4.1. Genetic and Phylogeographic Structure
Genetic structuring analyses based on paired
ФST, Bayesian analysis, PCA, AMOVA, and ML trees determined the most likely number of clusters for the three marine species sampled along the southern Caribbean Sea (Colombia sector). However, only
N. tessellata presented one population (
K = 1), and its high dispersal potential reflected in a larval lifespan of more than 60 days [
54,
55,
56] may be the primary explanation for the absence of genetic structuring. The overall
ФST value was low and coincides with those determined for other marine species associated with the southern Caribbean ecoregion, which were estimated with various molecular markers (mitochondrial genes and SNPs, and microsatellite loci), mainly in the fishes
Stegastes partitus [
39],
Lutjanus synagris [
40],
Caranx hippos [
42] and
Micropogonias furnieri [
82], the rock boring urchin
Echinometra lucunter lucunter [
44], the southern white shrimp
Litopenaeus schmitti [
83], and the Caribbean sharpnose shark
Rhizoprionodon porosus [
84]. Although some
ФST values were significant, the levels of genetic differentiation between localities were too low to prove any phylogeographic break (
FST < 0.05), and the high dispersal potential of these species may be a reason for the lack of structure (PLD > 10 days). Other studies that extended the sampling coverage to other regions of the Great Caribbean documented that species with high PLD do not show a phylogeographic pattern [i.e.,
Echinolittorina ziczac and
Cenchritis muricatus, [
41];
Panulirus argus, [
85]], and some species were described to have a panmictic population pattern [
Sparisoma viride,[
21];
Clibanarius tricolor, [
22];
Sparisoma aurofrenatum, [
46]]. Overall, the timing of the larval period of
N. tessellata defined the phylogeographic structure with a single population. It also exhibits high genetic connectivity between localities due to the functioning of the Caribbean current and the Panama-Colombia countercurrent, which could generate larval dispersal along the southern Caribbean in the presence of the barriers evaluated (
Figure 1a).
Concerning
A. rivasi and
C. pica, pairwise
ФST, Bayesian analysis, and PCA allowed the identification of three populations (
K = 3), which were confirmed with AMOVA. The delimitation of the three populations is influenced by the putative barriers evaluated. For example, based on the
ФCT statistic of the AMOVA, it was possible to measure the level of genetic differentiation between the samples classified on either side of the barriers. For both species, the first population was in Cabo de la Vela and the second was in Santa Marta. The populations are separated by the barrier defined as a combined effect of the absence of the rocky bottom at a distance greater than 300 km from the coastline (ARB) and the permanent upwelling in La Guajira (hereafter ARB+PUG). This barrier significantly affects
A. rivasi (
ФCT = 0.406) and
C. pica (
ФCT = 0.224), but in a minimal proportion in
N. tessellata (
ФCT = 0.005). Finally, the analysis determined a third population between the localities of Cartagena, Isla Fuerte, and Capurganá, separated from the second by the barrier of the Magdalena River plume (MRP). This barrier significantly affects
A. rivasi (
ФCT = 0.420) and, to a lesser degree,
C. pica (
ФCT = 0.076): for the latter, it is a barrier permeable to gene flow.
Cittarium pica presents a PLD of less than six days [
52,
53]. This time may be sufficient for larvae produced in Cartagena to be transported to Santa Marta via the Caribbean Counter Current (CCC). This current, which originates from the Panama-Colombia Gyre [
48,
86], transports surface waters over the continental shelf to the north and northeast to the La Guajira front when the northeasterly trade winds weaken [
48,
86,
87]. The CCC may also be responsible for maintaining high genetic connectivity between Capurganá and Cartagena, where the third populations of
C. pica and
A. rivasi are found. It is assumed that the larvae of both species produced in the reef systems of the Cartagena (Tierrabomba, Barú Island, Rosario Islands, and San Bernardo archipelago), Isla Fuerte, and Capurganá sectors are transported by CCC [
87]. This current has been proposed as the oceanographic factor that facilitates the exchange of genetic information between populations of several marine species that are not significantly understructured in the southwestern Caribbean [i.e.,
S. partitus, [
39];
L. synagris, [
40];
L. schmitti, [
83];
R. porosus, [
88];
C. hippos, [
42];
Orbicella faveolata, [
38];
P. notialis, [
45];
Mugil liza, [
89];
Acropora palmata and
A. cervicornis, [
47];
Micropogonias furnieri, [
82]].
