Appendix:

Definition of a ‘stressor’:

We define a biotic stressor based on previous definitions used by Ban et al. (2014), among other authors: any environmental change in a factor that causes some deleterious response by an organism or population of interest. Other authors (e.g., Townsend et al. 2008, Piggott et al. 2015) require that the stressor is anthropogenically influenced, or exceeds “normal” levels of occurrence. We do not adopt those modifications, however, we note some similarities between vermetids and another "stressor", the crown-of-thorns starfish (COTS, Acanthaster planci). Ban et al. (2014) refer to COTS as a biotic stressor because their populations periodically exceed their normal range, their increases have been linked to anthropogenically-elevated nutrients levels in coastal waters (Birkeland 1982, Fabricius et al. 2010), and they have deleterious effects on corals. Vermetid gastropods also appear to fit this more stringent definition of a biotic stressor and may offer a parallel to COTS:

a) Vermetids can harm corals. Although our study did not find a negative effect of vermetids in isolation, previous studies in Mo'orea and other systems have found that vermetids reduced coral growth and survival and alter coral morphology (e.g., Colgan 1985, Zvuloni et al. 2008, Shima et al. 2010, Shima et al. 2013).

b) Vermetids may respond to anthropogenic activities (Zvuloni et al. 2008, Shima et al. 2010). Conversations with local Polynesians (who harvest vermetid gastropods for consumption and sale) indicate that vermetid populations had increased dramatically in Mo'orea since the 1980s and up until the time of our study (Shima et al. 2010, Brown et al. 2016). This local knowledge was supported by unpublished data from Y. Chancerelle and B. Salvat (cited in Shima et al. 2010) that showed densities of C. maximum increased approximately 200-fold in the lagoons of Mo'orea from 1997 to 2008.

c) This increase may have been due to anthropogenically driven eutrophication of coastal waters (as suggested by Zvuloni et al. 2008). Mo'orea has experienced increased coastal development in recent decades, including an increase in pineapple farms and shrimp aquaculture farms, which has contributed to nutrient discharge onto the reefs via terrestrial runoff (Duane 2006, Lin and Fong 2008). Populations of sessile filter feeders in other coral reef systems have been recorded to increase in response to anthropogenically elevated nutrients and increased particulate organic matter (Smith et al. 1981, Costa et al. 2000). Vermetids (sessile filter feeders) may similarly benefit from anthropogenic eutrophication due to increased availability of foods like phytoplankton, zooplankton, suspended detritus, and particulate organic matter (sensu Zvuloni et al. 2008). This food availability would not only benefit adult Ceraesignum maximum vermetids, but also their larvae, which are obligate planktotrophs that display an intermediate larval feeding strategy (Phillips 2011). Because terrestrial run-off is typically high in both nutrient and sediment loads (Fabricius 2005), it is plausible that vermetid densities and sedimentation may be positively correlated, though we recommend future work to assess what thresholds of sedimentation levels may benefit or harm vermetid gastropods.

Possible mechanisms by which vermetids affect corals:

The negative effects of vermetids on corals is most likely produced by a mechanism involving the vermetids’ mucus net, either through direct contact with or indirect covering of the coral’s surface (see Fig. A1 below). Several mechanisms have been proposed:

a) Corals increase mucus production upon contact with vermetid mucus (and, consequently, suffer energetic costs, since corals can exude up to 50% of zooxanthellae-derived carbon as mucus [Davies 1984, Wild et al. 2004]).

b) Vermetid mucus might host distinct microbial communities, or exposure to the net may shift the coral microbiome, leading to deleterious effects on corals.

c) Vermetid mucus nets might reduce water flow (and thus the exchange of gases, nutrients, or waste products) or access to suspended particulates (i.e., heterotrophic food sources).

d) Vermetid mucus might prevent the removal of sediments or particulate matter from the coral tissue surface (as indicated by the present study).

e) Vermetids mucus nets may contain harmful a chemicals harmful to corals, or may alter the water chemistry underneath beneath the net in a disadvantageous way that is harmful to corals.

These mechanisms, in isolation or in combination, may have driven the results we observed in our experiment. However, the mechanisms underlying vermetid-coral interactions are currently unclear and remain a subject of ongoing investigations.

