Polychaetes associated with decaying wood in Térraba mangrove, South Pacific, Costa Rica

The present study shows spatial patterns in the faunal assemblage of decaying wood and sediments, with emphasis on the polychaetes. The survey was executed across a salinity gradient in a tropical mangrove estuary of Costa Rica. To capture the organisms we analyzed decomposing logs found in the Térraba mangrove and sediment samples were taken with a corer in the sand bottom. Seven different phyla were found in the sediment samples. Of the 192 individuals found in the sediment samples, 18 were polychaetes belonging to nine families and 11 species. Analyses of decaying wood resulted in 2 564 individuals distributed in five phyla. Polychaetes accounted for 429 individuals belonging to eight families and 16 species. Although, polychaetes were more abundant in decaying wood, and their diversity was lower. The abundance of polychaetes in decaying wood was negatively correlated with the number of individuals of Mollusca, Hexapoda and Crustacea. A change in the composition of polychaetes in decaying wood was found along the salinity gradient of this estuarine zone. Dissimilarities in the composition of benthic fauna in decaying wood and sediments in the Térraba mangrove showed that biodiversity was increased by the microhabitats inside the mangrove forest. Finally, several genera or species of polychaetes are new records for Costa Rica and the Central American Pacific Coast. Rev. Biol. Trop. 63 (Suppl. 1): 61-74. Epub 2015 April 01.

In spite of this known high biodiversity, there are few studies that include detailed quantification and identification of organisms (Skov, Vannini, Shunula, Hertnoll, & Cannicci, 2002).Jiménez (1994) indicated that taxonomic and comparative studies are lacking, especially in the Central American region.Polychaetes are one of the main groups in soft bottoms, and some inhabit mangrove roots and decaying wood (Alongi, 1990;Molina-Lara & Vargas, 1995;Glasby, 1999;Dean, 2004).The main studies concerning intertidal fauna in sediments or mangroves have been developed in the Gulf of Nicoya, Costa Rica (Vargas, 1987;Perry, 1988).
The Térraba-Sierpe mangrove is classified as a wetland of international importance (RAMSAR site), and it is the biggest mangrove forest in Costa Rica (Cordero, 2000), representing almost 40% of the total mangrove forests of the country (Lahmann, 1999).Chicas (1995), Vega (1994), and Echeverría-Sáenz, Vargas and Wehrtmann (2003) have worked in this mangrove with commercial ichtyofauna, populations of Anadara (Bivalvia), and taxonomic diversity of decapods, respectively.However, the region of Térraba-Sierpe lacks studies of other marine invertebrates.
In this paper, we present data on the faunal assemblage, with emphasis on polychaetes, present in decaying wood.Additionally, the relationship of this fauna of decaying wood with the salinity gradient in a tropical mangrove of Costa Rica is analyzed

MATERIALS AND METHODS
Study area: Térraba-Sierpe is located in Puntarenas province on the South Pacific coast of Costa Rica (Cordero, 2000).This mangrove has an area of 16 700 ha (Lahmann, 1999).The most common tree species at Térraba are Rhizophora mangle, Rhizophora racemosa, Pelliciera rhizoporae, and Mora oleifera (Mainardi, 1996).The average annual precipitation and temperature are 3638mm and 26.5ºC, respectively (Cordero, 2000).Because of its high precipitation and the great amount of freshwater throughout the year, Térraba, together with Sierpe and Golfito, are the structurally most developed mangroves of the country with canopies that reach 40m in height (Jiménez & Soto, 1985;Polanía, 1993;Silva, 2009).geographic coordinates were taken with the help of a Garmin eMap GPS.Time, collecting technique, salinity (PSU) using a WTW LF 340 refractometer, and the substrate type (sediments or decaying wood) were also annotated.

