Introduction

Reproduction (sexual and asexual) is critical for the maintenance of coral reef ecosystems, as well as their repopulation and recovery following disturbance (Harrison 2011; Glynn et al. 2017a, b). Sexual reproduction involves gamete production, fertilization, the transport, settlement, and survival of the resulting larvae (Fadlallah 1983; Harrison and Wallace 1990; Harrison 2011; Glynn et al. 2017a, b). Although corals can reproduce asexually (Fautin 2003; Baums et al. 2006), recruitment resulting from sexual reproduction can modulate marine population dynamics (Gaines and Roughgarden 1985; Doherty and Fowler 1994; Palma et al. 1999).

Sexual recruitment can be classified as self-recruitment when the larvae settle near the natal population contributing and favoring the maintenance of the natal population. In contrast, subsidiary recruitment refers to larval recruitment at a distinct enough location to such that they are effectively a different population (Harrison and Wallace 1990; Black et al. 1991; Cowen 2002); this strengthens genetic links between regions and enhances connectivity. It is important to notice that both types of recruitment promote genetic diversity, which favors adaptation to environmental change (Fadlallah 1983; Harrison and Wallace 1990; Harrison 2011). Biophysical models tend to favor local recruitment (Steneck 2006, Wood et al. 2014; Lequeux et al. 2018) because of a combination of oceanographic conditions, larval behavior, and larvae high mortality rates associated with planktonic conditions (Cowen et al. 2000; Strathmann et al. 2002; Levin 2006). However, coral larvae under favorable conditions (e.g., low predation pressure and high water flow) have the potential (energy reserves) for long-distance dispersal (Fabricius and Metzner 2004). Seasonal cycles of sexual reproduction and recruitment depend not only on environmental factors such as sea surface temperature (SST), light, and nutrients (Harrison and Wallace 1990; Mendes and Woodley 2002; Carpizo-Ituarte et al. 2011; Rodríguez-Troncoso et al. 2011; Santiago-Valentín et al. 2018), but also biotic conditions, namely larval (1) availability (which represents a culmination of gamete production and fertilization), (2) dispersal capacity, and (3) settlement cues (Pawlik 1992; Hadfield and Paul 2001); the latter are chemicals that signal to larvae that suitable substrate is nearby and can also induce metamorphosis. Conspecific corals, crustose coralline algae (Morse and Morse 1991; Morse et al. 1994; Heyward and Negri 1999), and even rubble (Negri et al. 2001; Golbuu and Richmond 2007) have been known to have associated biofilms that emit settlement cues, whereas turf algae, macroalgae, and cyanobacteria generally inhibit larval settlement (Kuffner and Paul 2004; Birrell et al. 2005, 2008; Kuffner et al. 2006; Doropoulos et al. 2016, 2017).

The Tropical Eastern Pacific (TEP) is characterized by marginal environmental conditions that limit the development of coral reefs (Glynn 1976; Cortés 1997; Glynn and Ault 2000). In this region, coral communities have experienced mass coral bleaching and reduction of live coral cover by 50 to 90% since the 1980s (Glynn and D’Croz 1990; Reyes-Bonilla et al. 2002). These losses are often attributed to anomalous temperatures, such as those brought upon by El Niño–Southern Oscillation (ENSO) events (Glynn and Ault 2000), but anthropogenic disturbance at local scales has clearly shaped these ecosystems as well (López-Pérez et al. 2012). The Islas Marias Biosphere Reserve, which is located in the Northeastern Mexican Pacific (NMP), represents an important source of larval for the entire west coast of Mexico and into the Gulf of California (Glynn et al. 1996, 2011; Reyes-Bonilla and López-Pérez 1998; Pérez-Vivar et al. 2006; López-Pérez et al. 2015). Thus, sexual reproduction of scleractinian corals at this site is essential for the maintenance of not only the local population, but also other reefs of the NMP. It is unfortunate, then, that prior studies have not documented adult coral reproduction in this region. Herein, we provide evidence of successful sexual reproduction in species of the three most common genera of scleractinian corals (Porites, Pocillopora, and Pavona) by tracking gametogenesis over space and time, as well as by documenting recruitment to both natural and artificial substrates across a variety of temperature and light regimes at Islas Marias.