The phylogenetic analysis confirmed the previous phylogeographic proposal for
A. rivasi and
C. pica. For
C. pica, only two clades were configured in the ML tree, one on each side of the ARB + PUG barrier. Nevertheless, clade 2 shows how the samples from Santa Marta are more closely related but separated from the rest of the localities and formed a subclade within it (53% support). This ML tree suggests that
C. pica shows evidence of the effect of a single biogeographic barrier (ARB+PUG). In contrast, for
A. rivasi, the simultaneous impact of the two barriers evaluated (ARB+PUG and MRP) is observed. These results are interesting because the action of two barriers was identified for a reef species such as
A. rivasi less than 400 km from the coastline. Interestingly, species of the genus
Acanthemblemaria have a proposed PLD of 22 to 25 days [
51]. If this is true for
A. rivasi, under this dispersal scenario, it would be affected by only one of the two barriers, as in the case of
C. pica. However, this fish could have a PLD of fewer than ten days, although this must be investigated. Another reason for explaining the phylogeographic pattern of
A. rivasi is that
Acanthemblemaria fishes are characterized by parental care of eggs by males, low fecundity, and inhabiting invertebrate skeletal orifices embedded in rocks or corals in reef areas with high wave energy [
50,
51,
90]. In addition, these species show larval retention near coral reefs [
91,
92]. The reproductive and ecological characteristics mentioned above could generate high larval retention and high biological recruitment in the local populations of Cabo de la Vela and Santa Marta, which, added to the effects of ARB+PUG and MRP barriers, define the high level of genetic structuring observed.
The combination of physical, ecological, and biological aspects suggests a vicariant event for
A. rivasi because a new species distributed from Santa Marta to Venezuela was recently proposed [
A. aceroi, [
93]]. However, the high level of genetic differentiation observed among the three clades (Cabo de la Vela, Santa Marta, and Cartagena+Isla Fuerte+Capurganá; AMOVA:
ФCT = 0.495,
p < 0.05) suggests a taxonomic revision, which should include the mitochondrial molecular markers, meristic characters, and morphology to determine the existence of a possible additional species in the
A. rivasi complex.
4.2. Identification of Phylogeographic Breaks
The phylogeographic analysis of the three marine species determined the action of two putative barriers across the southern Caribbean (Colombia sector). For the first time, evidence is provided of a phylogeographic break caused by the Magdalena River Plume (MRP), mainly for
A. rivasi. The Magdalena River annually delivers 142 x 106 tons year
-1 of sediment to the Caribbean Sea. The MRP extends to 6.5 km offshore and is characterized by a high turbidity concentration (178.6 ± 78.7 mg L
-1), comparable to estimates for the Amazon and Yangtze rivers [
94]. Furthermore, MRP manages to dilute salinity from 10.8 ± 3.4 at the mouth to 28.4 ± 0.4 in the adjacent marine sector [
95]. These physicochemical conditions and their influence on the Caribbean Sea suggest that the PMR must act as a biogeographic barrier for marine species that are limited in their dispersal, mainly those whose pelagic larvae are unable to survive when attempting to cross this area. Perhaps they cannot tolerate the decrease in salinity, higher temperature (≥ 2 °C) than the marine waters off Santa Marta-Cabo de la Vela, and the high turbidity of the waters, which are possible factors that regulate the success of biological dispersal. The MRP likely operates as a filter for
A. rivasi larvae that attempt to cross it through the action of the CCC. However, the possibility that populations formed on either side of the plume have adapted to the different environmental conditions is not excluded. For example, those larvae that cross the MRP may be selected against because the marine waters on the western side are warmer and less salty than those on the eastern side [
96].
