Abstract
Many larval sessile marine invertebrates exhibit settlement preferences, and larval behavioral responses to cues at settlement can ultimately influence the distribution of adults and an individual's lifetime fitness. Two epifaunal bryozoans, the invasive Membranipora mem- branacea and the native Electra pilosa, commonly co-occur on kelp species in the subtidal habitats of Nova Scotia, Canada. Outbreaks of M. membranacea have been linked to mass defoliation of the kelp canopy; however, E. pilosa has not been associated with any significant effect on its host substrate. To examine whether larvae of M. membranacea and E. pilosa exhibit settlement prefer- ence for a particular location along the blades of the kelps Saccharina latissima and Laminaria digitata, abundances of newly settled colonies were quantified at different locations along the kelp blade. Algae were sampled at 2 sites on the southwestern shore of Nova Scotia (The Lodge and Feltzen South) from September 2009 to October 2010, over one complete cycle of the annual life cycle of M. membranacea, and thus over a wide range of bryozoan percent cover. Settlers of both bryozoans were significantly more abundant towards the younger, more proximal regions of blades of both kelps across all sampling periods. These patterns did not vary seasonally with increasing colony density. Both M. membranacea and E. pilosa larvae showed preferential settle- ment, suggesting that they can detect small-scale differences in habitat quality at the scale of a single kelp blade.
Highlights
Spatial patterns in the distribution and abundance of sessile marine invertebrates with planktonic larvae are controlled in part by the non-random settlement of recruiting larvae onto substrates suitable for adult life (Hadfield 1986, Chia 1989)
The opposite was observed at Feltzen South, where peak abundance of settlers of E. pilosa was an order of magnitude greater than that of M. membranacea, but only on L. digitata (Fig. 1b)
Settlers of Electra pilosa were most abundant towards the proximal end of the blade; E. pilosa settlers were distributed across a wider age range of blade tissue than settlers of M. membranacea
Summary
Spatial patterns in the distribution and abundance of sessile marine invertebrates with planktonic larvae are controlled in part by the non-random settlement of recruiting larvae onto substrates suitable for adult life (Hadfield 1986, Chia 1989). In Nova Scotia, M. membranacea larvae typically settle between May and July with peak settlement occurring in September (Saunders & Metaxas 2007). Larvae of M. membranacea settle predominantly on kelp substrates (Yorke & Metaxas 2012), where they metamorphose into a pair of filter feeding zooids (ancestrula) from which additional zooids bud asexually, forming sheet-like colonies. These colonies rapidly encrust the kelp blade, and colony growth and reproduction continue throughout the summer months (June to August), with maximum colony abundance (percent cover) occurring in fall (September to October) (Saunders & Metaxas 2009a). Colonies begin to senesce in late fall and early winter (November and December), a process that is characterized by stasis and/or shrinkage, with
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