Methods

Study area

Islas Marias Biosphere Reserve (Fig. 1) is an archipelago located on the western coast of Mexico that is characterized by a dry season from November to April and a rainy season from May to October (Bullock 1986). Northwesterly winds prevail most of the year, and seasonal upwelling occurs in the Central Mexican Pacific (CMP) from December to April coinciding with the dry season (Fiedler and Talley 2006; Palacios-Hernández et al. 2010). The islands are located in an oceanographic transitional zone with highly variable, mixed-water conditions resulting from the convergence of three ocean currents: the California current, the Mexican coastal current, and the Gulf of California current (Fig. 1A); promoting a highly variable annual temperature range from 18 to 35 °C (Wyrtki 1965; Pennington et al. 2006; Kessler 2006; Palacios-Hernández et al. 2010; Pantoja et al. 2011). This includes inter-annual thermal anomalies driven by ENSO events that have a detrimental effect on coral communities (Glynn et al. 2000; Glynn 2001; Reyes-Bonilla et al. 2002).

Fig. 1
figure 1

Study area. A General ocean circulation pattern of the Central Mexican Pacific and location of the Islas Marias Archipelago. B Sample site location around Isla Maria Cleofas. CL Cleofas II, JG Japanese Garden, BR Baby Reef, GCC Gulf of California current, CCM Mexican coastal current, CC California current, 1—San Juanito, 2—Maria Madre, 3—Maria Magdalena, 4—Maria Cleofas

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The archipelago is composed of four islands—Maria Madre, Maria Magdalena, Maria Cleofas, and San Juanito (Fig. 1); 16 scleractinian coral species can be found there (39% of the total coral species inhabiting the Mexican Pacific; López-Pérez et al. 2015). The four most abundant genera are Pocillopora (31.5% cover across all five species), Porites (27% cover across two species), Pavona (5% cover across four species), and Psammocora (5% cover across two species; Reyes-Bonilla 2003; López-Pérez et al. 2015). Maria Cleofas (Fig. 1B) is the closest to the coast (~ 100 km from the state of Nayarit, Mexico) and is characterized by the highest live coral cover (38.5%; López-Pérez et al. 2016).

Gamete development

Histological studies were conducted to confirm the presence of female and male gametes; for this purpose, we collected five fragments from each coral species Porites panamensis, Porites lobata, Pavona gigantea, Pavona clavus and the three most abundant of species of pocilloporid (Pocillopora cf. eydouxi, Pocillopora cf. effusus, and Pocillopora cf. verrucosa; Schmidt-Roach et al. 2014) distributed in a depth range between 6 and 12 m, in both June and July of 2016 coinciding with the high temperature and light levels documented during these months. These have been hypothesized to trigger gametogenesis by others working in this region (Glynn et al. 1991, 1994, 1996; Carpizo-Ituarte et al. 2011; Rodríguez-Troncoso et al. 2011; Santiago-Valentín et al. 2018). Each of the fragments was fixed in formalin and decalcified in a solution of 10% acetic acid for 24 h. Coral tissues were dehydrated in a graded ethanol series, cleared in xylene, and embedded in Paraplast® using a Leica EG1160 tissue embedding system following the protocols of Santiago-Valentín et al. (2015). The samples were cut to 6 µm thickness with a Leica RM2125RT semiautomatic rotary microtome, stained using the Masson trichrome technique (Lynch et al. 1972), and finally mounted with synthetic resin onto glass slides. All samples were examined and photo-documented using a Carl Zeiss AxioScope® optical microscope. The presence of gametes and the reproductive stages in the coral tissues were determined according to the criteria proposed by Glynn et al. (1994), Carpizo-Ituarte et al. (2011), and Rodríguez-Troncoso et al. (2011).

Coral recruitment

Larval recruitment was evaluated on both artificial and natural substrates. The former consisted of ten terracotta settlement tiles (25 × 28 × 2 cm; 0.856 cm2 each), which were secured with cable ties to a steel bar embedded in the substrate near healthy colonies within the reef, with a distance of approximately three meters between each one, in a range depth of 7–9 m at both Cleofas II and Japanese Garden. The tiles were immersed for 15 months (June 2016–August 2017) and monitored in situ using a UV-FL-1 Dive Light (Nightsea®) torch with a yellow filter to detect the presence of recruits, which were identified in situ at genera level; recruit size was measured using a plastic gauge (Foy®) with an accuracy of 0.01 mm.