Taxonomy of massive Porites in Mo'orea:

Accurate field identification of many corals is difficult due to variable morphology and lack of reliable field traits. Corallite structure can be used as a diagnostic tool in some cases, but requires removal from the field, bleaching, and microscopic examination. Edmunds (2009) inspected 20 juvenile Porites colonies in Mo'orea (similar to our experimental corals and collection habitat, but in a different location) and determined that 85% were P. lutea and 15% P. lobata based on corallite structure. However, in a detailed and expansive molecular phylogenetic analysis of Porites, Forsman et al. (2009) found that corals identified as the same ‘species’ that had the same morphology and skeletal structure could be deeply divergent. Thus, even corallite structure is an unreliable diagnostic character in this group.

Therefore, we suspect that the massive Porites, used in our studies and identified on the basis of gross morphology (i.e., mound-like in shape), were likely a mix of P. lutea and P. lobata, and possibly two rarer species, P. australiensis and P. solida (Edmunds 2009). Although our experimental corals may consist of different, albeit closely-related, species, and certainly different genotypes, the individual corals were randomly assigned to treatment groups; therefore, taxonomic or genetic variation should not bias our results, although it could add additional within-treatment variation.

Experimental coral collection and blocking:

Juvenile massive Porites corals (less than 5 cm in diameter) were collected from a shallow (1 – 2 m depth) area of the backreef near our study site. Care was taken to ensure that the collected corals were juvenile colonies, rather than fragments of larger colonies: they were small in size, spherical in shape, lacked jagged edges that are characteristic of coral colony fragments, and were attached to reef pavement surfaces that were not immediately adjacent to larger coral colonies. Collected corals were placed individually into seawater-filled plastic bags, and transported to the lab in a large cooler. Corals were kept in a flow-through water table prior to field deployment.

All corals were approximately spherical in morphology, but we anticipated that small differences in coral shape could influence sediment retention. Therefore, we defined blocks by matching corals based upon minor morphological characteristics: e.g., size and the degree of convexity/concavity, as well as presence of dimples/crevices/divots in the coral. However, vermetid mucus, rather than variation in coral morphology, proved to play a much greater role in sediment retention, as demonstrated by Fig. 2 of the manuscript and the absence of a statistically significant blocking term. Also, when we returned to the sites after 48 hours, we observed that sediment-only treatment corals had been cleared of sediments, while all of the corals that were exposed to vermetid mucus nets and sediments still had retained sediment.

We also note that although we used coral morphology to define blocks, these differences were very small (and had no demonstrable effect on coral growth). Thus, our study targeted one particular size-class and life-stage. We speculate that vermetids may have different effects on different size-classes of corals in two distinct ways. First, large mounding corals are more likely to develop concave areas over time (e.g., cracks/dimples/divots/indentations) that could accumulate more sediment than our small juvenile corals (Stafford-Smith and Ormond 1992), which may yield stronger deleterious impacts in combination with vermetid mucus, based on the results of this study. Second, and conversely, larger corals have a greater surface area and are, thus, likely to experience less relative vermetid mucus coverage than small juvenile corals. Partial mucus coverage could allow larger adult colonies to use energy reserves supplied from unaffected tissue to reduce harm to tissue covered by mucus-sediment-conglomerate (Oren et al. 1997). Quantifying the effects of vermetids on corals of different sizes would be a fruitful avenue for further research.

Study site habitat characteristics:

In our study area, the distance from the reef crest to shore is about 800-1000 m. The lagoon includes approximately 600 m of shallow (approx. 2 - 4 m depth) backreef (comprised mostly of large living or dead colonies [‘bommies’] of massive Porites) intermixed with sand, a deeper (20 – 30 m depth), 150 m wide channel, and then about 50 m of fringing reef along the shoreline. Our study site (-17.475444°, -149.805234°) was approximately 200 m inshore from the reef crest.

The bommies upon which we conducted our study occurred at a consistent depth (~2.5 m) along a 150 m transect that was roughly parallel to the reef crest and perpendicular to prevailing water currents. The sediments at the study site were composed mainly of medium calcareous sands intermixed with fine particles (see also Schrimm et al. 2004). Vermetid densities on the north shore of Mo'orea and in the vicinity of our study site can exceed >100 m-2 (Shima et al. 2010).

