**1. Introduction**

Coral reefs are recognized as one of the most important marine ecosystems on the planet, since they host the highest biodiversity among marine environments [1]. The complex topography created by the living organisms, such as cnidarians and sponges, provides a three-dimensional structure that supports an incredible diversity of organisms, well suited for species interactions and associations [2]. Unfortunately, this fundamental environment is experiencing severe degradation due to the impacts directly related to climate change and anthropogenic activities [3]. As these ecosystems disappear, scientists find themselves racing against time to increase our knowledge of cryptofauna ecological interactions and their potential role in the survival and resilience of the reef ecosystem [4]. For example, hermatypic corals have evolved crucial microbial symbiotic relationships in order to maintain their health status, improve energy production, cope with environmental changes, complete nutrient recycling, have a defense mechanism for predators, or as a

**Citation:** Gobbato, J.; Magrini, A.; García-Hernández, J.E.; Virdis, F.; Galli, P.; Seveso, D.; Montano, S. Spatial Ecology of the Association between Demosponges and *Nemalecium lighti* at Bonaire, Dutch Caribbean. *Diversity* **2022**, *14*, 607. https://doi.org/10.3390/d14080607

Academic Editors: Harilaos Lessios and Michael Wink

Received: 25 June 2022 Accepted: 26 July 2022 Published: 28 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

protection from potential pathogen agents and coral feeding organisms [5–8]. Additionally, stony corals have also developed a symbiotic association with several distinct phyla that are involved such as Cnidaria, Porifera, Echinodermata, Annelida, Arthropoda, and Mollusca [9].

Hydrozoans are an example of a group of organisms that has been able to develop a plethora of symbiotic relationships with several marine organisms [10,11], including scleractinians and sponges [12–14]. Currently, there are records for a total of 20 hydrozoan families and 50 genera involved in symbiotic associations with different animals worldwide [15]. Sponges emerged as a suitable host due to the constant water filtration, which results in the continuous presence of nutrients that are available to its symbionts [15,16]. In particular, there are six families of hydrozoans that are generally found in relation to sponges (Cytaeididae, Corynidae, Cladonematidae, Tubulariidae, Sphaerocorynidae, and Campanulariidae) [16]. Worldwide, a total of 26 species of hydrozoans have been identified as epibionts of sponges; however, little information is known about most of these associations [16,17].

Bonaire coral reef systems have recently been recognized as one of the most biodiverse, robust, resilient, and healthy ecosystems in the South Caribbean region [18]. In this context, the island serves as an interesting hotspot to study hydrozoan–sponge associations, since sponges are one of the dominant benthic groups on the reef, second only to corals [19]. Recently, several studies have been conducted identifying novel symbiotic relationships between the reef organisms, such as the zoantharian *Parazoanthus axinellae* epibiotic on the sponge of the genus *Axinella* [16], *Pteroclava krempfi* with alcyonaceans [20], the sponge *Agelas conifera* and the agariciid corals *Agaricia agaricites* and *Helioseris cucullata* [21], the coral-gall crab *Opecarcinus hypostegus* and the agariciid *Agaricia undata* [22], crabs of the genus *Platypodiella* and zoantharians of the genus *Palythoa* with the sponge *Niphates digitalis* [23], sponges, scleractinians, ascidians and zoantharians with polychaetes *Spirobranchus* [24,25], and the *Stylaster*–*Millepora* association first reported in Bonaire [26]. Nevertheless, coral reef-associated fauna remain strongly understudied, and the total number of species of micro- and macro-invertebrates involved in association with other reef organisms in this region remains largely unknown, despite the potential benefit that these cryptic associations may have on the survival and resilience of the coral reef ecosystems [4,8]. One of these understudied organisms is *Nemalecium lighti* (Hargitt, 1924), a common thecate hydroid species belonging to the Haleciidae family that can be found all year round in all tropical waters, constituting one of the most abundant hydroid species [27,28]. *N. lighti* can be usually found on reef rock substrate, on corals, and on sponge surfaces, where it can better exploit the presence of planktonic particles to feed in the water column [29,30]. Its presence seems to have no influence on the functionality of the feeding strategy of the sponge host, as already demonstrated for other hydrozoans species [16,30], but see [31]. Therefore the impact of these associations on the sponges appears negligible, or even beneficial in some cases, as it may act as protection from predators thanks to the hydrozoan nematocysts [30,32].

In light of this, there are few studies that have examined the spatial ecology of crypto invertebrates associated with sponges [33,34]. Therefore, the goal of this study was to investigate and characterize the association of *Nemalecium lighti* with sponges in the coral reefs of Bonaire Island, with particular attention focused on determining the host range, prevalence, and distribution of this association. The results obtained provide a foundation for additional studies aimed at bridging the gap in our understanding concerning the cryptofauna diversity and its fundamental ecological role in coral reef ecosystems.

#### **2. Materials and Methods**

Underwater surveys were conducted between May and August 2021 to investigate the prevalence and distribution of *Nemalecium lighti*–sponge associations (Figure 1) in the reef system around Bonaire Island (12◦12 N, 68◦35 W), an area which is entirely protected since 1979 as part of the Bonaire National Marine Park (BNMP) [18].

**Figure 1.** Two examples of the association between demosponges and *Nemalecium lighti* in Bonaire reef system: *N. lighti* associated with (**a**) *Scopalina ruetzleri* and (**b**) *Ircinia* sp.

Along the west coast of the island, 16 different sites were chosen randomly based on their SCUBA shore-diving accessibility (Figure 2 and Table 1).

**Figure 2.** Map of Bonaire, Dutch Caribbean (12◦12 N, 68◦35 W) highlighting the dive sites investigated for sponges–*Nemalecium lighti* association in this study. Map made from OpenStreetMap loaded into QGIS.


**Table 1.** Coordinates, maximum and mean value of prevalence of association between sponges and *Nemalecium lighti* for each of the dive sites considered for the analyses in the study area.

Quantitative analyses were conducted by SCUBA diving, randomly placing three belt transects of 25 m × 2 m at two different depths for each site (total = 96 transects), resulting in 16 "shallow" stations between a 5–9 m depth and 16 "deep" stations between a 10–15 m depth.

Every sponge individual encountered within our transects, including without the presence of *Nemalecium lighti*, was counted. The prevalence was calculated as the number of sponges associated with *N. lighti* divided by the total number of sponges counted at that specific time and place. In addition, the taxon-specific prevalence for each sponge's genus was calculated as the number of sponge hosting associations for each genus, divided by the total number of counted sponges belonging to the same genus, according to Montano et al. 2016 [20]. All sponges were photographed in situ and were identified at the genus level using the relevant literature [35]. Sponges were included in the dataset and counted only when 50% of the individual or more lay within the belt transect area. Furthermore, the potential relationship between the association and the host size was evaluated through a comparison of the observed prevalence with that of five sponge size classes (C1: 5–10 cm; C2: 10–20 cm; C3: 20–30 cm; C4: 40–50 cm; C5: > 50 cm). The size of the sponges was estimated by placing a tape measure on the side of each specimen.

All the data obtained were tested for normality with Kolmogorov–Smirnov tests. In case the normal distribution and homogeneity of variance was violated, Kruskal–Wallis and Mann–Whitney *U* tests were performed to analyze the mean differences between the sites, depths, and dimensions of the sponge host. Data are presented as the arithmetic mean ± standard error unless stated otherwise. All the statistical analysis performed for this study were conducted using IBM SPSS 27 Software (IBM SPSS 27, New York, NY, USA).