Methodology
The first goal was the collecting of decapod species (Echeverría-Sáenz et al., 2003), however, in the present study we identified other faunal groups found in these samples.To capture the organisms that lived in the decaying wood we collected one fairly decomposed log (approximately 60cm long x 12-15cm diameter) from each field trip (Fig. 1).This log was packed in a plastic bag directly in the field to prevent animals from escaping.Once in the lab, the log was broken down into smaller pieces which were then put in a bowl with a formalin solution (5%).To remove the associated fauna the larger pieces were washed and sieved through a 1mm mesh, and all the remaining mixture of small pieces of wood, and animals, were separated using a dissecting microscope.
Sediment samples (Fig. 1) were taken with a core that consists of a pump made with PVC (5.5cm diameter, 15cm in the sediment).This pump was placed on top of the exit holes of organisms and the mud was sucked up, and the resulting mud core was sieved through a 1mm mesh.We also used a shovel to remove mud (5.5cm diameter, 15cm in the sediment) when the holes were not evident was sieved through the 1mm mesh.All organisms were fixed in a 5% formalin solution, and later stored in 70% alcohol.
All samples were processed, and organisms were identified after each field trip at the Museo de Zoología and CIMAR of the Universidad de Costa Rica.Polychaetes were identified with the keys of Fauchald andReimer (1975), Salazar-Vallejo, León-Gonzalez andSalaices-Polanco (1988), Glasby (1999), Dean (2001a) and de León-González et al. (2009), and original descriptions cited in the results.Voucher specimens have been deposited at the Museo de Zoología, Universidad de Costa Rica (MZ-UCR).
The free software PAST was used in the data analysis (Hammer, Harper, & Ryan, 2001), The Shannon-Wiener diversity index (H^), equitability of Pielou (J), and T statistictest for Shannon-Wiener were calculated for the polychaetes of sediment and decaying wood samples.The 95% confidence limits of Shannon-Weiner values were calculated using the Jackknife technique (Krebs, 1999).The similarity of species identity was measured with Sorensen presence/absence index (Krebs, 1999).
The Cramer´s V was used to measure the relationship of polychaetes with the other taxonomic groups found only in the decaying wood samples.The results were plotted with a 2-axis Correspondence Analysis of the taxa/ station abundance data.The direction and rate of increase in number of individuals of each taxonomical group were represented by vectors (Quinn & Keougth, 2003).Finally, a Principal Component Analysis of a correlation matrix (Quinn & Keougth, 2003) was carried out with the abundance data of taxa found in decaying wood in relation to the salinity level (PSU) measured in the field during the sampling event.The goal of this technique was to show the similarity in the distribution of taxa along the salinity gradient.

RESULTS
Sediment samples yielded 192 individuals belonging to seven phyla; Annelida, Arthropoda, Chordata, Echinodermata, Mollusca, Nemertea and Sipuncula.Polychaetes were found in only four of the 10 sediment samples and accounted for 18 individuals belonging to nine families and 11 species.On the other hand, 2 564 individuals were found in the eight decaying wood samples belonging to five phyla; Annelida, Arthropoda, Cnidaria, Mollusca and Nemertea.Polychaetes occurred in all wood samples with 429 individuals belonging to 16 species in eight families.Additionally, there were two Oligochaeta specimens in the sediment samples and 183 individuals in the decaying wood samples.The annelid worms represented 10.4 and 23.9% of the total abundance of organisms in sediment and decaying wood samples, respectively (Fig. 2).Hexapoda (mainly Diptera larvae) and Pholadidae (Bivalvia) were present only in the wood samples.Echinodermata and Sipuncula were absent in wood samples (Fig. 2).
The decaying wood of Boca Zacate (Fig. 1) presented low numbers of invertebrates (Fig. 3B) but in stations F and H at least 49% of them were polychaetes, representing the stations with the greatest percentage contributions of these worms (Fig. 3A).Stations A and B of Boca Coronado (Fig. 1) had nearly 190 individuals and polychaetes were also dominant in the most external station A (Fig. 3A) but only represented 18% of the abundance in the most internal station B (Fig. 3B).The stations A, B, and E had high numbers of capitellids (31-76 ind.) and oligochaetes (27-85 ind.), but low numbers of other polychaete families.Capitellids and oligochaetes were scarce in the rest of the stations (<10 ind.).Additionally, the most internal stations in the mangrove (E, D, and C) had the highest total abundances of invertebrates (Fig. 1, 3B).The polychaetes were most abundant in station E (117 ind.), but represent only 33% of the fauna.In station D, a high number of insect larvae were found, and the most abundant polychaete was Namaneries sp. with 76 individuals.The abundance of polychaetes in decaying wood was negatively associated with the number of individuals of Mollusca (V=0.78,p<0.001),Hexapoda (V=0.75, p<0.001), and Crustacea (V=0.64,p<0.001).Station C, which had a high abundance of these groups, also had low total polychaetes abundance (Fig. 3A, B).Some of the association patterns observed between species in the decaying wood could be explained by the salinity gradient.Hexapoda, Namanereis worms, capitellids, and Acari showed their maximum abundance at low salinity stations (0-0.2PSU) (Fig. 4).Crustacea, Mollusca, and Oligochaeta dominated in salinities between 4 and 10 PSU.Six species of polychaetes occurred at 13 PSU.This section   of the gradient contained the highest number of polychaete taxa (15).A. succinea was most abundant between 13 and 18 PSU while the rest of the polychaetes were present in the higher salinity sites (18-26 PSU) (Fig. 4).
of the body (Fauchald & Reiner, 1975).This is a new record of the species for Costa Rica.

