Echinoderm diversity of a tropical estuary in the largest river basin of the Costa Rican Pacific , Eastern Tropical Pacific

Echinoderms within the Eastern Tropical Pacific have mainly been studied in association with coral reefs. Investigations on echinoderms associated with soft-bottoms and estuaries are still scarce. The present study reports on the echinoderm species inhabiting the soft-bottom sediments of shallow-brackish waters adjacent to the largest river basin along the Pacific coast of Costa Rica, Térraba-Sierpe wetland. Nine species were recorded, three of them new records for Costa Rica: Luidia columbia, L. latiradiata, and L. superba. The most common species were L. columbia, Astropecten armatus, A. regalis, and L. latiradiata. This contributes towards current knowledge on the biodiversity of Térraba-Sierpe wetland and should be considered as a baseline upon which to monitor the effects of future impacts on this important mangrove area.


INTRODUCTION
Echinoderms are a diverse group of marine invertebrates, currently comprised of approximately 7000 known species (Pawson 2007;Appeltans et al. 2012).They play a key role in shallow-water habitats as herbivores, organic matter recyclers and predators of several benthic organisms (Jangoux 1982;Jangoux & Lawrence 1982;Menge 1982;Birkeland 1989).This mainly marine group is one of the most common and conspicuous in littoral environments across all latitudes, including the tropics (Lawrence 1987).
In Costa Rica, echinoderms have been studied sporadically (Alvarado & Cortés 2009).A total of 183 species have been reported for the Pacific coast (Alvarado et al. 2013).Most studies focused on echinoderms associated with coral reefs: Cocos Island (Deichmann 1941(Deichmann , 1958;;Clark 1948;Hertlein 1963;Guzmán & Cortés 1992;Lessios et al. 1996;Guzmán & Cortés 2007;Alvarado & Chiriboga 2008), Culebra Bay (Alvarado 2008), Marino Ballena National Park (Alvarado & Fernández 2005), and Caño Island (Guzmán 1988).Echinoderms associated with soft-bottom habitats have been less studied; the only information available is from the Gulf of Nicoya.Several authors provided a list of echinoderm species collected from this tropical estuary (Maurer & Vargas 1983;Maurer et al. 1984) and described aspects of their population dynamics (Vargas & Solano 2010).The lack of knowledge regarding echinoderm species from estuarine environments is not uncommon.Alongi (1990) had already mentioned that the diversity of subtidal fauna inhabiting soft bottoms has been poorly studied in tropical estuaries.
Térraba-Sierpe represents one of the most important wetlands in the Central American region and contains the largest mangrove forest in Costa Rica (Jiménez 1994).It is known for its diversity of fishes (Chicas 1995), decapods (Echeverría-Sáenz et al. 2003), and polychaetes (Sibaja-Cordero & Echeverría-Sáenz 2015).Moreover, this mangrove supports important local ark-clam fisheries (Vega 1994) and is a probable nursery ground for several elasmobranch species (Clarke et al. 2014).However, the soft-bottom habitats in brackishwaters outside Térraba-Sierpe have been poorly studied.Therefore, the present study aimed to provide an inventory of echinoderm species associated with soft-bottom sediments in Térraba-Sierpe, providing a unique opportunity to broaden our knowledge on euryhaline echinoderms in the ETP.

Study site
Térraba-Sierpe is an estuarine delta system located in the southern Pacific coast of Costa Rica (08°51′ N, 083°33′ W).The delta's two main tributaries are the Térraba and Sierpe rivers with drainage areas of approximately 5077 km 2 and 790 km 2 , respectively (see Umaña et al. 2015).This estuary probably presents a strong degree of connectivity with nearby coral reefs at Caño Island, Marino Ballena, and Corcovado (Quesada-Alpízar & Cortés 2006).
Annual mean precipitation in Térraba-Sierpe oscillates between 1500-6000 mm (Rojas 2011).Suspended sediments and nutrient concentration increase during the rainy season, accompanied by a decrease in salinity (Lizano et al. 2001;Umaña & Springer 2006).Brackish waters have been detected in demersal habitats off the estuaries during the rainy season (Picado 2015).The primary productivity in the estuarine region is low, with the highest values recorded in sites with high marine influence (0.79 ± 0.50 gCm -2 ) (Umaña et al. 2015).More recently, mangrove tree mortality has been detected along the seaward zone, probably related to changes in sea level and sediment dynamics (Lizano 2015).

