Martinique is a volcanic island in the Caribbean that is part of the Lesser Antilles. It is located between 14˚23' and 14˚53' north latitude, and between 60˚50' and 61˚15' west longitude, between Dominica to the north and Saint Lucia to the south. It is surrounded by the Caribbean Sea to the west and the Atlantic Ocean to the east. Martinique has a geomorphology of the seabed that depends on its geological history [22]. It is characterized by coral reefs in the Atlantic part (from Sainte-Anne to Sainte-Marie). Barrier reefs are located from the Caravelle to Vauclin, covering 25 km and encompassing the Loup Garou cay, the Pinsonnelle cay and the Pariadis cay [23]. Inside this area, many fringing reefs and islets have formed such as the Eau islet, the Boisseau islet, Rats islet. In the south of Martinique, in Sainte-Anne, there are micro-lagoons at Cap Chevalier [23]. The Caribbean area (from Prêcheur to Anses d’Arlet) has slopes of volcanic origin. Formations of gorgonians, corals, sponges on rocks are present and are called non-bio-constructive reefs, i.e. they do not come from the formation of limestone skeletons. Generally, this area consists of boulder fields, then a rock slope, a rock platform and finally a wall (sub-vertical slopes). The further north you go, the steeper the slopes. The bay of Fort-de-France is not of volcanic origin and is composed of many coral structures such as the Vache caye [23]. It is certainly protected, but very much anthropized. The Diamant/Sainte-Anne zone, located in the Caribbean zone, is called the southern zone. It is made up of coastal reef platforms (structures that do not form a lagoon) and cayes (shoal) such as the Cherry Cay or the Trois Rivières Cays. The depth of the seabed is influenced by these different geological constructions, which explains why the Atlantic zone is flatter than the Caribbean zone. In Martinique, there is a great diversity of benthic biocenotic composition [24]: coral community, seagrass beds, algal community, bare soft bottom community, mixed community. Marine phanerogams are almost everywhere in Martinique and very little represented in the northern Caribbean area [24]. The dominant species are Thalassia testudinum and Syringodium filiforme. Other marine phanerogams are present such as Halophila stipulacea and Halophila decipiens [24][25]. The type of substrate that is mainly found in Martinique is sand. Other substrates are present on this island: rocks, mud, muddy sand, coral, detrital (pebbles, coral debris) [24].

The sites sampled are located throughout Martinique and are thirty-three in number (Figure 1). They are spread over different municipalities:

North Caribbean Zone: Prêcheur (Pointe Lamarre), Case-Pilote (Fond Bellemare, Vétiver);Bay of Fort de France (2 sites);Southern Caribbean zone: Anses d’Arlet (La Baleine, Grand Anse, Petite Anse, Town);

Figure 1. Map of sampled sites in Martinique.

Southern Zone: Diamant (Anse Cafard, La Cherry, Tête de Singe, Town), Sainte-Luce (Kaye Baré, Trois Rivières, Anse Mabouya, Anse Mabouya beach), Marin (Pointe Borgnese), Sainte-Anne (Pointe du Marin, Anse Caritan, Cabrits islet);Atlantic zone: Sainte-Anne (Hardy islet, Chevalier islet, Anse Michel), François (Caye Pinsonelle, Grande Basse 1, Grande Basse 2), Robert (Rats islet, Eau islet, Boisseau islet), Trinité, Sainte-Marie (Anse Azérot, Saint Aubin islet).

The study ran from January 2023 to November 2024. These sites were located at depths between 0 and 24 metres. The method used is the transect method. On a 60 m² transect with a length of 30 metres, the white sea urchins on both sides of the line (1 metre on each side) were counted. The type of substrate (soft, hard), the geomorphology of the seabed (barrier reefs, rocky slopes, cays, etc.), the benthic floristic species present and their state of health were also inventoried. The health of reefs and seagrass beds was assessed according to the principles of Bouchon et al. [26]. A macroscopic analysis of the sand was carried out in order to classify them into several classes: fine sand, medium sand and coarse sand. The density of white sea urchins was calculated over several months and for each habitat type. Concerning the rock-boring sea urchin, four sites that are part of the sites sampled for the white sea urchin were considered, two in the Caribbean zone (Diamant, Trois Rivières) and two in the Atlantic zone (Trinité, Robert). The study ran from March to December 2024. The number of white sea urchins and rock-boring sea urchins was counted on a 60 m² transect over three periods of April-June, July-September and October-December.