Family Lumbrineridae
Scoletoma uncinigera (Hartmann-Schröder, 1959) (Fig. 5C) Remarks: Prostomium entire, the five pairs of maxillae (Fig. 5C) agree with the description of Hartmann-Schröder (1959).The maxillae II is in similar length to maxillae I, as in the Fig. 158 of Hartmann-Schröder (1959), for this species.The first parapodia lacks branchiae.The postsetal lobe is digitiform; the aciculae are yellow, and only multidentate, simple hooded hooks are present.These characteristics indicate that the specimen is S. uncinigera (Hartmann-Schröder, 1959;Carrera-Parra, 2009).This species has been reported from El Salvador by Hartmann-Schröder (1959).This is a new record of the species for Costa Rica.

Remarks:
The lack of pharyngeal papillae and paragnaths, and the reduced parapodia indicated this specimen is of Namanereidinae, and the pygidium with multi-incised rim characterized then as genus Namalycastis (Fig. 5E).The specimen is similar to the redescription of N. kartaboensis by Glasby (1999).The body has 128 setigers and is uniform in width anteriorly and tapers gradually in the posterior.The dorsum is convex, and the ventrum is flat.A narrow longitudinal groove is present on the prostomium.The specimens have two pairs of black eyespots with the posterior pair slightly smaller that the anterior (Fig. 5D).Antennae are smooth and extend to the tip of the palpophore.Jaws with a single robust terminal tooth, five subterminal teeth and seven proximally ensheathed teeth.Tentacular cirri with cirrostyles weakly segmented.Dorsal cirri decreasing slightly in length posteriorly.Notopodial sesquigomph spinigers are present from setiger 4-5, supra-neuroacicular setae include sesquigomph spinigers, in postero-acicular fascicles, and heterogomph falcigers, in preacicular fascicles.Supra-neuroacicular falcigers in setiger ten with smooth blades (serrations are absent in blades).This character is only present in Namalycastis brevicornis Glasby, 1999 and N. kartaboensis, but the blade 5.0-7.7×longerthan width of shaft head is characteristic of N. kartaboensis as in the specimen found (Glasby, 1999).The Sub-neuroacicular setae include heterogomph spinigers in post-acicular fascicles and heterogomph falcigers in preacicular fascicles, with smooth blades in the mid-posterior region.Sub-neuroacicular spinigers in mid-posterior parapodia with blades having coarse serrations prox imally.
The species N. kartaboensis is been previously reported from Surinam and Guyana (Glasby, 1999), and by this reason, the specimen found is identified as N. cf.kartaboensis.This is a new record for the Pacific coast of Central America.