For the evaluation of recruitment on natural substrates, five quadrants (1 m2) were spaced at 5-m intervals on three 25 m belt transects run parallel to the coast. In each quadrant, taxonomic classification, and assessment, was made of the presence and size of the recruits as described above in June 2016, July 2016, and April, May, and August 2017 (n = 5 survey times) at Baby Reef, Cleofas II, and Japanese Garden (Fig. 1B). Seawater temperature (SWT) was measured in situ every 25 min during the study period using HOBO® Pendant temperature loggers placed near the sample colonies. Day length was calculated as the time difference (in h) between sunrise and sunset as described by Stull (2000).

The variation in recruitment density (recruits m−2) between genera (Porites, Pavona, and Pocillopora), site (Cleofas II, Japanese Garden, and Baby Reef), and month (see above.) was tested with permutational analysis of variance (PERMANOVA) since the data did not meet the assumptions of parametric statistical analyses. A Euclidean distance matrix of the recruitment density data was constructed. A type I, fixed-factor, sum of squares (type III) model was used, and 10,000 permutations were generated using PRIMER ver. 6.1.11 + PERMANOVA ver. 1.0.1 (Anderson et al. 2008; Clarke and Gorley 2006). Pairwise tests were performed when overall model factors were significant (p < 0.05). A multiple linear regression analysis was performed to examine the relationship between recruit density and temperature and day length with SigmaPlot ver. 1.1.

Benthic characterization

The benthos was characterized by evaluating six evenly distributed 1 m2 quadrants at each of five transects (30 m2 per site) at the following sites: Baby Reef (from 5 to 12 m deep), Japanese Garden (5–15 m deep), and Cleofas II (3–6 m deep) allowing the characterization of different reef conditions along the study area. The percent cover data were square-root transformed, and a Bray–Curtis similarity matrix was constructed. To test for differences between sites, a one-way analysis of similarity (ANOSIM) was conducted with PRIMER ver. 6.1.11. Linear regression analysis was performed with SigmaPlot to examine the relationship between recruit density and the cover of the other benthic groups, though coral and rubble cover were excluded since their variance inflation factors were high; they also tended to be multicollinear.

Molecular systematics

Due to the difficulties in taxonomically classifying recruits, species-level genotyping assays were carried out in 13 recruits randomly collected during sampling (five in Cleofas II and eight in Baby Reef). Genomic DNA from adults (n = 2 fragments per coral species) and recruits was extracted using the Wizard® SV genomic DNA purification system (Promega). Partial sequences of the internal transcribed spacer ITS (ITS1-5.8-ITS2) and cytochrome c oxidase subunit 1 gene (cox1) were amplified with PCR using the following primers: ITS4 (5′-TCCTCCGCTTATTGATATGC-3′) and ITS5 (3′-GGAAGTAAAAGTCGTAACAAGG-5′; Cruz-Barraza et al. 2012; ~ 600 bp) and the degenerate primers LCOI490 (5′-GGTCAACAAATCATAAAGAYATYGG-3′) and HCOI21908 (5′-TAAACTTCAGGGTGACCAAARAAYCA-3′; Folmer et al. 1994; ~ 600 bp). Please see Santiago-Valentín et al. (2019) for PCR details. PCR products were visualized on Tris–acetate–EDTA–agarose (2%) gels, purified using the Wizard SV gel and PCR clean-up system (Promega) and sequenced at Macrogen Inc® (Seoul, Korea). Sequences were manually edited to obtain a consensus sequence using Geneious (ver.4.8.5) and then analyzed using BLAST. Each gene sequence was submitted to the National Center for Biotechnology Information (NCBI): cox1 (MN005652–MN005661) and ITS (MK946633–MK946642). In order to determine the relationship among samples (adults and recruits), a maximum likelihood (ML) tree of K2P distances was created (Kimura 1980) with MEGA7. To corroborate the taxonomic identity of the samples using the cox1 gene, sequences were downloaded from the GenBank, according to the access numbers: P. panamensis NC024182; P. lobata LT558153; P. clavus DQ643836; and P. damicornis LC331996). As outgroups to root the tree, Gorgonia flabellum (GQ342418.1) and Dendronephthya gigantea (AF320104.1) were used for the cox1 and ITS trees, respectively.

