Geographic distribution of Mytilopsis species
In a total, 143 occurrences of Mytilopsis species were retrieved from the literature survey (n = 45 for native and n = 98 for introduced areas). All the Mytilopsis records by specific locality are fully presented in the Supplementary Material S1. Among all the five Mytilopsis species, M. leucophaeata and M. sallei are the most widespread, contrasting to the other three species (i.e. M. adamsi, M. trautwineana, and M. africana) that showed more restricted geographical distribution. Mytilopsis adamsi were reported in few localities on its native range, while its introduced range varied from North America (Mexico) to the south of Africa (Mauritius), and Asia (Philippines and Thailand), with an overall latitudinal variation from 23°N to 20°S (n = 8 occurrence records). Mytilopsis trautwineana (n = 2 occurrence records) was reported as associated with shrimp farms in Cartagena, Colombia, a non-native area for this species, despite other unprecise records, as a generically mention for to the eastern Pacific coast of Colombia and Ecuador (Aldridge et al. 2008), within its native area. Mytilopsis africana were generically reported to the west coast of Africa, from Ivory Coast to Gabon, from 5°N to 0°, on the native range (Loeuff, 1999).
The native range of M. leucophaeata comprised brackish systems in the Chesapeake Bay (USA) and Gulf of Mexico (latitudinal variation from 39°N to 18°N), while its invaded range extends from South America (Brazil), Eurasia (from Spain to Iran), and north of Africa, following a latitudinal variation from 60°N to 23°S (Fig. 1; n = 88 occurrence records). The longitudinal variation was from 96°W to 76°W on native range, and from 74°W to 49°E on the invaded area. The native range of M. sallei comprised the Gulf of Mexico, Caribbean Islands, and the North of South America, with a latitudinal variation from 28°N to 10°N. However, its invaded range included the North of Africa (Egypt), Asia (from India to Japan), Oceania (Australia), and Pacific Islands (Fiji and Hawaii), following a latitudinal range from 35°N to 18°S (Fig. 1; n = 42 occurrence records). The longitudinal variation was from 88W to 61W, and the introduced range was almost all the globe around, from 30E to 34W (considering the records for M. cf. sallei).
Habitat environmental conditions in native and non-native areas
Characterization of environmental conditions associated with the native and non-native geographical distribution range of Mytilopsis species is presented as Supplementary Material S2. The multivariate analysis (PCA) applied on the environmental dataset (i.e. latitude, longitude, water temperature, dissolved oxygen, chlorophyll a, salinity, and transparency) evidenced preferential conditions for native and non-native Mytilopsis populations (Fig. 2). The first two PCA axes were selected by broken-stick (BS) method, with axis 1 and 2 explaining, respectively 36.6% (BS eigenvalue = 2.829) and 22.4% (BS eigenvalue = 1.829) of total variance. PCA axis 1 accounted for most of the differences between native and non-native geographical locations. Most of the samples from non-native populations (i.e. red color in Fig. 2) were located at the right side of the biplot and correlated with high values of water transparency (r = 0.856). This difference in environmental conditions between samples from native and non-native systems was overall supported by canonical analysis of principal coordinates (CAP trace statistics, p = 0.035). Moreover, PCA axis 2 accounted for the sample’s separation among Mytilopsis species. Most of the samples from M. leucophaeata distribution were located at the lower side of biplot and correlated with higher values of dissolved oxygen, both for mean (r = − 0.260) and range (r = − 0.520) values, latitude (r = − 0.774) and temperature range (r = − 0.649); while samples from the other three Mytilopsis species (i.e. M. sallei, M. adamsi, and M. trautwineana) were exclusively located at the upper side of biplot and correlated with higher values of salinity (r = 0.269), mean temperature (r = 0.407), longitude (r = 0.513), and chlorophyll a (r = 0.383). These differences in the environmental conditions between samples of M. leucophaeata distribution and samples from the other Mytilopsis species was statistically significant (CAP trace statistics, p < 0.0001).
Populational traits of Mytilopsis within native and introduced systems
The maximum values found for the populational traits (i.e. density and shell length) of Mytilopsis species are presented in Table 1. Moreover, all the compiled values (mean, minimum and maximum) of population traits for four Mytilopsis species (except by M. africana for which no data was found) evaluated are presented in Supplementary Material S1.
Table 1. Maximum values of density and individual size found for Mytilopsis species on native (N) and introduced (I) areas. No information was found for M. africana. A fully detailed compilation of populational traits is shown in Supplementary Material S1.