The Shapiro-Wilk test was used using the XLSTAT software to test the normality of the sample. As the data did not follow the normal distribution, nonparametric statistical tests were used. Differences in white sea urchin densities according to substrate type (soft, hard of biogenic origin and hard of volcanic origin) were analyzed using the nonparametric Kruskal-Wallis test (XLSTAT®). If the null hypothesis was rejected, a Dunn’s pairwise multiple comparison was used to determine which substrate types differed from each other. Density was expressed as an average ± standard error. For this test, the significance level was set at p < 0.05. The Mann-Whitney U test was tested using XLSTAT software to compare the densities of rock-boring sea urchins and white sea urchins for each period (April-June, July-September and October-December).

First, white sea urchins are located in various seagrass communities (Figure 2). They lived in the exclusive beds of Thalassia testudinum and in the exclusive beds of the invasive Halophila stipulacea seagrass beds on both muddy sand and sand. Also, they were present in mixed seagrass communities (Thalassia testudinum/Halophila stipulacea or Syringodium filiforme/Halophila stipulacea or Thalassia testudinum/Syringodium filiforme) on sand with low to medium grains. No white sea urchins were observed in the bare loose substrate (mud, muddy sand, sand) and in the meadows of halophila (Halophila stipulacea, Halophila decipiens) on coarse sand. This plain of Halophila seagrass beds was located after the rocky slopes in the Caribbean coast (Case-Pilote, Anses d’Arlet) at depths greater than 15 metres. White sea urchins were more numerous in mixed meadows of Thalassia testudinum and Syringodium filiforme on sand and less in invasive meadows of Halophila stipulacea, alone or with Syringodium filiforme. However, at Pointe Borgnese, when this invasive species is associated with the Thalassia testudinum seagrass, the number of sea urchins was close to that observed in mixed seagrass beds. Tripneustes ventricosus generally preferred sand as a substrate than muddy sand. Pointe Borgnese was the site with the most white sea urchins and a greater diversity of marine phanerogams.

Figure 2. Number of white sea urchins per site according to the type of substrate and their biocenotic composition in marine phanerogams.

In Fort-de-France, Anses d’Arlet (Grande and Petite Anse) and Sainte-Anne (Cabrits islet, Hardy islet and Chevalier islet), despite the presence of meadows of Halophila stipulacea on muddy sand or mixed meadows of Thalassia testudinum and Syringodium filiforme, they were devoid of sea urchins (Table 1).

Concerning the hard substrate of volcanic origin, white sea urchins have been found at Anses d’Arlet, in the fields of boulders with brown algae and only one in the rocky slope. This slope was composed of corals, sponges, gorgonians, turf and was very degraded with the presence of Sargassum sp. In the North Caribbean zone (Prêcheur, Case-Pilote), they were absent. It was composed of rock block fields with encrusting corals and turfs, slopes and rock platforms with mixed populations (corals, sponges, gorgonians, turfs).

Table 1. Characteristics of sampled sites with no white sea urchins.

Finally, in hard substrates of biogenic origin (of coral origin), white sea urchins generally lived on coastal reef platforms, composed of coral debris, dead and/or live corals. At the level of the Chéry Caye located in Diamant, the white sea urchins were particularly visible on the shoal and less so in the meadows of Thalassia testudinum located between the cay and the coast. The seagrass area after the cay was devoid of Tripneustes ventricosus. In addition, at the level of the barrier reef, the Pinsonnelle Caye had no white sea urchins. The sea urchins thrived in the reef flats of the fringing reefs in François and Robert. On the other hand, none were observed in Trinité and Sainte-Marie. The particularity with the Eau islet in Robert compared to the nearby islet Boisseau is that the Eau islet has a more marked back-reef depression. In addition, it is in this area that there were the most white sea urchins. And when we head out to sea, and therefore towards the reef front, the more the number of sea urchins decreased (Figure 3). The back-reef depression was composed of mixed populations of gorgonians, coral patches, seagrass beds, rocks, brown algae while the reef flat had Sargassum polyceratium. The reef front was made up of degraded corals and gorgonians.

In the end, white sea urchins significantly prefer the soft substrate, unlike the hard substrate of volcanic origin (P = 0.0002) and the hard substrate of biogenic origin (P = 0.015) (Figure 4). When comparing the two types of hard substrate, it flourished particularly in hard substrates of coral origin (P = 0.028).

In the Caribbean, the population of rock-boring sea urchins increased between April and December in Trois Rivières or remained more or less stable in Diamant (Figure 5). On the other hand, over the same period, white sea urchins were rare in Sainte-Luce or decreased in Diamant.

Figure 3. Number of white sea urchins as a function of the difference in level of the fringing reef at the Eau islet in Robert.

Figure 4. Number of white sea urchins per square meter (mean ± standard error) according to the type of substrate and the typology of the seabed.

Figure 5. Number of white sea urchins and rock-boring sea urchins between the months of April-June, July-September, October-December in Trois Rivières, Diamant, Trinité and Robert.

