Characterization of tidal pool sites

Three groups of tidal pools were examined at 16 localities on the west, east, and south coasts of La Réunion island (Fig. 2). The tidal pools in Boucan-Canot (BC), L'Hermitage les Bains (HB), Trois-Bassins (TB), and Saint-Leu (SL) are located within the shallow lagoon (depth 0.1–2.0 m). In these areas, nearshore platforms of lava tuff substrate form shelves with crevices and pools, resulting in ranges of tidal pool habitats. During flood and ebb, the water streams moderately in and out of the pools, which are mostly characterized by sandy bottoms. The other pools directly face the open sea, and are thus exposed to stronger wave action. At these sites, the water streams in with more physical power during high tides. Among these, the pools at Grande Anse (GA) and Manapany-les-Bains (MB) are artificially created swimming pools in former tidal pool areas, which still function as natural tidal pools. Both of these were already in this condition in 1995, when the study started.

The pools were visited in September 1995 (RF), December 1998/January 1999 (RF), February 2020 (RF, JPQ), October/November 2022 (RF, JPQ), 2010–2022 (CC), and continuously from January 2016 to March 2023 as part of the CUMARO Phase 1 project (JPQ). The following characterization of the studied tidal pools refers to the condition in October/November 2022. Information concerning surface area and depth refers to the condition at the lowest ebb-tide level.

West coast

Cap de La Houssaye (CH) (Fig. 3). About 20 pools are located at the black volcanic rock point of Cap La Houssaye, a famous diving site close to the sandy bay of Saint-Paul. The three examined pools have a total surface of 800 m², mostly shallow (10 cm) except for the main pool below the cliff (maximum depth 1.2 m), with a bottom of bare black volcanic rock and boulders. The main pool receives water influx only from swells. Coordinates of the southern, principal pool: 21°01′5.86″S, 55°14′16.23″E.

Boucan-Canot (BC) (Fig. 4). Five shallow pools are located close to the artificial swimming pool on the rocky volcanic point (Cap Champagne), located on the sheltered side of the point, opposite the swell direction, with a total surface of 120 m² and a maximum depth of 20 cm. The bottom consists of black volcanic rock covered with brown algae (predominantly Sargas-sum spp.) and with a few live coral colonies. In summer, during low tide and poor swell conditions, these pools are strongly affected by heat stress. Coordinates of the southern, principal pool: 21°01′35.56″S, 55°13′33.32″E.

In addition, pools on the adjacent reef crest 130 m east-northeast of Cap Champagne were sampled at extremely low tide; these pools had a maximum depth of 1.2 m and a bottom of coralline rock and gravel, with live corals and few brown algae. Coordinates of the collection site: 21°01′34.78″S 55°13′36.14″E.

L'Hermitage les Bains (HB) (Fig. 5). The fringing reef of L'Hermitage-les-Bains is located on the west coast, comprising reef flats with tidal pools (with a depth of 0.1–0.5 m) which are isolated from the open sea during low tide. These habitats are considered as highly patrimonial, with diverse fish and invertebrate communities supported by live corals dominated by Acropora Oken, 1815 colonies and a bottom of carbonate gravel. They are now in the core zone of the Natural Marine Reserve of La Réunion (established 5 Mar. 2007) and can no longer be accessed. Coordinates of the examined pool: 21°04′44.67″S, 55°13′11.49″E.

Trois Bassins (TB) (Figs. 6–7). Located on the coast of Trois Bassins and close to a river mouth that seasonally releases fresh water into the ocean, this system comprises five connected pools which are separated from the open ocean by a volcanic rocky bar that allows swell to feed the pools with sea water (Fig. 6). This black volcanic pool complex is considered the most important and diverse on the west coast of La Réunion (unpublished data), with a total surface of 1,500 m². Various habitats are found here, including rocky flats that emerge at low tides, and deep pools with a depth of 3 m or more with a bottom of round boulders or hard substrate communities dominated by hard corals and macroalgae. Coordinates of the southern, principal pool: 21°07′35.28″S, 55°15′50.61″E.

A system of deep volcanic rock pools 1.6 km northwest of TB was also sampled (Fig. 7). The main pool consists of two basins, the first with a depth of 0.5 m at low tide, the second with a depth of 2.5 m. The shallower pool is covered by brown and green algae, the deeper pool has a substrate of black rock and gravel, the rocks covered by hard corals, macroalgae, and a few live corals (genus Acropora). Coordinates of the principal pool: 21°06′54.61″S, 55°15′17.71″E.