Species
|
Density (ind. m-2)
|
Size (mm)
|
N
|
I
|
N
|
I
|
M. leucophaeata
|
14,150
|
204,000
|
22.0
|
31.7
|
M. sallei
|
900
|
83,000
|
25.6
|
35.0
|
M. adamsi
|
-
|
569,000
|
-
|
32.0
|
M. trautwineana
|
-
|
-
|
-
|
24.0
|
(-) no data available
The density of Mytilopsis species in introduced aquatic systems was significantly higher than in native ones (Table 1; t-test, p = 0.012). Significant relationships between the density of native and invasive populations and environmental data were only found for two variables: values of longitude (Linear AIC = 44.26; F = 8.11, p < 0.01) and temperature range (Non-linear AIC = 47.88; F = 6.56, p = 0.017) (Fig. 3). A non-linear model for Mytilopsis density in relation to the range in water temperature was selected by AIC, where invasive populations (red color) showed higher densities, but more restricted to a lower temperature range (i.e. lower variation), with a tendency to decrease after a range of 15°C (Fig. 3b). A linear model was chosen by AIC for Mytilopsis species density in relation to the longitude of sampling site, where M. leucophaeata was more restricted to lower longitudinal areas and lower density within its native range and extended its longitudinal distribution and density on the introduced range (Fig. 3a). In contrast, the distribution of M. sallei was more restricted to higher longitudinal areas, where this species is invasive, with higher densities (Fig. 3a). Independently of the distribution range, populations of M. sallei showed significantly higher densities than M. leucophaeata (t-test, p = 0.025).
No significant difference was found for false mussel size (in shell length) between the most widespread species, M. leucophaeata and M. sallei (t-test, p = 0.207). However, significant relationships (GAMs) between size and environmental data were found for two variables: water transparency range (Non-linear AIC = 480.18; F = 9.87, p < 0.01) and temperature range (Non-linear AIC = 641.34; F = 8.75, p < 0.01) (Fig. 3). A unimodal response was found for the relationship of mussel size and with the range of temperature (Fig. 3d) and transparency (Fig. 3c), in which a maximum shell length was reached close to 15°C of temperature variation and 200 cm of water transparency, and then decreasing towards higher temperature range (i.e. broader variation) and water transparency.
Co-occurring taxa with native and invasive populations of Mytilopsis spp.
Information on the co-occurring epibenthic taxa with Mytilopsis populations in native and invaded systems were restricted to four species: M. adamsi (n = 3), M. leucophaeata (n = 25), M. sallei (n = 30), and M. trautwineana (n = 1). No information about co-occurring species with M. africana was found in the literature. A significant difference was found in the proportion of epibenthic taxa that co-occur with Mytilopsis populations between native and invaded areas (t-test, p < 0.0001). Mytilopsis has been recorded occurring with the greatest variety of epibenthic taxa in non-native areas. Seventeen epibenthic groups of ten different phyla were found to co-occur with Mytilopsis populations (Fig. 4), in which four groups have co-occurred exclusively with non-native false mussels (Nematoda, Nemertea, Platyhelminthes, and Porifera). Bivalvia was the most frequent taxa associated with Mytilopsis populations, regardless of the origin of the false mussel populations (i.e. native or non-native) and considering all the co-occurring groups, followed by Gastropoda for native populations and Cirripedia for non-native ones (Fig. 4).
Substrate colonization by native and non-native Mytilopsis populations
Non-native Mytilopsis populations colonized a wider variety of substrates, including the artificial ones, in their invaded systems compared to individuals within their native geographical distribution (Fig. 5). In the native area, M. leucophaeata was found mainly on soft sediment samples (42%), over benthic fauna (31%), and rocks (17%); while in the invaded systems, the most colonized substrates were human constructions (28%), rocks (24%), and plastic materials (13%). On the native range, M. sallei was found attached on mangrove roots (56%), rocks (19%), and sediment samples (19%); whereas in the non-native areas, the species occurred mainly on human constructions (35%), wood fragments (13%), and plastic materials (12%). No information regarding substrate colonization was found for M. adamsi within its native occurrence, but the species was mostly found attached on plastic materials (34%) within invaded systems. No information about substrate occurrence was found for M. africana and only two records for M. trautewiana, thus not displayed on Figue 5.
A significant difference in the colonization of natural and artificial substrates was found between native and non-native populations of Mytilopsis (CAP trace statistics, p < 0.001; Fig. 6). Non-native populations of Mytilopsis used significantly more artificial substrates for colonization in introduced areas. Most samples of substrate colonization within the invaded systems (red color) were located at the left side of CAP axis 1 and highly correlated with artificial substrates: human construction (r = − 0.64), rope and mesh netting (r = − 0.48), plastic material (r = − 0.47), and vessels (r = − 0.37). However, most samples of substrate colonization within the Mytilopsis native systems (green color) were located at the right side of CAP axis 1 and highly correlated with natural substrates: soft sediments (r = 0.47) and benthic fauna (r = 0.26).