In the Atlantic zone, in Trinité, there were no white sea urchins (Figure 5). For rock-boring sea urchins, their density ranged from 0.7 to 1.1 individuals per square metre. Between October and December, due to weather conditions, no data were recorded. In Robert, the population of Tripneustes ventricosus and rock-boring sea urchins was almost at the same level over the period from April to September (Figure 5). However, an increase in the number of rock-boring sea urchins was observed in November while the number of white sea urchins decreased.

In the end, the population of Echinometra lucunter is higher than the white sea urchins between the months of April and December at all the sites sampled. These rock-boring sea urchins were significantly larger only for the months of July-September (P < 0.001) and October-December (P < 0.0001). These sea urchins have been found in biogenic hard substrates, on reef flats and coral patches.

White sea urchins are more numerous in mixed seagrass communities in good condition of Thalassia testudinum and Syringodium filiforme. These seagrass beds are part of their food preference [3][4][16]. According to our observations, white sea urchins also manage to develop on the meadows of Halophila stipulacea. It is an invasive species that comes from the Red Sea. It was first observed in Martinique in 2006 and continues to spread [25]. It is generally visible in anchorage areas [25][27] such as in Anses d’Arlet. Nevertheless, the number of sea urchins is lower compared to other seagrass beds, which can be explained by the fact that Halophila stipulacea has a lower nutritional value than that of Thalassia testudinum and Syringodium filiforme [28]. In the bay of Fort-de-France, at the Cabrits islet and Grande Anse, the reasons that may explain the lack of white sea urchins are as follows: first, the seagrass beds are very degraded and the second, there is a strong nautical activity and/or agricultural production nearby. Similarly, the Hardy islet and Chevalier islet are areas with a lot of poaching. In addition, white sea urchins may also be present on other communities of marine phanerogams in Martinique, such as in the exclusive beds of Syringodium filiforme [29].

White sea urchins seem to dislike hard substrates of volcanic origin. These habitats are more or less devoid of food for white sea urchins. Only the commune of Anses d’Arlet is colonised by white sea urchins. The town of Anses d’Arlet was composed of fields of brown algae blocks, dominated by Dictyota sp. This algae is part of the diet of the white sea urchin [3][30]. At the site of La Baleine (Anses d’Arlet), the only sea urchin present had a pile of greenish-brown threads on its test. This has never been observed before, nor described in the literature. In addition, according to divers, there are usually no white sea urchins in this area.

The southern zone is an area with the most white sea urchins. It is composed of hard biogenic substrate. It is in this area that fishing for white sea urchins is more important. At the Chéry Caye, our observations were able to show the importance of this coral structure for the white sea urchin. Indeed, the cays facilitate the metamorphosis of the larvae and therefore allow a larger population on these shoals [16]. On the other hand, in the Atlantic zone, an extinction of the population of Tripneustes ventricosus has been observed. According to fishermen, Trinité and the cays of François were once rich in white sea urchins. In Robert and François, they live hidden in the crevices of reef flats or shallow coral patches (1 - 2 metres). This behaviour is usually visible when the light is too bright [2]. It is accentuated when the depths are shallow. Numerous shells of dead young sea urchins have also been found near the sites studied in François. The high temperatures of recent years may have had a negative impact on the white sea urchin population. Sea urchins do not tolerate high temperatures [31].

The presence of white sea urchins depends on the type of substrate, the biocenotic communities and therefore on the geomorphology of the seabed. The population of Tripneustes ventricosus is concentrated in Anses d’Arlet, in the southern zone and in the Atlantic part. In the North Caribbean (Prêcheur-Case-Pilote), they are absent. In this area, food for white sea urchins is scarce, the slopes are too steep for the white sea urchin to develop. It is also very rich in corals, gorgonians and fish. Nevertheless, according to fishermen and divers, there are white sea urchins in Prêcheur at depths of about 40 metres.

In the Caribbean, in Jamaica, Haley and Solandt [14] also noted this difference in density between the front and back of the reef. They found that the population of white sea urchins was larger and more stable at the back reefs. The latter was composed of seagrass beds and corals.

The rock-boring sea urchins, Echinometra lucunter are significantly larger than the population of white sea urchins. Moreover, in Trois Rivières (Kaye Baré), a significant increase was observed between 2022 and 2024 (personal observation). At the beginning of July, there was a strong swell in Martinique, particularly in Diamant, caused by the passage of Hurricane Beryl. It seems that the rock-boring sea urchins were not affected by this swell, unlike the white sea urchin, which, on the contrary, accentuated its decline. This may be due to the fact that rock-boring sea urchins are smaller in size and tend to lodge in small crevices. This sea urchin is also a species that is resistant to heat stress, unlike the Tripneustes ventricosus. The slightest thermal variation can lead to numerical erosion of the latter [31]. A further study over time will confirm these results and understand the reasons for its resistance to environmental stresses.