Data collection
Sampling was conducted on a monthly basis from March 2013 to January 2014 in areas off Sierpe and Coronado estuaries at two depth levels: 5-10 m and 15-20 m (Figure 1; Table 1).The sampling was carried out alternating between the two estuaries during each month.There were three sampling stations at each depth level (Figure 1; Table 1).At each station a 20-minute tow was conducted using an artisanal trawl (1.27 cm cod-end mesh and 7 m mouth opening).Bottom salinity was measured using a SeaBird 19 Plus V2 SeaCAT profiler CTD.All collected specimens were stored on ice, transported to the laboratory of the Centro de Investigación en Ciencias del Mar y Limnología (CIMAR) in San José, where they were preserved in 70% ethanol.In this sampling and in the same area, parallel surveys on fish fauna were carried out using gill nets (9 cm mesh size) (Figure 1; Table 1).Moreover, fishing bottom long-lines (50 m length, 50 hooks, baited with sardines) were placed in the same area including depths up to 50 m (Figure 1; Table 1).An octocoral with brittle stars was tangled in a gill net and one starfish was caught in a bottom long-line feeding on the bait.These echinoderms were also included in this list of species.

RESULTS
Echinoderms were present in 23 of the 66 trawls (34.8%).Bottom salinity at sampling sites ranged from 25 to 34 parts per thousand (Table 1).A total of 225 specimens of echinoderms were recorded in four classes, four orders, six families, six genera, and nine species as listed below.
Material examined.Table 2; Figure 2A, B. Body flattened, slightly depressed in the middle of the disc.Oral plates with one terminal spine.Arms long and slender, sharping at the distal end.Both sides of arms with three rows of large paxillar zones, square-shaped.Middle region of arms and disc with small paxillar zones.Each inferomarginal plate with three spines, arranged in a characteristic marginal row.Each ambulacral plate with four spines.Each angle of man- Platasterias latiradiata -Gray (1871): 136.
Thin, depressed body with five broader rays near proximal ends, gradually narrowing towards blunt distal ends, narrow in their insertion with disc determining deep indentations between them.Margins of rays with a closed series of semicylindrical spines with blunt ends.Upper marginal plates, abortive, represented by paxillae.Simple papillae.
We found some specimens of the porcelain crab Minyocerus kirki Glassell, 1938 (probably as epibionts) on the surface of some specimens of L. latiradiata.New record for Costa Rica.Material examined.Table 2; Figure 2E, F. Arms large and sturdy, that sharpens towards the edge with a blunt point.Arms remarkably longer and larger compared to  other Luidia species.Adambulacral plates with one thin spine, saber-shaped, followed by one analogous spine, longer and robust but less curved.Conical spines arranged irregularly and zigzag on paxillae of third and fourth row of the edge of arms.Tube feet well developed and turgid, without suction cups.Mouth plates narrow, with 11 spines along the middle suture.
One specimen was caught feeding on sardine bait from a bottom long-line.New record for Costa Rica.

Astropecten regalis
Body flattened, with wide arms that sharpen towards the points.Abactinal radial surface with medial protuberance and two longitudinal hollows.Supramarginal plates with very small granuliform spines.Spine-free interradius.Madreporite presents longitudinal grooves.Inferomarginal plates with robust and corrugated spines.
Body compressed dorso-ventrally.Disc covered by fine granulation, reaching base of arms.Radial shields long, oval, Material examined.Table 2; Figure 4C, D. Disk covered by scattered grains of different sizes.Oral and adoral shields appear to be fused, forming a continuous ring; covered by skin.Tentacle scales absent.Six arm spines with well-developed hooks at tip, with third one being the longest.Color is mainly rosaceous, with variations within pale pink and purplish tones.
All specimens of O. mirabilis were collected living on a colony of the octocoral Leptogorgia laxa Hickson, 1928 that was caught with a gill net.Many individuals showed regenerated structures.
Test thin and fragile.The apex of test near extremity of odd ambulacra.Interambulacral lunulae remarkably small and variable in shape, compared with other species from the ETP.Anterior border of interambulacral lunulae closer to distal border of posterior petals than to center of abactinal system.Material examined.Table 2; Figure 6A, B, C, D. Depressed thin cylindrical body.Mouth in terminal position with 20 tentacles.Ambulacral feet distributed in five rows, ventral surface in cylindrical shape, dorsal surface palliform shape.Spicules from body wall as small tables, with basal part generally conic and with spines.Border with a system of spines that form a double Maltese cross shape.
A total of seven species were collected in Coronado, while eight species were obtained in Sierpe (Table 2).The most common species in the sediments was Luidia columbia with 82 individuals.A total of 56 individuals of Ophiothela mirabilis were recorded from a single octocoral colony.Other common species (> 10 individuals) were Astropecten regalis, A. armatus and L. latiradiata.The remaining four species were relatively uncommon with less than 10 individuals.