Pointe des Chateaux (PC) (Fig. 8). A large tidal pool is situated 100 m southeast of Pointe des Chateaux, approximately 2.4 km northwest of Saint-Leu. The examined pool has an estimated total surface of 170 m², with a maximum depth of 1.5 m; it continues with channels towards the reef crest of the fringing coral reef. The bottom consists of volcanic and coralline rock, boulders, gravel, brown algae, and dead and live corals. Coordinates of the principal pool: 21°09′01.29″S, 55°16′22.60″E.

Saint-Leu (SL) (Fig. 9). Rock pools were sampled 1.3 km south of Saint-Leu in 1995 and 2022; these pools were situated on a boulder field at the mouth of Ravine du Cap, on the reef flat near the end of the fringing reef, with a total surface of about 400 m², a depth of 10–30 cm, and a bottom of sand, gravel, and boulders. In 1995, they contained numerous sea urchins and were in sub-optimal condition, pointing to some degree of eutrophication from the former sugar factory at Stella Matutina; in 2022, the water and habitat quality appeared much improved. Coordinates of the main pool: 21°11′04.94″S 55°17′10.27″E (Fig. 9).

Another reef top pool area was sampled 1.6 km south of Saint-Leu in 1998; it was situated at the reef crest of the fringing reef, with a total surface of about 300 m² and a maximum depth of 1.2 m. The bottom consisted of volcanic and coralline rock, sand, gravel, few live corals, few algae, and hosted numerous sea urchins. Coordinates of sampled intertidal area: 21°11′15.33″S 55°17′10.90″E.

Pointe au Sel (PS) (Fig. 10). Several tidal pools are located on the northern side of Pointe au Sel, approximately 2.2 km south of Saint-Leu. The examined pools have a total surface of 60 m², are mostly shallow (10–30 cm depth) except for the main pool (maximum depth 1 m), and have a bottom of bare black volcanic rock, boulders, and black gravel, with little vegetation of brown macroalgae. The main pool receives a significant underground freshwater influx. Coordinates of the principal pool: 21°12′06.79″S, 55°16′51.97″E

Etang Salé (ES) (Fig. 11). Located along an 800 m rocky shore close to Le Gouffre de l'Etang Salé, four main pools were examined during several explorations. These pools have a total surface area of 1100 m² and a maximum depth of 4 m at low tide. The pool bottoms are covered by bare rocky substrate with seasonal brown algae communities (dominated by Cystoseira myrica). The water flow in these pools strongly depends on swell conditions. On the opposite, northern side of Etang Salé lies a rocky point north of a black sandy beach, with a tidal pool of 2000 m² and a maximum depth of 50 cm at low tide. The bottom is covered with black sand or rubble, and rock substrate with coral and algal communities. Coordinates of the southern-central pool: 21°16′47.30″S, 55°20′19.24″E.

East coast

Sainte-Rose (SR). Small tidal pools were sampled 100 m east of Saint-Rose harbour. The total surface is estimated at 15 m², with maximum depths of 0.3 m. The pools are exposed to high surf throughout the year. The bottom consists of black volcanic substrate covered with few brown macroalgae. Coordinates of the main pool: 21°07′29.80″S, 55°47′21.33″E.

Pointe Corail (PC) (Fig. 12). Located on the volcanic southeast coast at Sainte-Rose, Pointe Corail is a 150 m long rocky point which harbours a complex of numerous tidal pools submitted to strong swell conditions most of the year. The total surface is estimated at 400 m², with depths of 0.1–0.5 m except the southern pool (1.2 m). The bottom consists of black volcanic substrate except the southern pool, where black sand has accumulated. Coordinates of the main pool: 21°08′21.12″S, 55°49′03.00″E.

Port Ango (PA) (Fig. 13). Located on the volcanic southeast coast at Sainte-Rose, Port Ango is a 180 m long rocky coastline with two complexes of numerous tidal pools separated by a rocky point, with a total surface estimated at 1,370 m². These pools are shallow (0.1–0.6 m deep) and subjected to strong swell conditions most of the time. They are dominated by macroalgae, with scarce colonies of hard corals observed mainly in the southern complex. Coordinates of the southern pool: 21°09′00.78″S, 55°49′38.56″E.