Results

Gamete development

Gametogenic development was observed in coral species sampled with the same patterns in species of the same genera. (Full detail of reproductive activity by species is included in Table 1). Immature Porites oocytes (stages I, II, and III) were detected in June while mature oocytes (stage IV) were observed in July; spermiaries in stages III and IV were observed in June and July. Pavona colonies showed stages II–III oocytes and stages III–IV spermiaries were observed in July. Immature morphotypes Pocillopora spp. oocytes (stages I–II) were detected in June and July (Table 1). The species with the highest percentage of reproductive activity (gamete presence) were P. panamensis (72%), followed by P. clavus (50%) and Pocillopora cf. verrucosa (30%).

Table 1 Gametogenesis and recruitment for three coral genera: Pocillopora (three morphotypes), Pavona (two species), and Porites (two species)
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Recruitment

On the artificial substrates, ten Porites and two Pavona recruits were recorded, and the highest number of recruits was observed in April 2017 (5 Porites + 1 Pavona), 11 months after tile installation. The recruits varied in size: Porites recruits ranged from 2 to 7 mm, while Pavona recruits averaged only 0.4 mm. A mortality rate of 42% recruits was reported in May and August of 2017. Cleofas II had the highest recruitment (eight Porites and two Pavona), while only two Porites recruits were recorded at Japanese Garden.

On natural substrata, a total of 383 coral recruits throughout the period of study were recorded, with a mean monthly density of 1.82 ± 0.23 recruits m−2 (± SD for this and all following recruit data). Three coral genera were identified among recruits Porites, Pavona, and Pocillopora (Fig. 2), and recruitment density varied across genera (Table 2). Porites was most dominant (0.60 ± 0.45 recruits m−2), followed by Pavona (0.52 ± 0.24) and Pocillopora (0.28 ± 0.06; Table 1 and Fig. 3). Recruitment density also differed significantly across the three sites (Table 2); highest recruitment was observed at Baby Reef (2.57 ± 0.46 recruits m2), followed by Cleofas II (1.81 ± 0.21) and Japanese Garden (1.24 ± 0.46; Fig. 3 and Table 2). No significant differences in recruit density were detected over time (Table 2). However, significant differences within genera over time were observed (Table 2); highest Porites recruitment was documented in June (1.40 ± 0.05 recruits m−2), while for Pavona it was in August (0.87 ± 0.2 recruits m−2). Although Pocillopora showed less variation, the highest density of recruits was found in July (0.36 ± 0.03 recruits m−2). The results show that the peak of recruits at Cleofas Island was recorded during the period with the highest value of daylight hours (13.24 h) and SWT (27.67 °C). However, the monthly recruitment data did not correlate with SWT or day length (R = 0.5659, N = 5, p = 0.68; Fig. 4).

Fig. 2
figure 2

Coral genera studied at Isla Maria Cleofas. Adult colonies (scale bars = 20 cm) of Porites (A), Pavona (B), and Pocillopora (C). Recruits of Porites (D) and Pavona (E) attached to coral rubble colonized by crustose coralline algae and a Pocillopora recruit (F) attached to rocks colonized by crustose coralline algae (scale bars = 1 cm). Recruits of Porites (G), Pavona (H), and Pocillopora (I) under ultraviolet fluorescence (scale bars = 0.5 mm)

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Table 2 Permutational analysis of variance (PERMANOVA) testing the effects of genera, study site, and survey month on variation in recruit density at Isla Maria Cleofas during 2016–2017
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Fig. 3
figure 3

Recruit density of each of three coral genera (Porites, Pavona, and Pocillopora) at each site. A Pooled across the three study sites of Isla Maria Cleofas, B Cleofas II, C Japanese Garden, and D Baby Reef. Bars depict mean ± SD

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Fig. 4
figure 4

Relationship between recruitment density in each sampling month (ind m2), temperature (°C), and day length (hours)