Remarks:
The lack of pharyngeal papillae and paragnaths, and reduced parapodia indicated that this specimen is Namanereidinae.The specimens are placed in Namanereis by the presence of acicula, without notosetae in the parapodia.Also, the genus have a tripartite pygidium (Fig. 5G), with two smaller lateral lobes and a smaller, pointed dorsal lobe; and the anus is terminal (Glasby, 1999).Specimens have 28-83 setigers with a mode of 67.The worms are uniform in width, tapering in the far posterior; convex dorsally and flat ventrally.The prostomium has an anterior cleft or shallow dorsal depression.The antennae are cirriform and smooth and slightly shorter than the palpophore.Two pairs of black eyespots present with the posterior pair smaller.Three pairs of tentacular cirri are present (Fig. 5F).This is the first record of this genus for the Pacific coast of Central America.The individuals presented heterogomph falcigers and spinigers with serration on the shaft.This is a possibly non described species for the region.
Lepidonotus spiculus (Treadwell, 1906) Remarks: At least 26 segments, twelve pairs of elytra with macro and microtubercles, the former conic and large.The specimens presented a thorny and unidentate neuroseta (Fig. 5H, I) (Salazar-Silva, 2006).This is a new record of the species for Costa Rica.

DISCUSSION
The mangrove habitats: The logs deposited in mangrove floors represent shelter and food sources for various taxa.Decaying wood, as well as the roots of the mangrove trees, are hard substrates that are colonized by borer species and later by species that use this newly generated living space.These are three dimensional structures that accumulate sediment in their holes and crevices but since the logs are exposed to the water current during the tidal cycle, they remain well oxygenated.In this way, a high abundance of several groups can be found in the decaying wood.On the contrary, the fine grain soft bottom in the mangrove is anoxic below the 2cm superficial layer (Vargas, 1987).Infaunal species are reduced in their abundance in anoxic sediment (Alongi, 1990;Little, 2000).These conditions probably explain the higher abundance and diversity of fauna found in decaying wood when compared to sediment samples.
Some taxa, such as the sedentary tube worms Diopatra ornata and Owenia collaris, were only found in the mangrove sediments studied.These species are not able to make their tubes in the decaying wood habitat.Also, Echinodermata and Sipuncula were absent in wood substrates of the Térraba mangrove, possibly due to their incapability to bore in the wood and by the low salinity measured in the sampling sites.Finally, the high percentage of crustaceans in the sediment samples is explained by the fact that the methodology was directed towards the capture of this type of organisms (Echeverría-Sáenz et al., 2003).