Coulée de Lave 1977 (CL) (Fig. 14). 600 m south of the southern end of the lava flows that reached the ocean in 1977, a large pool of 190 m² is located below the hiking track that leads from Anse des Cascades to Coulée de Lave 1977, close to a number of other, small pools. The main pool has a maximum depth of 2 m, with rich species diversity as the pool is protected from the south-easterly swells that hit the coastline throughout the year. Coordinates of the main pool: 21°10′14.79″S, 55°50′00.03″E.

South coast

Grande Anse (GA) (Fig. 15). On the southern part of the white sandy beach of Plage Grande Anse, an artificial swimming pool was created on the fringing reef a few decades ago, offering excellent conditions for marine life to develop. The total surface of the pool is 4,100 m², and large live coral colonies have grown on volcanic rock boulders together with the associated fauna and flora. The maximum depth of the central pool is 2 m, with sand and hard substrate. Coordinates of the pool: 21°22′12.58″S, 55°32′53.76″E.

Manapany-les-Bains (MB) (Fig. 16). In the rocky bay of Manapany-les-Bains, an artificial swimming pool was created. This pool, which has a surface of 2,700 m²and a maximum depth of 2 m, is affected by a significant underground freshwater inflow that reduces the development of marine communities in the inshore parts of the pool. The bottom of the pool consists of black sand and rubble; the rocks towards the ocean are covered with brown algae and few hard corals. Coordinates of the pool: 21°22′24.05″S, 55°35′17.35″E.

Vincendo (VI). Situated south of the mouth of Vincendo River is a volcanic cape with numerous shallow tidal pools, with an estimated pool surface of 3,100 m². These pools with hard, black volcanic rock substrates were explored during low tide and no swell conditions. The benthic live cover of flora and fauna was low due to heavy hydrodynamic conditions (and seasonal freshwater flows). The maximum depth was 0.5 m. Coordinates of the pool: 21°22′45.11″S, 55°40′37.09″E.

Saint-Philippe (SP) (Fig. 17). The pool of Saint-Philippe is located on the south coast of La Réunion, frequently affected by heavy south-west swell all year long, which makes this pool difficult to explore. Located 100 m west of the ‘harbour’ wharf, the pool surface is 1,870 m², with water depths varying from 0.1 to 2.5 m. Two deep pools were sampled in 1998, with a substrate of black volcanic rock, boulders, brown algae, and few live corals. Additional, small pools occur on the southern part of this coastline. Coordinates of the sampled main pool: 21°21′51.93″S, 55°45′59.33″E.

Data compilation

In order to get a complete list of intertidal fish species of La Réunion, all available literature was searched, extracting data on fishes recorded from intertidal habitats and their localities. Beside the published records from La Réunion (Fricke 1999; Fricke et al. 2009), data from Europa Island (Fricke et al. 2013), Glorieuses Islands (Durville & Chabanet 2009), Mauritius (Arndt & Fricke 2019), and several further places were also considered for those species that are known from La Réunion but for which published intertidal records from La Réunion were lacking so far. Additionally, material from the following museum collections was included: USNM (National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA), SNMS (Staatliches Museum für Naturkunde, Stuttgart, Germany) and MNHN (Muséum National d'Histoire Naturelle, Paris, France). The complete list of species and sources is given in the Results (Table 1). Collection acronyms follow Fricke & Eschmeyer (2022). Family authorships are according to Van der Laan et al. (2014); genus and species classification follows Fricke et al. (2022). New records from La Réunion are species recently found as new or previously misidentified species; in the latter case, previous records/synonymies are provided.

Fishes may occur in the intertidal zone for quite different reasons, or in various periods of their life. Therefore, it is necessary to classify different ‘types’ of intertidal fishes (Arndt & Fricke 2019). The definition of these terms follows Thomson & Lehner (1976) and Griffiths (2003):

R – permanent residents, spending most of their life (juvenile to adult) in tidal pools or the intertidal zone. They are often highly adapted for intertidal life by possessing specialized behavioural or physiological adaptations.

O – opportunists (also secondary or temporary residents), living in the intertidal zone/tidal pools during specific life history stages or seasons. They are also widely distributed in the subtidal zone.

T – transients (or tidal visitors), using the intertidal zone, including tidal pools, transiently for foraging. They may end up accidentally trapped in pools as the tide goes out.