The MRP was proposed as a barrier across the southern Caribbean [
34]. However, many studies were inconclusive because some of them did not consider localities in Colombia [
32,
33,
35,
36,
37], and when it was possible, they only sampled one side of the MRP [
34,
38]. The hypothesis of the MRP as a barrier was rejected when studies included both sides of MRP and selected species with pelagic larvae enduring longer than 12 days (
S. partitus,
L. synagris,
L. schmitti,
C. hippos,
M. incilis,
P. notialis,
M. liza,
M. furnieri, A. cervicornis, and
A. palmata). Nevertheless, other world scenarios describe marine species with phylogeographic breaks caused by large river plumes. The Amazon River plume, for example, has been the subject of biogeographic studies on reef fishes, demonstrating the separation of the fish fauna of the Caribbean Sea from Brazil [
12]. Recent reviews also document it as a barrier to dispersing species from several taxonomic groups, generating a spatial pattern in marine species diversity between the Greater Caribbean and Brazil [
97]. From a phylogeographic point of view, some species are not affected by the Amazon River plume; for example, the reef fishes
Chaetodon striatus [
98] and
Abudefduf saxatilis [
99], showed genetic flow between localities in the Caribbean Sea and northern Brazil. The current from northern Brazil flowing toward the Caribbean, passing through the Amazon River plume platform, promotes the genetic connectivity of
A. saxatilis between both sectors [
99]. In contrast, the Amazon River plum has been shown to act as a barrier for the reef fishes
Chromis multilineata and those of the genus
Halichoeres, demonstrating levels of genetic differentiation between samples from the Greater Caribbean and localities from Brazil and the Central Atlantic islands [
100,
101]. This was also observed in the striped snapper
L. synagris when comparing samples from Colombia with Brazil [
24]. However, another phylogeographic break caused by a river plume has been documented. The limpet
Cellana toreuma is distributed along the coasts of China, where the Yangtze River plume causes a phylogeographic break for this species [
102].
The other finding was the effect caused by the absence of the rocky coastline for more than 300 km between Cabo de la Vela and the TNNP sector, which operates for
A. rivasi and
C. pica. The rocky coastline constitutes the specific habitat for both species, which heterogeneous distributions in the southern Caribbean [
44,
103]. However, the absence of the rocky coastline in this 300 km sector coincides with two upwelling areas. One is almost permanent in La Guajira, with strong upwelling during December-May and a weak effect during June-August when the temperature reaches a minimum of 24 °C (~72°W,
Figure 1A; see review in [
104]). The other upwelling develops between Santa Marta and TNNP and is usually seasonal (December to March), with a minimum of 24 °C [
48,
86,
104]. In both cases, surface currents transport upwelled water offshore [
48,
86,
104], which is then added to water transported by the Caribbean Current toward the Central Caribbean [
48,
104] (
Figure 1a). This oceanographic feature may be responsible for transporting larvae offshore [
104] and regulating the genetic exchange between the Cabo de la Vela and Santa Marta localities. Therefore, it could be a combined effect which generate the phylogeographic break in the two species. This barrier had not been documented with scientific support [
44,
46] and is interesting because the two species evaluated contrast in their larval life histories, a sign of the generalized effect of the barrier on the communities of marine organisms dependent on the rocky bottoms across the southern Caribbean [Aspect III, [
4]]. A similar situation occurs in Norway with the rocky shore fish
Symphodus melops [
105] and southeastern Australia with the barnacle
Catomerus polymerus, and the limpet
Cellana tramoserica, [
5]], where these marine species associated with the rocky shore exhibits a break ingenetic connectivity due to a biogeographic barrier formed by sandy coastlines.
Some investigations have explained how upwelling zones affect the genetic and phylogeographic structures of marine species. For example, upwelling events at Cape Blanco (Oregon) and Cape Mendocino (California) affect the genetic structure of the barnacle
Balanus glandula [
106] and five species in the rocky intertidal community [
19], respectively. Along the southeastern Pacific coast, the gastropod
Crepipatella dilatata [
107] and the beach isopod
Excirolana hirsuticauda [
108] exhibit a break at 32 S, a transition area characterized by upwelling. In addition, the fish
Pomatomus saltatrix exhibits this phenomenon in the upwelling area of the Benguela Current [
109], and the fish
Sebastes thompsoni ocurrs between two sectors of the East Sea, which may be related to current patterns such as eddies and upwelling [
110].