Polychaetes:
The differences in the species reported from sediments and decaying wood in Térraba mangrove can be related to the ability of organisms to bore in the wood or by salinity preferences.Polychaetes presented lower diversity in the decaying wood than in the sediment samples with the main difference being the dominance of the families Capitellidae and Nereididae in the decaying wood.The family Nereididae has been found in several mangrove roots in previous surveys, in Costa Rica (Dean, 2001a).The species Alittas succinea was abundant in decaying wood and was present in lower abundances, in the sediments.In Punta Morales, Gulf of Nicoya, the same worm (as Neanthes succinea) was found in mangrove roots and the near intertidal rubble bottom.Another nereid, Platynereis dumerilii (Audouin & Milne Edwards, 1834), was collected only in the decaying wood, but the previous record of this species was found in a rocky intertidal tide pool, in Golfo de Papagayo (Dean, 2001a).Perinereis seridentata (Hartmann-Schröder, 1959) is a species collected on intertidal mud and mangrove roots of Golfo Dulce.The Perineis species found in Térraba mangrove was collected in wood and sediment samples.Nereis oligohalina (Rioja, 1946) was found only in the decaying wood of Térraba.Moreover, N. oligohalina was collected in mangrove roots at Jicaral, Gulf of Nicoya, and sediments of Golfito mangrove (Dean, 2001a).Another nereid species, Nereis (Neanthes) micromma Harper, 1979, was found only in sediments of Térraba mangrove but Dean (2001a) reported this species from mangrove roots in Punta Morales, Gulf of Nicoya.Nereididae specimens of the same species of the present study were reported in similar mangrove habitats of other tropical sites (Londoño-Mesa, Polanía, & Vélez, 2002;Molina-Lara & Vargas, 1995).The results represent an extension of the knowledge of the natural history of these species in the Costa Rican Pacific coast.
The subfamily Namanereidinae was found in the decaying wood of Térraba mangrove.This subfamily is known to live in the littoral and supralittoral areas, in association with decaying wood and leaves of mangroves systems (Glasby, 1999).One of the species collected in Térraba was Namalycastis cf.kartaboensis, which was previously reported from muds in the Surinam River.Another species of the same genus, Namalycastis borealis Glasby, 1999, has been reported associated with mangrove roots in Belize.Similar associations with decaying vegetative matter have been noted for Namalycastis abiuma (Grube, 1872) (circumtropical species) and Namalycastis indica (Southern, 1921) from India (Glasby, 1999).
Another species of Namanereidinae, Namanereis sp., was found in the present survey.Worms of the same genus such as Namanereis amboinensis have been associated with dead and rotten vegetation in Belize, and in other places it has been found in decaying vegetation of coastal waters.The association with decaying vegetal matter is also characteristic of the circum-global Namanereis littoralis spp.gp., Namanereis catarractarum (Feuerborn, 1931) in the Indopacific, Namanereis hummelincki (Augener, 1933) in the Caribbean, and Namanereis pontica (Bobretzky, 1872) in the Black sea and Mediterranean (Glasby, 1999).
Namalycastis and Namanereis from Térraba could possibly play a role in the degradation of vegetation.The species Namalycastis borealis was reported feeding in wood by Glasby (1999).Possibly, similar to fresh water Namanereidinae, the mangrove populations use decaying wood and roots as food sources but are also facultative predators of insect larvae and other annelids (Benbow & Burky, 2001).
Capitellids and oligochaetes are subsurface deposit feeder worms (Fauchald & Jumars, 1979), and their presence in mangrove zones was previously reported by Dean (2001b).The logs contain a high percentage of organic matter and sediment was accumulated in their holes and crevices.In this way, decaying wood is a perfect habitat to opportunist worms as capitellids and oligochaetes (Rizzo & Amaral, 2001;Dean, 2001b).
Benthic fauna and salinity gradient: Kaller and Kelso (2006) found that insects and molluscs are the main components in decaying woods, of coastal freshwater streams, in South-Western Louisiana.The adaptations of these groups to wood boring may have allowed them to dominate the decaying wood in low salinity regimes.The lower salinity in some stations of the Térraba mangrove showed increases in the presence of freshwater insects in the wood, but they are absent in the sediment.Marine sediment is usually composed of fine sand, silt, and clay and the freshwater fauna of streams, other than some dipterans are not adapted to inhabit the fine sediments of estuaries (Kaller & Kelso, 2006).
Only some polychaetes (Capitellids and Namanereidinae) were found in logs located at salinities below 5 PSU, but they were highly abundant.In the Urías estuary, Sinaloa, Mexico, the capitellids were most abundant in the sediment of the inner estuary (and bordered by mangrove forest), than in the mouth of the estuary (Méndez, 2002).Also, a high percent of the species of Namanereididae is reported for low salinity habitats (Glasby, 1999;Benbow & Burky, 2001).
The annelids at a generic level found in the mangrove habitats of the present study (Lepidonotus, Diopatra, Alitta, Perineis and Namalycastis) are the same found by Metcalfe and Glasby (2008) in the tropical regions of Australia.They found a change in the assemblages from seaward (high abundance and species richness) to landward, in the mangrove of Darwin Harbour.Most of the organisms at same generic level showed a reduction to low salinity in Térraba mangrove.
To conclude, the decaying wood and sediments in the mangrove of Térraba, showed dissimilarities in the composition and identity of their fauna.In this way, the biodiversity was increased by the microhabitats inside the mangrove forest.The polychaetes play diverse roles in the food chains in this place, where some species possibly degrade decaying wood and other species are deposit feeders or carnivores.The association with other invertebrates, and estuarine gradient, results in a high dynamic community.
Vargas, who helped with the separation of individuals from decaying wood.Also, the Museo de Zoología provided the space to work with the samples.

Fig. 3 .
Fig. 3. A) Correspondence analysis showed the ordination of the decaying wood samples (A-H), and the abundance by groups (vectors); B) Contribution of the polychaetes in the total abundance by station.The stations of the decaying wood are sort from low to high total abundance.

Fig. 4 .
Fig. 4. Principal Component Analysis based on the correlation matrix of the taxa abundance, in the salinity gradient.Lines are the vectors of the direction in that the abundance of each taxa increased.The color and width of the vectors indicate the salinity range (PSU) in that the taxa reach a peak in abundance.Térraba mangrove, Costa Rica.

TABLE 1
Polychaetes and total number of individuals in the decaying wood and sediment samples.