Sediment application and sediment characteristics:

Sediments in the backreef habitats of Mo'orea have been characterized for composition and analyzed for organic matter and nutrient content by Schrimm et al. (2004). The sediments found at the two barrier reef (backreef) sites of Schrimm et al. 2004 (sites T1 and T8) that are most comparable to our study site were comprised mostly of fine and medium grain sizes (i.e., 63 μm to 450 μm), and were microscopically identified as being composed of “carbonated, sometimes siliceous, biogenic fragments such as coral and coralline algae fragments, whole or broken mollusk shells and Halimeda fragments. Other constituents were represented by foraminifers, echinoderms, crustaceans, bryozoans, serpulids, alcyonarians and sponges fragments” (Schrimm et al. 2004). Another estimate of seafloor grain sizes in this locale come from the Moorea Long Term Ecological Research program. Bergsma 2010 (LTER Data) collected seafloor surface sediments from 16 sites in the Vaipahu lagoon and sorted them into size classes using sieves, and found the following average percent composition of grain sizes, by mass: <0.125mm: 2.3%, 0.125-0.25mm: 5.6%, 0.25-0.5mm: 20.4%, 0.5-1.0mm: 27.0%, 1.0-2.0mm: 27.8%, 2.0-3.36mm: 7.9%, >3.36mm: 8.9%. We compare these findings to the grain sizes of sediments collected from sediment traps by Stewart et al. 2006. They deployed sediment traps at a location near our study, laid flush to the seafloor substratum (thereby including sediments from lateral translocation, resuspension, and precipitation from the water column, to comprehensively represent the sediments that affect corals) and compiled grain size classes from 19 measurements over 24 days: 0.05-0.1mm: 20.2%, 0.25-0.5mm: 18.1%, 0.5-2.0mm: 23.4%, 2.0-40mm: 38.3%. Together, these studies reveal that sediments from the seafloor in this lagoonal area (what we used, and what Bergsma [2010] and Schrimm et al. [2004] measured) are a combination of the fine to coarse grain sizes that naturally land on corals (Stewart et al. 2006); they do, however, differ in their relative size composition.

In our study, every 1 - 2 days, approximately 11 g dry mass of sediment from the surface of the nearby seafloor was applied to each coral in the +sedimentation treatment. We applied sediments for a total of 47 days over the course of the 54-day experiment. However, much of this sediment immediately fell off of the coral. To assess the mass of sediment that remained on the coral tissue immediately after application, we conducted a laboratory study using the study corals upon completion of the field experiment. We first used photographs taken from a direct overhead perspective in the field to position the corals in a similar orientation in the lab. We then applied the sediment as we did in the field, and some of this sediment promptly fell off of the coral. The sediment that remained on the coral tissue was collected, transferred to an aluminum tin, dried in an oven for 24 hours, and weighed. We did this 3 times for each coral. We also used digital analysis of the photographs taken overhead in the field (which included an object of known size for scale) to measure the shadow area of each coral: the horizontally planed area of the coral tissue that would be capable of retaining sediment (see example in Fig. A2 below). We then used the retained sediment mass, frequency of application, and coral shadow area to calculate the daily sediment deposition rate (the mass of sediments that settle on the horizontal surface of the coral), which averaged to 54.2 ± 21.2 (mean ± SD) mg cm-2 day-1.

The sediment that remained on corals after application in the lab appeared visually similar to what remained on the coral after a typical in-situ sediment application; however our measured lab sediment accumulation rate was probably not a precise measurement of what was occurring in the field, nor was that our intention. Our lab measurements of sediment accumulation represent the maximum sedimentation rate that our field applications could provide (i.e., if there were minimal water motion); our in situ sedimentation rates were likely lower, but comparable, given that our sheltered, lagoonal study site experiences relatively low flow rates (compared to an exposed coastal environment). We sought confirmation that our application was ecologically relevant and did not exceed natural sedimentation rates.

Average sedimentation rates on coral reefs in the Indo-Pacific range from 0.1 to 228 mg (dry mass) cm-2 day-1 (Marshall and Orr 1931; Smith and Jokiel 1975; Schuhmacher 1977; Randall and Birkeland, 1978), with reefs that are not affected by human development having much lower rates than other sites (Rogers 1990). The sedimentation rate from a nearby backreef site in Mo'orea averaged 12.6 mg cm-2 day-1, but had a maximum recorded rate of 89.9 mg cm-2 day-1 over a six-year period (Alldredge 2012). Thus, our supplemental sedimentation rate of 54 mg cm-2 day-1 was within the observed range of sedimentation rates in the Indo-Pacific in general (Pastorok and Bilyard 1985, Rogers 1990) and in Mo'orea in particular (Alldredge 2012).

