Locations of green-lipped mussel attachment within macroalgal substrates on a mussel reef in New Zealand
Description
The goal of this study was to develop and test a tool that has the potential to aid restoration practitioners in the selection of suitable natural substrates for overcoming substrate limitation at mussel reef restoration sites in New Zealand and potentially elsewhere across the globe. One of the objectives of this study was to compare the relative performance of various macroalgae species with differing morphology as attachment substrates for three size classes of juvenile green-lipped mussels or Perna canaliculus on two remnant mussel reefs. To aid an understanding of why certain morphological characteristics of macroalgae support the presence of juvenile P. canaliculus at different ontogenic stages, the location of attachment within each macroalga species was characterised for each mussel size class. This data details the presence of three different size classes of juvenile green-lipped mussels or Perna canaliculus (<10 mm, 10 - <20 mm, and 20 - <30 mm in shell length) attached to different locations within macroalgae that were sampled on two remnant intertidal mussel reefs in northeastern New Zealand (Waipu Cove and Pakiri Beach). The six possible locations of mussel attachment on different macroalgae species were; the holdfast, stipe, lower branches, lower branch nodes, upper branches, and upper branch nodes. The presence of juvenile mussels in each size class attached to each location within each macroalga species is reported binomially (i.e., present: 1, absent: 0) in the "Juvenile Mussel Presence" column. For smaller juvenile mussels <10 mm, attachments to the branches and branch nodes of macroalgae were far more common than for juveniles ≥10 mm, suggesting that juvenile mussel attachment is more likely to be influenced by the branching morphology of macroalgae earlier in juvenile development. For juveniles <10 mm SL, attachments to the upper branches and nodes of macroalgae were more common at Waipū Cove than at Pākiri Beach, where smaller juveniles were more often attached to the lower portion of macroalgae. Lastly, macroalgal holdfasts were the most common location of attachment in this study for juvenile mussels ≥10 mm, and for juveniles smaller than 10 mm, it was as common of an attachment location as the branch nodes of macroalgae. These results suggest that juvenile mussels attached to macroalgae are likely to be influenced by a macroalga’s holdfast morphology throughout all stages of development but especially in later development.
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Between February and June of 2024, sampling was conducted in the intertidal during low tide on two remnant intertidal mussel reefs in northeastern New Zealand (Waipu Cove and Pakiri Beach), in which a series of 13 m alongshore transects were laid across the width of the mussel reef to ensure that a range of available macroalgae substrates were sampled for attached juvenile green-lipped mussels or Perna canaliculus (<30 mm in shell length). At 0.5 m intervals along each transect, 10 × 10 cm quadrats were placed over the substrate to standardise the area of macroalgae searched for cryptic juvenile mussels, resulting in 26 quadrats per transect. Each macroalga species with a holdfast located within the quadrat was searched along the entire length of its thallus for any P. canaliculus juveniles attached to it by byssal threads. Once a juvenile mussel was located, its shell length was measured using callipers. For each macroalga species within a quadrat, six possible locations were examined for the presence or absence of attached juvenile mussels within three size classes, i.e., <10 mm, 10 – <20 mm, and 20 – <30 mm in shell length. The specific location of juvenile P. canaliculus attachment within each macroalgal substrate was recorded to contribute to a comparison of the relative importance of certain morphological features of macroalgae to different developmental stages of juvenile mussels. The six locations within each macroalga were; the holdfast, stipe, lower branch nodes, lower branches, upper branch nodes, and upper branches. A branch node was defined as the 1 cm2 area around the point of connection between branches or between a stipe and branch. Holdfasts, stipes, and lower and upper branches were defined as 1 cm2 areas without branch nodes. To evaluate how the location of mussel attachment within substrates varied with mussel size class at each site, the presence/absence of attached juvenile P. canaliculus among macroalgae species was compared using a generalised linear mixed effects model with the binomial family function and logit link, used specifically for data with quasi-binomial distributions, in R. The effect of the factors of site, macroalga species, mussel size class, location of mussel attachment within macroalgae, and their interaction on juvenile mussel presence were assessed with quadrat incorporated as a random effect. Where the interaction of these factors was not significant, the interaction was removed and the model was re-run to assess the significance of the main effects alone using the greater statistical power gained from conserving degrees of freedom. Where the interaction of these factors was significant, post hoc comparisons were performed using Tukey tests to isolate differences in pairs of means through the emmeans function in R. Analysing the data binomially (i.e., presence as 1 and absence as 0) allowed the model to account for differences in sample size among macroalgal substrates.