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Benthic characterization

A total of 13 species were recorded across the genera Pocillopora (seven species), Porites (two), Pavona (three), and Psammocora (one), with an overall coral cover of 11.56%. The genus that contributed most to cover was Pocillopora (8.52%), followed by Pavona (2.18%) and Porites (1.05%; see the contribution to coverage by species in Table 3). The benthic composition differed across sites (ANOSIM; R = 0.185, p = 0.01). Pocillopora was most abundant at Baby Reef (12.22%); its cover at Cleofas and Japanese Garden was very similar (7.15 and 7.56%, respectively). The relative dominance of the various pocilloporid species differed across sites; at Cleofas II, Pocillopora verrucosa was most abundant (4.61%), whereas at Japanese Garden and Baby Reef Pocillopora eydouxi (2.33%) and Pocillopora effusus (6.89%), respectively, were most commonly observed. The genus Pavona was most abundant at Japanese Garden (5.22%), and P. clavus (3.83%) was the most dominant pavonid species at all three sites (See Table 3). Of the three target genera, Porites has the lowest coverage across the three sites, though its cover differed significantly across sites; this was driven by a difference between Baby Reef and Cleofas II (0.97 and 0.90%, respectively) over Japanese Garden (0.58%). P. panamensis (0.39%) was the most commonly documented poritid at Japanese Garden and Baby Reef, while P. lobata (0.57%) dominated Cleofas II; in contrast, P. lobata was not present at Baby Reef (Table 3).

Table 3 Benthic cover (%) at each site of Isla Maria Cleofas during 2016–2017
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There was a positive correlation between the density of recruits and the benthic groups (R = 0.49, n = 18, p = 0.27). Meanwhile, recruit density correlated negatively with sediment (R = − 0.09; p = 0.027), rubble (R = − 0.07; p = 0.03), and the “other” group, which featured organisms such as barnacles, octocorals, and encrusting organisms (R = − 0.23; p = 0.019). Coral recruit density was not affected by macroalgae (R = − 0.009; p = 0.601) or turf algae (R = − 0.046; p = 0.133) cover. There was a positive correlation between cover of crustose coralline algae and coral recruitment (R = 0.057; p = 0.019); Baby Reef was characterized as the site with both highest larval recruitment and cover of crustose coralline algae (CCA; 26.8%), and the low presence of sediments (1.8%). In contrast, the lowest recruitment was recorded at Japanese Garden, which presented the highest sediment coverage compared to the other sites (9.4%; Fig. 4).

Molecular analysis

ML trees of K2P distances (Fig. 5) were reconstructed for two genes, cox1 mtDNA (n = 7 670-bp sequences) and ITS nDNA (n = 5 474-bp sequences), and the recruit sequences were clustered with those of the sampled adults. Sequence comparisons revealed 100% nucleotide similarity between adults (identified morphologically) and recruits for cox1 (Fig. 5A) and ITS (Fig. 5B). The molecular analysis revealed that, independent of the loci used, recruits belonged to five species: P. panamensis, P. lobata, P. gigantea, P. clavus, and Pocillopora damicornis.

Fig. 5
figure 5

Neighbor-joining tree of DNA sequences from adults and recruits of Porites panamensis, Porites lobata, Pavona gigantea, Pavona clavus, and PocilloporadamicornisA Partial fragment of the cytochrome oxidase subunit 1 gene (cox1). B Internal transcribed spacer (ITS). Numbers on internal nodes represent bootstrap values (100 replicates)

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Discussion

Successful sexual reproduction in corals depends on the sequential processes of gamete production, fertilization, larval transport, and larval settlement (Harrison 2011; Glynn et al. 2017a, b); the former and latter processes were evaluated here for scleractinian corals of the Northeastern Mexican Pacific (NMP; Islas Marias). The gametogenic period of Central Mexican Pacific (CMP) corals is shorter in duration, compared to other regions of the Tropical Eastern Pacific (TEP; Santiago-Valentín et al. 2018). This observation has been attributed to variation in temperature and day length (Carpizo-Ituarte et al. 2011; Santiago-Valentín et al. 2018). To delimit the sampling time of gametogenic development, previous records from the CMP, which conclude that during April to August and/or temperatures above 26 °C, promote gamete development (see Carpizo-Ituarte et al. 2011). In this study, it was not possible to relate gametogenic development with temperature; however, mature eggs were observed in colonies of Porites, and mature sperm in Pavona and Pocillopora colonies, during June and July which presented temperatures higher than 27 °C. That being said, coral reproduction was not assessed during the cold season when gametic development is absent, and so the relationship between temperature and gametogenesis should be corroborated by studies in which corals are tracked at other points in the year. Regardless, the presence of gametes in advanced stages of maturation is direct evidence of the reproductive ability of scleractinian coral colonies in the Islas Marias region.