Variable effects of vermetid gastropods on coral growth:

While several studies have found vermetids to have deleterious effects on coral growth (e.g., Colgan 1985, Zvuloni et al. 2008, Shima et al. 2010, Shima et al. 2013), we were surprised to find no significant effect of vermetids on coral growth in the absence of added sediments. Our lab has observed (but not yet published) other studies that show variable effects of vermetids on corals. We are currently investigating the cause of this variation. Several explanations are possible:

a) Spatial or temporal variation in environments: Environmental characteristics may shift in space or time, which could alter coral-vermetid interactions (e.g., pH, salinity, water flow, temperature, nutrients). For example, sedimentation rates in Mo'orea vary considerably through time (Alldredge 2012), and, combined with the results of our study, suggest that variation in ambient sedimentation rates could produce temporal differences in vermetid effects. Densities of vermetids in our field study may also have been lower than those used in previous studies (e.g., Shima et al. 2010), although the high consistency of mucus coverage that we measured in our experiment suggests that spatial and temporal variation in vermetid density might not have been a contributing factor.

b) Phenotypic plasticity: It is also possible that corals responded phenotypically following prolonged exposure to vermetids (e.g., resulting in changes in their physiology, morphology or microbiomes) which could have increased their resistance to vermetids.

c) Selection: Given the increasing and high densities of vermetids in the region during the 2000 - 2010 time period, the corals that we used in our experiment may have experienced selection in which more vulnerable phenotypes may have died (or been in poor shape and thus not used in our study). As a result, our study may have used corals that were better able to withstand exposure to vermetids compared to studies conducted 6 years earlier (e.g., Shima et al. 2010).

Our results emphasize the need to better examine potential factors influencing coral growth in response to vermetids.

Heterotrophic feeding by juvenile corals on sediments:

The lack of a negative effect of sediments (in the absence of vermetids) was unexpected, given the many studies demonstrating detrimental effects of high sedimentation on corals (see Rogers 1990). The convex morphology of our experimental corals may have facilitated the clearing of sediments (Lasker 1980, Stafford-Smith and Ormond 1992) from corals receiving the sediment-only treatment. Indeed, we observed that these corals quickly shed large sediments, while retaining fine sediments on the coral surface. This fine sediment was likely small enough to be ingested by the corals’ polyps. Corals typically benefit from heterotrophic feeding (Drenkard et al. 2013), because photosynthetically fixed carbon from zooxanthellae do not provide corals with all required nutrients (Dubinsky and Jokiel 1994), and juvenile (vs. adult) corals are especially metabolically-demanding (Hughes and Connell 1987). Heterotrophic feeding by corals on organic matter within sediments has been either directly observed or implicated in the works of Anthony (1999), Rosenfeld et al. (1999), Anthony (2000), Mills and Sebens (2004), and Anthony et al. (2007). Furthermore, another field experiment in this study system found a positive effect of added nutrients on the growth rates of the corals Porites rus and Acropora pulchra (Gil et al. 2016), indicating that corals in our study system may be nutrient-limited in this backreef habitat.

The sediments found at the site of Schrimm et al. 2004 that is most comparable in habitat to our study site (T8) were of biogenic origin. The sediments (by dry mass) were comprised of 0.25% organic carbon (of which 83% was hydrolyzable organic carbon), 0.020% nitrogen, and contained 0.64 and 1.26 mg g-1 of amino acids and carbohydrates respectively: any of which could have served as a potential food source for the coral colonies (Mills and Sebens 2004).

Appendix Figures:

Figure A1. Photograph of a vermetid gastropod with an extended mucus net. Note the discoloration of the underlying coral tissue. (Photo credit: Osenberg-Shima-Frazer labs)

Figure A2. A photograph of an experimental coral, taken from a direct overhead orientation (Photo credit: Julie Zill). ImageJ software is being used to calculate coral shadow area (yellow outline).

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