The above demonstrates the importance of further investigating the effects of the MRP and ARB+PUG barriers on other species inhabiting the rocky shores and reefs of the Southern Caribbean. Ideally, future research should use a multispecies approach that includes species from different taxonomic groups to test whether the effects of the barrier are widespread throughout the marine community associated with this ecosystem [
4,
111]. In addition, research should also be based on the multilocus approach [
4] to evaluate whether barriers have generalized genomic effects on marine species.
4.3. Population Model
One of the assumptions in population genetics is that genetically structured species fit the isolation by distance (IBD) model, in which samples from nearby locations are less genetically different than those farther apart [
112]. However, highly structured species may exhibit false-positives when testing the IBD model for spatial autocorrelation between genetic and geographic distances [
76,
77]. Some species experienced abrupt separations in the past that are evidenced by high levels of genetic divergence in the present. Thus, Mantel test analyses and Mantel correlograms performed on
A. rivasi and
C. pica determined that none of them fit the IBD model. In contrast, these analyses demonstrated that they fit the hierarchical population model [
77]. This model coincides with those species that show an abrupt genetic change in the face of a biogeographic barrier in a geographical area [
1,
77,
113]. For example, the regression plots could show a linear relationship between geographical and genetic distances, although in some cases not to a significant degree, as was observed in
C. pica.
All of this may occur due to the genetic relationships between subpopulations within each clade that bias the test by suggesting a fit to the IBD model. For
A. rivasi and
C. pica, it was possible to detect the effects of barriers on the relationship between the two distances (
Figure 4). In the correlation plots of these species, three relationship groups are observed. One is found toward the upper part of the graph, where the pairs of data from Cabo de la Vela are related to the rest of the localities. Another group is toward the lower part of the graph, where the relationships of the localities of Cartagena, Isla Fuerte, and Capurganá are observed, and another is in the center, including the relationships of the Santa Marta data with those of Cartagena, Isla Fuerte, and Capurganá. For
N. tessellata, the Mantel test analyses showed a low correlation between geographic and genetic distances, which was not significant. The slope values of the linear regressions between the two variables tended to be zero. These results, those of the AMOVA, and the low value of
ФST suggest that
N. tessellata fits the model of open or panmictic populations across the southern Caribbean [
1].
Species that do not fit the IBD population model have been investigated across the geographical area studied, principally those species with high PLD values and low levels of genetic differentiation that were estimated from microsatellite loci, as in the cases of
S. partitus [
39],
L. synagris [
40],
M. incilis [
43],
E. lucunter lucunter [
44],
Mugil liza [
89], and
C. hippos with mitochondrial genes [
42]. No effect of geographic distances on genetic distances suggests a process of genetic homogenization among the studied localities. However, the literature review identified that the shrimp
P. notialis is the only species that fits the IBD model across the southern Caribbean [
45]. For this species, a
K of three populations was determined, with the samples from La Guajira being the most genetically different from the rest of the localities in Colombia.
4.4. Phylogeographic Concordance between Species
Phylogeographic concordance factors allow us to quantify the degree of phylogeographic congruence among species to determine which species share a phylogeographic pattern. In the past, phylogeographic studies compared phylogenies among multiple taxa qualitatively and interpreted similarity as a shared response to a historical event [
4,
114,
115]. However, most work concludes that not all codistributed species exhibit identical phylogenies due to incongruence in tree topology and clade divergence times. This argument is posited because each species responds differentially to the factors responsible for the phylogeographic break and to the biological characteristics of the species involved [
5,
57,
116].
In this study, only two species presented phylogeographic congruence.