Coral settlement has been evaluated on artificial substrates (e.g., plexiglass and terracotta; Richmond 1985; Reyes-Bonilla and Calderón-Aguilera 1994; Medina-Rosas et al. 2005; López-Pérez et al. 2007; Cabral-Tena et al. 2018), as well as natural ones (Smith 1991; Reyes-Bonilla and Calderón-Aguilera 1994; Glynn and Leyte-Morales 1997; Glynn et al. 2000, 2011; Guzmán and Cortés 2007; Glynn et al. 2011), and, as documented herein, recruitment to the former tends to be low (or not at all; Birkeland 1977; Wellington 1982). This has been attributed to the use of inappropriate substrate material (Muñoz et al. 2018), as well as to competition and predation exerted by organisms such as macroalgae, barnacles, and sponges (Sammarco 1982; Richmond 1987).

The density of recruits on natural substrates was the hierarchy of the target genera followed according to their life history traits (Ritson-Williams et al. 2009). Porites panamensis are gonochoric and release sperm into the water column. Sperm is uptaken by females, egg are fertilized internally (Carpizo-Ituarte et al. 2011; Glynn et al. 1994; Rodríguez-Troncoso et al. 2011; Santiago-Valentín et al. 2019), and the larvae are then released in a pre-competence stage. Porites lobata is a gonochoric spawner (Glynn et al. 1994, 2017a, b). Pocillopora synchronously spawn and are hermaphroditic (Carpizo-Ituarte et al. 2011; Chávez-Romo and Reyes-Bonilla 2007; Glynn et al. 1991; Rodríguez-Troncoso et al. 2011), whereas Pavona are sequential hermaphrodites or gonochoric transmission spawners (Carpizo-Ituarte et al. 2011; Glynn et al. 1996; Rodríguez-Troncoso et al. 2011; Santiago-Valentín et al. 2015). Species representing these reproductive modes differ in colony size, gametic cycles, larval competency, dispersal distance, and dinoflagellate symbiont transmission (Richmond and Hunter 1990). Brooders are typically smaller than spawning corals and have multiple planulating cycles per year, as opposed to one or two cycles in broadcast spawners (Szmant 1986).

Also, brooding and spawning corals present differences in their dispersal potential (Baird et al. 2009); as larvae of brooders are released in an advanced stage of development, they tend to settle more quickly (Miller and Mundy 2003) and can recruit at high densities, as was observed herein and elsewhere in the TEP for P. panamensis (Szmant 1986; Glynn et al. 2017a, b). Indeed, Smith (1991) and Glynn et al. (2000) reported even higher values (12.92 ind m−2 yr−1; Table 4) at Isla Uva, Panama, than those reported here (and throughout text). In contrast, the Pavona recruitment rates at Islas Marias were higher than reported elsewhere in the TEP (Table 4). Pocillopora was characterized by the lowest settlement rates at the study sites, and even the rates at the site with the highest pocilloporid recruitment, Baby Reef (1.57 ind m−2 yr−1; Table 4), were lower than those documented at Isla Caño, Costa Rica (1.81 ind m−2 yr−1). Although no significant differences in recruit density were observed over time, peak recruitment months differed for Porites (June), Pavona (August), and Pocillopora (July). These temporal differences may reflect physiological differences (e.g., energy stores, maturation time), and/or external reproduction cues (Carpizo-Ituarte et al. 2011; Rodríguez-Troncoso et al. 2011, 2014; Santiago-Valentín et al. 2018).