Table 1.

List of intertidal fish species with their residency status and references to their tidal pool records in La Réunion. Genus and species classification follows Fricke et al. (2022); references in parentheses refer to species that occur in La Réunion but were recorded in tidal pools elsewhere. Abbreviations of residency status: O – opportunists, also known as secondary or temporary residents; R – permanent intertidal residents (R* - permanent volcanic rock pool inhabitant); T – transients or visitors. Abbreviations of tidal pools: BC – Boucan-Canot; CH – Cap de La Houssaye; CL – Coulée de lave; ES – Etang Salé; GA – Grande Anse; HB – L'Hermitage-les-Bains; MB – Manapany-les-Bains; PA – Port Ango; PC – Pointe Corail; PCH – Pointe des Chateaux; PS – Pointe au Sel; SL – Saint-Leu; SP – Saint-Philippe; SR – Sainte-Rose; TB – Trois Bassins; VI – Vincendo.

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Table 2.

Permanent resident species in volcanic rock pools at La Réunion. Location of records: CH – Cap de la Houssaye; CL – Coulée de lave; ES – Etang Salée; GA – Grande Anse; MB – Manapany-les-Bains; PA – Port Ango; PC – Pointe Corail; PCH – Pointe des Chateaux; PS – Pointe au Sel; SL – Saint-Leu; SP – Saint-Philippe; SR – Sainte-Rose; TB – Trois Bassins; VI – Vincendo.

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The assignment to one of these categories was established on the basis of literature references such as Murase (2013) and SINDORF et al. (2015), the authors' observations, and information about size as well as developmental stage of the examined fishes in different studies such as Ntiba et al. (1993), Durville & Chabanet 2009, Ghanbarifardi & Malek (2009), Gonzalez-Murcia et al. (2016), and Arndt & Fricke (2019). It is worth mentioning that there are no generally used or standardized abbreviations of the aforementioned categories in the existing literature, e.g., ‘PR’ means partial resident in Cox et al. (2011), but permanent resident in Sindorf et al. (2015). Permanent resident species are treated in more detail (Table 2), as these data are of special importance for habitat conservation. When prioritizing pools, it is important to take permanent residents into account, as this habitat is obligatory for them.

Field examination of fishes in tidal pools

The fishes of all selected tidal pools (see description above) were examined repeatedly since October 1995. The examination time was determined by the tides, usually starting half an hour before low tide and ending half an hour after low tide. Qualitative surveys of fish species in the pools were conducted by collection with anesthetics (clove oil) and visual census, at low tide. The pools were also repeatedly visited at night. Photographs for later analyses were taken during visual censuses. During the surveys at low tide, the number of permanent resident species was counted to estimate their abundance (Gibson 1999). The total number of fish specimens in each pool was also counted/estimated. Other species representing secondary residents or transients were simply photographed and registered without counting. All fish species observed in the tidal pools are included in Table 1.

Diversity of tidal-pool fishes

A total of 323 fish species from La Réunion are reported from intertidal habitats (Table 1). Among these are approximately 63 transients, 177 opportunists, and 65 permanent residents. The families with most species occurring in the intertidal zone include Gobiidae (42 species), Blenniidae (31 species), Labridae (20 species), Muraenidae (21 species), and Scorpaenidae (17 species). The category of some species, here provisionally listed as opportunists, could not be determined with certainty as not enough data were available. Moray eels (Muraenidae), gobies (Gobiidae), blennies (Blenniidae), roundheads (Plesiopidae), kelp blennies (Clinidae), triplefin blennies (Tripterygiidae), dragonets (Callionymidae), and also snake eels (Ophichthidae) include permanent intertidal residents that may spend their whole life in tidal pools. A few permanent resident species, including Damania anjouanae (Blenniidae), Enchelyurus kraussii (Blenniidae), Istiblennius dussumieri (Blenniidae), Diplogrammus infulatus (Callionymidae), Gymnothorax pictus (Muraenidae), Haliophis guttatus (Pseudochromidae), Enneapterygius tutuilae (Tripterygiidae), and Helcogramma fuscopinna (Tripterygiidae), are known from subtidal waters at La Réunion, but were recorded from tidal pools elsewhere.