Acanthemblemaria rivasi and
C. pica coincided in presenting phylogeographic breaks to the putative barriers evaluated but with a differential response. For example,
A. rivasi showed two phylogeographic breaks caused by the ARB+UPG and MRP barriers, while
C. pica was only affected by ARB+UPG. This explains why the Kendall concordance factor [
79] presented a value of 0.5, indicating some degree of incongruence with respect to the distribution of the topology categories of their dendrograms. Nevertheless, these species showed a high level of genetic and topological congruence as determined by the method of [
80]. The biological and genetic attributes of both species are likely responsible for the level of congruence observed: low dispersal potential attributed to a PLD of few (6 days in
C. pica) to several days (22 days in
A. rivasi); presenting egg parental care (
A. rivasi) and low fecundity (
A. rivasi); high
ФST values (> 0.08); a
K = 3 (
A. rivasi,
C. pica); and two (
C. pica) to three clades (
A. rivasi) delineated by the phylogenetic analyses.
N. tessellata is characterized as having high dispersal potential with a PLD greater than 60 days, high fecundity, no parental care, and is genetically unstructured (
ФST < 0.01;
K = 1; 1 clade).
4.4. Conservation Aspects
For
A. rivasi and
C. pica, three populations were identified: one in Cabo de la Vela, another in Santa Marta, and the third codistributed across Cartagena, Isla Fuerte, and Capurganá. This spatial pattern coincides with the zoogeographic subareas proposed by [
28] as a result of a study of the biogeography of marine gastropods in the southern Caribbean. This author proposed five subareas, three of which are distributed in the marine sector of Colombia. The first is located from the mouth of the Magdalena River to Costa Rica (Isthmian), which contains the Colombian localities of Cartagena, Isla Fuerte, and Capurganá. The second includes the marine sector from the eastern side of the mouth of the Magdalena River to the eastern side of Tayrona Natural National Park (Samarian), where the third subarea begins and reaches the Paraguaná Peninsula in Venezuela (Goajira). This subarea includes the Cabo de la Vela location. Overall, these three subareas are characterized by differences in the size of the continental shelf, in the predominant types of bottoms, in the conditions of quietude, transparency, temperature, and salinity of the water masses, and in the different types of habitats that they contain [see details in [
28]].
The physical, chemical and geological conditions of the three subareas exert substantial effects on the coastal marine species of the Caribbean of Colombia, which allowed us to observe a typical pattern of genetic differentiation between the samples collected in the Goajira subarea and the other subareas [i.e.,
C. mapale, [
27];
E. lucunter, [
44];
P. notialis, [
45];
R. porosus, [
88];
Sciades proops, and
Melongena melongena [
82]]. Thus, the marine sector of La Guajira in Colombia should be considered a particular management area for the conservation of marine species, mainly for those that are exploited by fishing activities, as in the cases of
C. pica,
C. mapale,
S. proops,
P. notialis and
R. porosus or those that are part of fragile marine ecosystems, such as coral reefs inhabited by
A. rivasi. For each species, La Guajira's population should be considered a genetic management unit (GMU) to prioritize implementing conservation and fisheries management measures [
117]. Two marine protected areas are established in La Guajira (Bahía Portete – Kaurrele National Natural Park and Los Flamencos Sanctuary), but they are not sufficient for conservation.
On the other hand, additional C. pica and A. rivasi populations were identified in the Samarian subarea, which should also be treated as a second GMU. This sector has the advantage of including marine protected areas such as Tayrona Park and Salamanca Island Road Park, where the main factors affecting species and ecosystem conservation, such as fishing and tourism activities, are regulated. The population associated with the Isthmian subarea is the largest and should be considered the third GMU for both species. Although the marine protected areas of Corales del Rosario and San Bernardo islands and Acandí are established there, the challenge for the fishery and environmental authorities is to improve the regulation of multiple human activities that affect the conservation of species and ecosystems outside these protected areas.
Finally, additional studies should be conducted to investigate the phylogeographic patterns of each species assuming all areas of its distribution. The results will serve as a basis for implementing a multinational approach to developing conservation strategies in which the countries that host the species will be committed to implementation.