x8x1cm box was placed bottom-up within a 10x10x5cm square plastic tub. The 5x5cm resin cast (positive) was placed, topographic detail face up, on the upside down box. The polyvinyl rock cast

(negative) was fitted to the resin cast (positive). (Figure 2.1a)

Flexil-S RTV-30C silicon and standard 5 % green catalyst (Jacobson Chemicals;www.jacobsonchemicals.co.uk) was poured into the tub containing the rock casts and box. This was left to set overnight in a fume cupboard. The following day the silicon was removed from the tub and the upturned box and resin ‘plug’ were removed from the silicon (Figure 2.1b).

Plate 4.2 Plate 4.4 Plate 4.6 Plate 4.3 Plate 4.5 Plate 4.7

Polyvinylsiloxane topography cast (negative)

Resin topography cast (positive) Upside down box

Silicone mould material Polyvinylsiloxane topography cast (negative) Silicone mould material approx. 5cm a b c _{Hardened filled} polyester resin topographic mimic

Figure 4.1Schematic diagram of the mould casting process. (a) shows the set-up after liquid silicon has been poured into the square tub. (b) shows the mould after it has been turned out of the tub. (c) shows the finished tile after it has hardened and been turned out of the mould. The drawings are not to scale.

The silicon mould was covered in J-wax and filled with polyester resin mixed with liquid catalyst hardener and standard black dye (product # 1010706; Trylon) was poured into the T-shaped square hole of each topographic mould. This was left to set for 3 h and then the tiles can be turned out and left to finish the curing process in a fume cupboard whilst more tiles were poured (Figure 4.1c). 4mm holes were drilled into two opposing sides on the border of the tiles once they had fully cured. The tiles were then soaked and leached in running tap water for 1 week.

For deployment in the field two pairs of 70x70cm Perspex sheets were used so that 16 tiles (four replicates of each of four topography casts) could be attached to it in a randomly allocated Latin square each day. At each of the 16 positions four pairs of holes were drilled parallel to the axis of the tile so that orientation of the tiles also

could be allocated randomly. Cableties secured the tiles to the Perspex mounting boards. Two rock blocks were selected at Fife Ness in 2005. A rope lattice (Todd 2003) was secured at both blocks so that the Perspex boards could be secured on the top surface of the blocks, lying horizontally. Each day the Perspex boards were replaced with a new set of cleaned tiles in a different Latin square formation. The settled tiles were taken back to the laboratory andSemibalanus balanoidescyprid densities per tile were recorded. The settlement ofBalanus crenatuswas very low (<1% of total) and so the presence of these cyprids was not recorded. When counted the tiles were scrubbed rigorously in tap water with a stiff bristle toothbrush to remove any trace of cyprids.