DISCUSSION
Here we report three new records of echinoderm species for Costa Rica: Luidia columbia, L. latiradiata and L. superba.Alvarado & Solís-Marín (2013) reported 36 asteroid species for the Costa Rican Pacific.Therefore, this study reveals that asteroid species richness in the Costa Rican Pacific is leastwise 8.3% (39 species) higher than previously reported.
It is important to note the presence of the sea star L. latiradiata, considered a living fossil related to extinct asterozoans (Fell 1962;Blake 1982).This species has been reported from other environments with estuarine conditions: Chiapas, Mexico (Caso 1970); Gulf of Fonseca, El Salvador (Barraza & Hasbún 2005); Corinto, Nicaragua (Caso 1970); close to Guapi River, Colombia (Neira & Cantera 2005), and Gulf of Guayaquil, Ecuador (Villamar & Cruz 1983).The limited number of records for L. latiradiata in the ETP is probably due to the few studies on echinoderms in estuarine areas.
The high abundance of Ophiothela mirabilis was related to its association with a single octocoral colony in the middle of a mangrove channel, and it was not collected in other areas.Recently, Hendler & Brugneaux (2013) recorded this species in association with two colonies of Leptogorgia miniata (Milne Edwards & Haime 1857) in brackish waters of the Amazon Barrier.The presence of this ophiuroid in the Térraba-Sierpe mangrove and the previous records near the Amazon suggest that brackish environments are probably not a biogeographic barrier for O. mirabilis.This is the first inventory of echinoderm species associated with soft-bottom sediments off Térraba-Sierpe.However, information on echinoderm diversity is available for the Gulf of Nicoya, another tropical estuary located further north along the Pacific coastline of Costa Rica.Maurer & Vargas (1983) reported the presence of four ophiuroid species in subtidal soft-bottoms, which were collected by a modified Smith-McIntyre bottom grab.Maurer et al. (1984) obtained a total of nine echinoderm species, which were collected with a semi-balloon shrimp trawl: two echinoids, five ophiuroids, and two asteroids, including A. armatus.Despite the geographical proximity of both areas (ca.150 km), only A. armatus was recorded in both the Gulf of Nicoya and Térraba-Sierpe (Maurer & Vargas 1983;Maurer et al. 1984;present study).The difference in species composition could be associated with the different gears used in each study and the different environmental characteristics of the two estuaries (such as productivity and salinity variations) (Maurer & Vargas 1983;Maurer et al. 1984;Picado 2015;Umaña et al. 2015).
Echinoderms are considered to be absent from brackish environments due to their stenohaline nature (Stickle & Diehl 1987, Dauvin et al. 2013).However, some estuaries from different regions have shown variable echinoderm diversity.For example, the subtropical estuary of the Yangtze river in China is inhabited by four echinoderm species (Chao et al. 2012); ten species of brittle stars were recorded in the subtropical estuarine region of Paranaguá Bay in southern Brazil (Barboza et al. 2015); only one species was reported in the Nahoon temperate estuary in South Africa (Bursey & Wooldrige 2002), while Josefson & Hansen (2004) mentioned 22 echinoderm species in a revision of several Danish temperate estuaries.Compared with these estuaries in other latitudes, Térraba-Sierpe has moderate echinoderm diversity (nine species).
Despite the recent increase of echinoderm studies across Latin America (Alvarado & Solís-Marín 2013), there are still some areas and topics that need more attention.Recording and publishing biodiversity with high taxonomic quality contributes to improve future efforts in research, conservation, and management, and is necessary to properly assess anthropogenic or natural impacts on marine ecosystems and possible effects on the ecosystem services due to biodiversity loss (Worm et al. 2006;Costello et al. 2013;Alitto et al. 2016).There is a project to build a hydroelectric dam in the main tributary river of Térraba-Sierpe wetland (Pérez 2011).Dams may influence the hydrological and sedimentological dynamics, and thus influencing the coastal systems and its biodiversity (Kowalewski et al. 2000).Therefore, the results of the present study complement our knowledge of Térraba-Sierpe wetland biodiversity and may contribute to future monitoring of impacts on this important mangrove area.

Figure 1 .
Figure 1.Study area in the Térraba-Sierpe mangrove, southern Pacific coast of Costa Rica, showing trawling sampling stations off Coronado and Sierpe estuaries at two depth levels: 5-10 m and 15-20 m.Stations 13 and 14 were sampled with a gill net and a bottom long-line, respectively.

Table 1 .
Information about depth, salinity, gear type used, and geographic coordinates of the sampling locations off Coronado and Sierpe estuaries in Térraba-Sierpe mangrove, southern Pacific coast of Costa Rica.

TAblE 2 .
List of echinoderm species, number of specimens, locality, depth range and voucher number, recorded in waters off Térraba-Sierpe mangrove, southern Pacific coast of Costa Rica.Localities are represented by C = Coronado; S = Sierpe.New records for Costa Rica = *.