Table 1 does not include several of the pelagic species and fishes that usually forage in open water or near the water surface (e.g., anchovies, herrings, carangids, barracudas, or needlefishes), though they can often be observed visiting intertidal habitats, especially mangrove creeks, during the juvenile stage (Bianchi 1985; Whitehead 1985; Whitehead et al. 1988; Aguilar-Perera & Appeldoorn 2007). Ntiba et al. (1993) recorded several representatives of the above-mentioned families in intertidal mangrove creeks in Kenya, e.g., Herklotsichthys quadrimaculatus (Rüppell, 1837), Spratelloides delicatulus (Bennett, 1832), Gnathanodon speciosus (Forsskål, 1775), Trachinotus baillonii (Lacepède, 1801), Trachinotus blochii (Lacepède, 1801), Chanos chanos (Fabricius in Niebuhr, 1775), Hemirhamphus far (Fabricius in Niebuhr, 1775), Lobotes surinamensis (Bloch, 1790), Sphyraena barracuda (Catesby, 1771), and Sphyraena jello Cuvier, 1829. All of these species are known to occur at La Réunion (Fricke 1999), but most have so far not been recorded from intertidal habitats of the island.

The tidal pool fish fauna presented here also includes 22 new records of fishes from La Réunion (see Table 1): Zebrasoma desjardinii (Bennett, 1836) (family Acanthuridae), previously known from the island but misidentified; Antennarius indicus Schultz, 1964 (family Antennariidae); Apogon dammermani Weber & Beaufort, 1929 (family Apogonidae); Atherinomorus pinguis (Lacepède, 1803) (family Atherinidae); Tylosurus melanotus (Bleeker, 1850) (family Belonidae); Entomacrodus vermiculatus (Valenciennes, 1836), Mimoblennius rusi Springer & Spreitzer, 1978, and Omobranchus elongatus (Peters, 1855) (family Blenniidae); Bathygobius meggitti (Hora & Mukerji, 1936), Cabillus lacertops (Smith, 1959), Eviota punyit Tornabene, Valdez & Erdmann, 2016, E. sodwanaensis Greenfield & Winterbottom, 2016, Fusigobius inframaculatus (Randall, 1994), and F. pallidus (Randall, 2001) (family Gobiidae); Plicomugil labiosus (Valenciennes, 1836) (family Mugilidae); Gymnothorax pseudothyrsoideus (Bleeker, 1853) (family Muraenidae); Scarus rubroviolaceus Bleeker, 1847 (family Scaridae); Parascorpaena aurita (Rüppell, 1838), Scorpaenodes corallinus Smith, 1957, Scorpaenodes possi Randall & Eschmeyer, 2002, and Scorpaenopsis macrochir Ogilby, 1910 (family Scorpaenidae); Enneapterygius ventermaculus Holleman, 2007 (family Tripterygiidae).

Permanent residents in volcanic rock pools

A total of 32 permanent residents were found, predominantly in volcanic rock pools (Table 2) as opposed to coralline rock pools (part of the coral reef), where much fewer permanent residents were found. Species numbers in the volcanic rock pools increased in proportion to the surface area of stones (rocks) covering the pool bottom. The pools with the highest number of residents were SL (19 species), PCH (13 species), CH (12 species), SP and TB (9 species each), PS (8 species), and SR (7 species). The most speciose families in volcanic rock pools were Blenniidae (11 species), followed by Gobiidae (6 species), Muraenidae (5 species), and Tripterygiidae (4 species).

Istiblennius edentulus was the most widespread resident species in volcanic rock pools (found in 13 pools), followed by Bathygobius fuscus and Entomacrodus striatus (8 pools), Enneapterygius philippinus (7 pools), and Istiblennius bellus (6 pools).

The abundance of B. fuscus decreased with increasing pool depth and increasing pool surface; it occupied very shallow areas of the pools, often very crowded during the lowest ebb-tide level. A similar ‘overcrowding effect’ was observed in I. edentulus, with a negative correlation between its abundance and pool surface area during low tide. Furthermore, there was a close relation between the abundance of I. edentulus and the algae cover on one hand, and between B. fuscus and pools with a long period of isolation during the tidal cycle on the other. The starry moray (Echidna nebulosa) and the ebony gregory (Stegastes limbatus) occur in large pools with a supply of boulders.

