Invasive tunicates fouling mussel lines: evidence of their impact on native tunicates and other epifaunal invertebrates

Vanessa Lutz-Collins

doi:10.3391/ai.2009.4.1.22

Abstract

Blue mussel Mytilus edulis aquaculture lines and associated gear provide habitat for sessile and mobile epifaunal fouling organisms. Due to food limitations and substrate space, these species are likely to interact among themselves and with mussels. In some areas of Prince Edward Island and elsewhere in Atlantic Canada, invasive sea squirts such as the vase tunicate, Ciona intestinalis, colonize mussel socks and rapidly become the dominant species in terms of abundance and biomass. A relevant question for these systems is which native epifauna are most affected by the growth of these invasive tunicates. Our study documents local variations in the abundance of vase tunicates and relates this information to the abundance patterns of three groups of native epifauna with distinctive levels of mobility: sessile tunicates of the genus Molgula, sedentary polychaetes and errant polychaetes. Following recruitment, large abundances of vase tunicates created a significant among-site variation pattern that was consistent over time irrespective of the season when the mussel socks were deployed. In contrast, native tunicates of the genus Molgula colonized the mussel socks in lower numbers and in a spatial pattern opposite to that of invasive tunicates. With the exception of one sampling period, sedentary polychaete colonization also displayed a negative relationship with invasive tunicates. Errant polychaetes displayed erratic patterns that were apparently unrelated to the other species studied. Overall, these results suggest that invasive tunicates have negative effects on many epifaunal species, particularly those that are sessile or have limited mobility.

Highlights

The deployment and growth of blue mussels in suspended line systems provides a hard, three-dimensional substrate that is quickly colonized by a variety of fouling and epifaunal organisms (Lutz et al 1991; Ellis et al 2002; Khalaman 2001)
In order to test the consequences of this hypothesis, we investigated the epifauna of mussel longlines deployed at two different seasons, and sampled in summer and fall when the invasive tunicates had established as the dominant species on the mussel socks
Spatial differences in average numbers of native tunicates (Molgula spp.) among sites were striking and directly opposite to the pattern for the average numbers of invasive tunicates: site C consistently showed the lowest average abundance of Molgula spp. (5.3 tunicates / 0.3 m mussel sock in August), whereas site A showed the highest average (110.9 tunicates / 0.3 m mussel sock, in the August sampling)

Summary

Introduction

The deployment and growth of blue mussels in suspended line systems provides a hard, three-dimensional substrate that is quickly colonized by a variety of fouling and epifaunal organisms (Lutz et al 1991; Ellis et al 2002; Khalaman 2001). Based on their post-colonization mobility, epifaunal communities are composed of sessile, sedentary and mobile organisms (e.g., Fauchald and Jumars 1979; Rosenberg 2001). This classification becomes relevant while addressing the effects of strong epifaunal competitors on community structure (Khalaman 2001), as the intensity of their effects will likely differ depending on the relative mobility of other members of the community

Methods

Results

Conclusion