Table 4 Comparison of coral recruitment rates in the Tropical Eastern Pacific
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Differences in recruitment density were observed across sites, and such differences could be attributed to depth, sedimentation, or any number of other abiotic factors (Harrison 2011; Glynn et al. 2017a, b). Environmental factors can directly influence the physiology of larvae or gametes (Wittenberg and Hunte 1992) or else change their substrate settlement behavior (Gilmour 1999). Herein, inter-site differences appear to be linked to CCA cover, as has been documented previously (Morse et al. 1994; Raimondi and Morse 2000), though it is unclear whether the biofilms present on these algae, or the algae themselves, are responsible for the observed settlement trends (Johnson et al. 1991; Webster et al. 2004). Coral larvae tend to show preference for settlement on CCA with the least potent antifouling defenses (Carleton and Sammarco 1987). Cleofas II had the highest amount of dead coral rubble, which would presumably make it conducive for larval settlement (Heyward and Negri 1999). However, there was in inverse relationship between rubble cover and recruitment herein, possibly due to the generally unconsolidated nature of the rubble and its tendency to be coated in sediments (which would smother coral recruits). Also, larval interactions with the biological inhabitants of reef communities can influence larval survival and settlement, and encrusting organisms (e.g., barnacles, sponges), algal turfs, macroalgae, and sediment can negatively impact the settlement of coral larvae (Kuffner and Paul 2004; Birrell et al. 2005, 2008; Kuffner et al. 2006). The high cover of these non-coral organisms at both Cleofas II and Japanese Garden could explain their low recruitment rate compared to Baby Reef. However, it is also important to characterize the impact of the environmental variables of each site that are promoting a change in the components of the habitat, and how the synergy of abiotic and biotic variables modifies coral recruitment.

In general, the taxonomic identification of scleractinian corals is based on the morphology of adult colonies, as well as their corallites (Veron 2000); however, in the early stages of development (i.e., larvae and recruits), these characteristics are not distinguishable between species (Hillis et al. 1996). Therefore, molecular markers were used herein to identify recruits, and at Isla Maria Cleofas. There was successful sexual recruitment of the five most abundant coral species in the study area: P. panamensis, P. lobata, P. gigantea, P. clavus, and Pocillopora sp; whether or not these recruitment levels are sufficient to maintain the current populations remains to be determined.

The five adult species present at the study sites differed in distribution, as has been documented elsewhere in the TEP (Cortés and Guzmán 1998; Glynn and Ault 2000; Maté 2003; Reyes-Bonilla et al. 2005). P. panamensis is a widely distributed endemic species from the TEP (Glynn 2003) whose reproductive biology has been documented previously (Smith 1991; Carpizo-Ituarte et al. 2011; Glynn et al. 1994; Rodríguez-Troncoso et al. 2011; Santiago-Valentín et al. 2018, 2019), while P. lobata has a more limited distribution in the Mexican Pacific (Carriquiry and Reyes-Bonilla 1997) and Central America (Glynn and Ault 2000). In contrast to these species, successful recruitment of P. gigantea and P. clavus along the Mexican coastline had not previously been reported. Regarding the pocilloporids, they are generally thought to predominantly maintain their population sizes by asexual reproduction; although histological studies have clearly shown gametogenesis (Carpizo-Ituarte et al. 2011; Chávez-Romo and Reyes-Bonilla 2007; Glynn et al. 1991; Rodríguez-Troncoso et al. 2011), the amount of recruitment documented by others in this region was relatively low (Glynn et al. 1994; López-Pérez et al. 2007; Medina-Rosas et al. 2005). Our data show that Islas Marias might be an important bastion of genetic variation for the genus Pocillopora.

The Islas Marias Archipelago is an essential stop for coral larvae migrating between the Gulf of California and the Mexican Pacific (Glynn et al. 1996; Reyes-Bonilla and López-Pérez 1998) via Revillagigedo (Pérez-Vivar et al. 2006), as suggested by biophysical connectivity models (López-Pérez et al. 2016). The current study provides empirical data on the reproductive ability of adult colonies, and, despite having only documented recruitment at local scales, there is the potential for gametes and/or larvae spawned from corals of Islas Marias to travel toward the Gulf of California or other sites along the Mexican Pacific. To date, the connectivity models have been built; using generalized data, however, they provide us with knowledge of a time line that a larva need to connect between populations; as such, a larva from Islas Marias could travel to the North Mexican Pacific and the CMP coastal reefs if is transported during at least 40 days. If the time is extended to 120 days, the larvae can reach insular sites such as Revillagigedo Island and the South Mexican Pacific (López-Pérez et al. 2016; Lequeux et al. 2018). Nevertheless, the lack of data regarding larval competency time generates a gap to accurately model coral population dynamics along the Mexican Pacific. Therefore, in order to build a specific model about connectivity future research should examine the relative contribution of self-recruitment and subsidiary recruitment, as well as their role in the recovery and maintenance of coral populations at Isla Maria Cleofas.