Comparison with other W Indian Ocean islands

Few publications have dealt specifically with intertidal fishes in the Western Indian Ocean. Durville & Chabanet (2009) found a total of 32 fish species in intertidal rockpools on the Glorieuses Islands north of the Mozambique Channel. The authors classified 19 of these species as ‘typical population’ also occurring during as adults in these habitats, whereas the remaining 13 species were observed as juveniles only. The examined pools were located high in the infralittoral zone, thus more or less isolated during long periods of the tidal cycle, and they had a surface area of approximately 2 m² during low tide, similar to the present study. Several taxa listed as permanent or temporary residents by Durville & Chabanet (2009) were confirmed in La Réunion pools, e.g., the temporary residents Kuhlia mugil, Chaetodon lunula, Stethojulis albovittata, Thalassoma purpureum, Acanthurus triostegus, Chrysiptera biocellata, Chrysiptera glauca, three species of Abudefduf, and the permanent resident Istiblennius edentulus. In contrast to our findings at La Réunion, Durville & Chabanet (2009) found juveniles and adults of the moray eel Gymnothorax pictus and several adults of the eel blenny Haliophis guttatus in their small pools. For the authors, these species met the criteria of ‘typical’ for intertidal pools, i.e., the species represented permanent residents in the Glorieuses Islands tidal pools. Gymnothorax pictus and H. guttatus seem to be rare in La Réunion, and no records from tidal pools have been published so far. However, Beckley (1985) confirmed that eel blennies may represent permanent intertidal residents and may occur in tidal pools in large numbers (Arndt & Fricke 2009).

Sindorf et al. (2015) recorded 55 species during an examination of intertidal fishes in a national park in Kenya, 21 of which were permanent residents. Twenty-five of these species also occur in La Réunion tidal pools; however, we assigned several of them to different resident categories (see remarks in Table 1). Five wrasses and seven damselfishes occurred in both Kenyan and La Réunion tidal pools, but not a single permanent resident of either gobies or blennies.

Two tidal pool studies refer to sites in the northern Indian Ocean. Ghanbarifardi & Malek (2009) examined pool communities along the Iranian coast (Persian Gulf and Gulf of Oman). The large majority of fish (93.5%) represented permanent residents of gobies and blennies; three species of either family were most abundant. The remaining 6.5% comprised eight species from six families, all of which are temporary residents. None of the abundant species along the Iranian coast occur in La Réunion (Ghanbarifardi & Malek 2009, table 2). Tsering et al. (2012) published a study from Goa (India). All of the examined pools were small (surface < 1 m²), but their depth ranged from 0.1–0.7 m. Seven fish species were recorded altogether, including gobies, blennies, and Abudefduf sordidus, which may represent permanent residents, but information referring to this category was not given by the authors. The pools from Goa were dominated by Istiblennius dussumieri, while in La Réunion the dominant species was I. edentulus.

Arndt & Fricke (2019) examined the intertidal fish fauna of the neighbouring island of Mauritius. They recorded a total of 292 fish species, with 62 species representing permanent intertidal residents. These numbers are lower than those at La Réunion, which has 28 species more than Mauritius. Although the majority of fish species are also found in the tidal pools of La Réunion, there were quite some differences. In the family Blenniidae, both islands share 23 species (77%), including 16 permanent residents (64 %), while 7 species (23%) are different, including 6 residents (20%). The two islands share 32 species (80%) of Gobiidae, including 6 permanent residents (15%); 14 species (35%) are different, including 4 permanent residents (10%). Similar figures were observed for Muraenidae and Tripterygiidae. Approximately two-thirds of the intertidal species are identical at La Réunion and Mauritius, while one-third is different. This difference may be due to the dominance of relatively recent lava rocks in tidal pools of La Réunion, while Mauritius tidal pools are dominantly formed by coralline rock or eroded lava rocks. We observed differences even in the most abundant tidal pool species. At Mauritius, Bathygobius coalitus, Istiblennius edentulus, and I. bellus were the most common species in shallow tidal pools, while Bathygobius fuscus was rare. At La Réunion, Bathygobius fuscus was dominant and much more common than B. coalitus, and Istiblennius edentulus was common, while I. bellus was usually rare (except at Saint-Leu, where it was the dominant species).

