Introduction
The genus Euapta belongs to the apodous holothurians and includes five species, with Euapta lappa (Müller, 1850) being the only species recorded in the Atlantic Ocean. The beaded sea cucumber usually occurs in the shallow subtidal up to about 20 m depth (Pérez-Ruzafa et al., Reference Pérez-Ruzafa, Entrambasaguas, Espejo, Marcos and Bacallado2002; Wirtz, Reference Wirtz2003) but has been recorded up to a depth of 1350 m (Martins et al., Reference Martins, Souto, Braga and Tavares2018). Euapta lappa is a nocturnal species that feeds on detritus (Hammond, Reference Hammond1982; Wirtz, Reference Wirtz2003; Brewin et al., Reference Brewin, Brown and Brickle2016). Their body is soft, highly flexible and can extend up to 1 m in length (Heffernan & Wainwright, Reference Heffernan and Wainwright1974; Hammond, Reference Hammond1982). The beaded sea cucumber inhabits a wide range of habitats, including sandy bottoms, seagrass, coral reefs or rhodolith beds (Pérez-Ruzafa et al., Reference Pérez-Ruzafa, Entrambasaguas, Espejo, Marcos and Bacallado2002; del Valle-García et al., Reference del Valle-García, Abreu-Pérez, Solís-Marín and Laguarda-Figueras2013; de la Cruz-Francisco et al., Reference de la Cruz-Francisco, Morales-Quijano and González-González2017; Martins et al., Reference Martins, Souto, Braga and Tavares2018).
Euapta lappa can be found throughout the tropics in the Atlantic Ocean. In the West Atlantic, it is known from the Gulf of Mexico and the Caribbean Sea, including the Bahamas, Barbados and Trinidad and Tobago (Deichmann, Reference Deichmann1963; Miller & Pawson, Reference Miller and Pawson1984; Felder & Camp, Reference Felder and Camp2009). More recently, it also has been recorded from Espírito Santo and Trindade Island, Brazil, which constitutes the most southern record of this tropical species in the Atlantic (Martins & Souto, Reference Martins and Souto2020). In the East Atlantic, it has been recorded from São Tomé and Príncipe northwards to the Selvagens Islands, including the archipelagos of Cabo Verde and the Canary Islands (Pérez-Ruzafa et al., Reference Pérez-Ruzafa, Entrambasaguas and Bacallado1999, Reference Pérez-Ruzafa, Entrambasaguas, Espejo, Marcos and Bacallado2002; Wirtz, Reference Wirtz2003; Entrambasaguas et al., Reference Entrambasaguas, Pérez-Ruzafa, García-Charton, Stobart and Bacallado2008). In recent years an increasing number of non-indigenous species (NIS) has been recorded at Madeira Island (Castro et al., Reference Castro, Carlton, Costa, Marques, Hewitt, Cacabelos, Lopes, Gizzi, Gestoso, Monteiro, Costa, Parente, Ramalhosa, Fofonoff, Chainho, Haroun, Santos, Herrera, Marques, Ruiz and Canning-Clode2022), with many representing northern range expansions (e.g. Wirtz & Berenger, Reference Wirtz and Berenger2017; Ribeiro et al., Reference Ribeiro, Neto, Moreu, Haroun and Neves2019; Schäfer et al., Reference Schäfer, Monteiro, Castro, Rilov and Canning-Clode2019; Castro et al., Reference Castro, Schäfer, Parretti, Monteiro, Gizzi, Chebaane, Almada, Henriques, Freitas, Vasco-Rodrigues, Silva, Radeta, Freitas and Canning-Clode2021). This new record of the beaded sea cucumber, E. lappa, at Madeira Island adds to the growing list of tropical species recorded in the study region.
Sighting
In October 2022, a single individual of Euapta lappa was sighted during a scuba night dive near Caniçal on the south coast of Madeira Island (32°44′03.0″N 16°44′27.1″W; Figure 1). The animal was observed and photographed on a rocky substrate at about 17 m depth (Figure 2). It had a length of ~50 cm and appeared to be feeding.
Figure 1. Distribution of Euapta lappa in the Atlantic Ocean downloaded from the Ocean Biodiversity Information System (OBIS; 09 March 2023) and the Global Biodiversity Information Facility (GBIF; 07 March 2023) marked as circles. The location of the record described in the present study is displayed as a star.
Figure 2. Photograph of the beaded sea cucumber (Euapta lappa) specimen seen during a night dive at Caniçal, Madeira Island (photographed by Eliani Rodrigues and Miguel Rodrigues).
