*Article* **Naturally Occurring Rock Type Influences the Settlement of** *Fucus spiralis* **L. zygotes**

**William G. Ambrose Jr. 1,\*, Paul E. Renaud 2,3, David C. Adler <sup>4</sup> and Robert L. Vadas <sup>5</sup>**

<sup>1</sup> School of the Coastal Environment, Coastal Carolina University, Conway, SC 29528, USA


**\*** Correspondence: wambrose@coastal.edu

**Abstract:** The settlement of spores and larvae on hard substrates has been shown to be influenced by many factors, but few studies have evaluated how underlying bedrock may influence recruitment. The characteristics of coastal rock types such as color, heat capacity, mineral size, and free energy have all been implicated in settlement success. We examined the influence of naturally occurring rock types on the initial attachment of zygotes of the brown alga *Fucus spiralis* Linnaeus 1753. We also assessed the dislodgment of zygotes on four bedrock types after initial attachment in laboratory experiments using wave tanks. Settling plates were prepared from limestone, basalt, schist, and granite, found in the region of Orrs Island, Maine, USA. The plate surfaces tested were either naturally rough or smooth-cut surfaces. We measured the density of attached zygotes after 1.5 h of settlement and subsequently after a wave treatment, in both winter and summer. The pattern of initial attachment was the same on natural and smooth surfaces regardless of season: highest on limestone (range 7.0–13.4 zygotes/cm<sup>2</sup> ), intermediate on schist (1.8–8.5 zygotes/cm<sup>2</sup> ) and basalt (3.5–14.0 zygotes/cm<sup>2</sup> ), and lowest on granite (0.8–7.8 zygotes/cm<sup>2</sup> ). Patterns of survivorship following the wave treatment were similar to those of initial settlement with the mean survivorship varying from 60.1% (SE = 3.8) (limestone, smooth substrate) to 31.8% (SE = 0.59) (granite, natural substrate), and with the highest mean survival on limestone, basalt, and schist, and the lowest on granite. Our results suggest that rock type has a significant effect on zygote attachment and persistence. Patterns of attachment were the same on smooth and rough surfaces, indicating that surface roughness is not the predominant factor controlling the difference in successful attachment among rock types. Other properties of bedrock, possibly grain size, surface free energy, or chemical interaction with the adhesives used by the zygotes, directly affect the attachment of these algal propagules. These results suggest that patterns of benthic community structure could be determined in part by the distribution of bedrock types.

**Keywords:** *Fucus*; rock type; settlement; attachment; mineral composition