In the present field study, the total number of permanent intertidal residents was low compared to the results of Durville & Chabanet (2009) or Sindorf et al. (2015), but similar to those reported for Mauritius by Arndt & Fricke (2019). This low number of residents could be due to the low depth and small size of several of the examined pools. Arndt & Fricke (2019) observed a decrease in tidal pool species at Mauritius between 1995 and 2018; they supposed there were fundamental reasons, e.g., environmental change due to eutrophication and industrial sewage, including heavy metal pollution, effects of eutrophication due to a high input of nitrate, and effects due to increasing population and tourism. At La Réunion, there seems to be a similar decrease of tidal pool species, although this may be difficult to quantify. A total of 44 species recorded subtidally from La Réunion have been reported from tidal pools elsewhere, but were not found in the intertidal region in La Réunion; this equals 15.4% of the total intertidal species.

A comparably low number of resident species does not necessarily mean low abundances of fish. Especially in smaller pools, the abundance of permanent resident species was quite high, with an average of 5.98 individuals/m² and a maximum value of 10 individuals/m²for Bathygobius fuscus at Sainte-Rose (SR). The abundance increased with decreasing pool size, peaking in pools with a surface area between 1–2 m² during lowest ebb-tide level. Lundquist & Pinkerton (2008), who examined tidal pools in New Zealand, estimated an abundance of 10 fish/m² across their intertidal study area, Bennett & Griffiths (1984) counted an average of 7.42 individuals per m² in South Africa, while in Central America, Gonzáles-Murcia et al. (2016) found mean total abundances of between 5 fish/m² in high shore pools and 12 fish/m² in pools at lower shore sections in El Salvador, but opportunists and transients were included in these studies.

Fish diversity in relation to tidal pool size

Larger tidal pools usually have more algae cover due to a lower abundance of herbivores, e.g., blennies. It is thus not surprising that these larger pools, which contain more algal and rock ledge cover, host a larger and more diverse population of fish. Pool depth, volume, and the variety of microhabitats such as the presence of shells, pebbles, and rock ledges strongly influence richness and total abundance of fish (Mahon & Mahon 1994; White et al. 2015). White et al. (2015) examined very small pools with an area of 20 cm² to 8 m² in New South Wales (Australia) and included all 27 recorded fish species in their analysis. In the present study, we used another approach focusing on the diversity of permanent residents. The species number increased with pool size. However, the results show that specific pool microhabitats are associated with the occurrence and abundance of particular species. After the exclusion of fundamental oceanographic parameters, because lagoon or wave-exposed open sea sites affected stone and sand coverage of examined pools differently, features like algae cover and the presence of boulders contributed mainly to the occurrence of certain permanent residents. We observed that the abundance of the two most common species, B. fuscus and I. edentulus, was negatively correlated with pool surface area, and that of B. fuscus also with pool depth. Deep and large pools tend to contain predators (White et al. 2015), and shallow and small pools with a long isolation period during the tidal cycle obviously may be beneficial for these permanent residents. This ‘overcrowding effect’ may be linked to the absence of predators in these very small pools. Comparison of the present data with results of a survey made in the same area in 1995–1999 suggests that a decline of resident species occurred during the last decades, probably linked to human influences such as eutrophication and water pollution.

Importance of pools for temporary residents

It is well known that a large number of intertidal fishes stay in intertidal habitats only temporarily and that most of them shelter in these narrow and more or less isolated habitats as juveniles but move to deeper water as adults. This can be observed in mangrove areas (Nagelkerken et al. 2000; Laegdsgaard & Johnson 2001; Ikejima et al. 2003; Mumby et al. 2004; Jaxion-Harm et al. 2012) and estuaries (Miller et al. 1985; Able 2005; Vasconcelos et al. 2008; Figueiredo & Pessanha 2016), in which vegetated habitats within estuaries tend to harbour higher densities of many fish species compared to unvegetated substrates (Sogard 1992; Arndt & Fricke 2019).

Tidal pools also offer shelter for juvenile temporary residents (Mahon & Mahon 1994; Gibson & Yoshiyama 1999, Durville & Chabanet 2009; Ghanbarifardi & Malek 2009; Murase 2015; Sindorf et al. 2015; Gonzalez-Murcia et al. 2016). In La Réunion, we identified 205 temporary residents (71.9% of all intertidal species in the study area, see Table 1). Wrasses (Labridae), gobies (Gobiidae), damselfishes (Pomacentridae), groupers (Serranidae), moray eels (Muraenidae), and surgeonfishes (Acanthuridae) comprise temporary residents that can be found most frequently in tidal pools of La Réunion.