Discussion
The first record of the beaded sea cucumber, Euapta lappa, at Madeira Island constitutes the northernmost record of this species in the Atlantic Ocean. While commonly found up to the north of the Gulf of Mexico and Florida in the Western Atlantic (~31°N latitude, Figure 1), in the Eastern Atlantic, the previous northern limit of the species was the record of a single specimen at the Selvagen Islands in 1999/2000 (Pérez-Ruzafa et al., Reference Pérez-Ruzafa, Entrambasaguas, Espejo, Marcos and Bacallado2002). At the Canary Islands, about 150 km south of the Selvagens, E. lappa has been described as a common species in cryptic habitats (Hernández et al., Reference Hernández, Clemente, Tuya, Sangil, Moro-Abad and Bacallado-Aránega2013). However, studies investigating the benthic communities at the Canary Islands describe an eastern-to-western gradient concerning the E. lappa abundance, with usually higher densities encountered at the western islands of the archipelago (Hernández et al., Reference Hernández, Clemente, Tuya, Sangil, Moro-Abad and Bacallado-Aránega2013). In the past, this trend was also observed for other species with tropical affinities, such as the spotfin burrfish Chilomycterus reticulatus or ephemeral benthic algae species (Sangil et al., Reference Sangil, Sansón, Afonso-Carrillo, Herrera, Rodríguez, Martín-García and Díaz-Villa2012; Espino et al., Reference Espino, Tuya, Rosario, Bosch, Coca, González-Ramos, del Rosario, Otero-Ferrer, Moreno and Haroun2019). This imbalance in observations of certain tropical species most likely follows the prevailing temperature gradient in the Canary Islands, with western islands being slightly warmer than eastern ones (Barton et al., Reference Barton, Aristegui, Tett, Canton, Garcia-Braun, Hernandez-Leon, Nykjaer, Almeida, Almunia, Ballesteros, Basterretxea, Escanez, Garcia-Weill, Hernandez-Guerra, Lopez-Laatzen, Molina, Montero, Navarro-Perez, Rodriguez, van Lenning, Vélez and Wild1998; Sangil et al., Reference Sangil, Sansón, Afonso-Carrillo, Herrera, Rodríguez, Martín-García and Díaz-Villa2012).
With ongoing climate change, the sea surface temperature in the NE Atlantic is increasing (Taboada & Anadón, Reference Taboada and Anadón2012). Seawater temperatures at Madeira are currently warming at a rate of about 0.2 °C per decade, slowly shifting the current temperatures at Madeira close to values typical at the Canary Islands a few decades ago (Schäfer et al., Reference Schäfer, Monteiro, Castro, Rilov and Canning-Clode2019). This warming promotes the progressive tropicalization of the area, with previously strictly tropical species expanding their distributional limit northwards to Madeira (e.g. Bianchi et al., Reference Bianchi, Morri, Sartoni and Wirtz1998; Wirtz & Berenger, Reference Wirtz and Berenger2017; Schäfer et al., Reference Schäfer, Monteiro, Castro, Rilov and Canning-Clode2019; Castro et al., Reference Castro, Schäfer, Parretti, Monteiro, Gizzi, Chebaane, Almada, Henriques, Freitas, Vasco-Rodrigues, Silva, Radeta, Freitas and Canning-Clode2021). In this context, an increasing number of tropical species has been recorded at Madeira, including crustaceans, algae and fish (Ribeiro et al., Reference Ribeiro, Neto, Moreu, Haroun and Neves2019; Schäfer et al., Reference Schäfer, Monteiro, Castro, Rilov and Canning-Clode2019; Castro et al., Reference Castro, Schäfer, Parretti, Monteiro, Gizzi, Chebaane, Almada, Henriques, Freitas, Vasco-Rodrigues, Silva, Radeta, Freitas and Canning-Clode2021).
As in most of these cases, the vector for the arrival of the beaded sea cucumber to Madeira is unclear. Holothurians commonly reproduce through planktonic larvae allowing them to travel great distances (Doll et al., Reference Doll, Caballes, Hoey, Uthicke, Ling and Pratchett2022; Yu et al., Reference Yu, Wu, Tu, Hong and Luo2022). However, as the main current systems in Macaronesia follow a north-to-south pattern around Madeira, the northward transport of larvae by natural means is greatly limited (Mason et al., Reference Mason, Colas, Molemaker, Shchepetkin, Troupin, McWilliams and Sangrà2011; Sala et al., Reference Sala, Harrison and Caldeira2016). Castro et al. (Reference Castro, Ramalhosa, Jiménez, Costa, Gestoso and Canning-Clode2020) showed that in Madeira's case, a great number of new arrivals to the archipelago could be linked to ship traffic. The location where Euapta lappa was observed is in close proximity (~650 m) to Madeira's main shipping harbour in Caniçal. In the case of the beaded sea cucumber, transport of larvae or juvenile stages by ships seems likely. Ultimately, the exact vector responsible for the arrival of the species remains unknown. Further studies including a genetic analysis of Euapta lappa at Madeira are needed to gain more insight into its origin and possible vectors of introduction. The size of the specimen observed at Madeira suggests that it is already an adult. The transport of such a large specimen by accident is quite unlikely. Given the typical nocturnal behaviour of E. lappa, a fast detection in a new habitat might be hindered and a single individual could have been overlooked for some time.
The present study presents the first record of E. lappa at Madeira Island. This new record constitutes a northward range expansion of the beaded sea cucumber in the NE Atlantic by about 300 km. Future monitoring efforts are needed to evaluate the possible establishment and future spread of this species in the Madeira Archipelago.
Acknowledgements
Thanks to Eliani Figueira Jardim Rodrigues and Miguel Ângelo Pereira Rodrigues for providing the images, video and information about the record used in the present study. Furthermore, I would like to thank Peter Wirtz for kindly confirming the species identification based on photographs and video and comments on an early draft of the manuscript.
Financial support
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
