A new species of Malletia (Bivalvia, Malletiidae) and new records of deep-water bivalves from Pacific Southern Colombia

Abstract In order to enhance the understanding of Pacific Colombia’s deep-water marine fauna, a benthic research cruise (2012 TUM Offshore 6 and 7) was conducted off the coast of the Department of Nariño, in southern Colombia. Biological, oceanographic and sediment samples from the continental shelf and slope were collected at depths between 350 and 941 m. A new species of Malletia obtained on that cruise is described and compared with other species from the eastern Pacific. Sixteen species of bivalve mollusks (belonging to 12 families and 15 genera) were identified. Five of them were the first records for Pacific Colombia (Jupiterialobula, Limatulasaturna, Lucinomaheroica, Cuspidariapanamensis, and Dallicordiaalaskana). Four of them had geographic distributions that now extend to Tumaco at the southern end of Nariño.


Introduction
Throughout the past decade, the search for hydrocarbon and natural gas reserves in Colombia (Pacific and Caribbean coast) has sparked an interest in the country's remote deep-sea regions. This has resulted in intensified deep-sea baseline studies, primarily along the continental shelves and slopes. Nevertheless, deep-sea studies face logistical and cost limitations, including the availability of research vessels and proper equipment for collecting samples.
Despite the increase in knowledge during recent years, the presently known range of many invertebrates groups inhabiting soft sediments, including mollusks, is still fairly fragmentary in remotes parts of the Colombian Pacific. There is a lack of published data on the biology, functional morphology, ecology, development and dispersal mechanisms for these invertebrates, as well as a lack of baseline faunal inventories. Thus, the true biodiversity of the Pacific Colombian deep-sea must be vastly underestimated.
As a result of recent Colombian expeditions, a rich benthic fauna inhabiting of the deep-seas of Pacific Southern Colombia has been discovered, but few species of mollusks have been described when compared with the mollusks from the coasts of the Colombian Atlantic (e.g., Ardila-Espitia and Diaz 2002, Simone and Gracia 2006, Gracia and Ardila-Espitia 2009. In the context of faunal inventories, the tropical west coast of America is well documented, with 890 species of bivalves presently recorded (Coan and Valentich-Scott 2012). For northwestern South America to Peru, a basic knowledge of deep-sea bivalve mollusks has been covered by a few recent publications (e.g., Gracia and Valentich-Scott 2014;Paredes et al. 2016). The investigations in Pacific Colombian waters have hitherto focused mainly on the coastal zones (e.g., Cantera et al. 1979, Cosel 1984, Díaz et al. 1997, Cantera 2010, López de Mesa and Cantera 2015 rather than zones farther offshore (e.g., Hertlein and Strong 1955). Gracia and Valentich-Scott (2014) documented the bivalves off the Department of Choco (Colombian North Pacific) where more than 38 species of bivalves were found, 34 of which were new records for the country.
The current work presents a systematic and annotated list of bivalve species collected in the southern Colombian Pacific region. Each entry includes the species' geographic and bathymetric distribution, plus additional remarks and observations. From the above, several species stand out as being first records for the country. We are also including the description of a new species uncovered in this survey. Our records represent a significant expansion in the knowledge of the Pacific Colombian bivalve fauna, but much more sampling and analysis is needed when one takes into account the large geographic extent of this region.

Study area
The present study was carried out in the tropical eastern Pacific Ocean (Fig. 1). The study area (TUM Offshore Blocks 6 and 7) covered 7,308 km 2 and extended from Sanquianga National Nature Park in the Department of Nariño to the Colombia-Ecuador border. The region is influenced by continental contributions from Tumaco Bay, as well as by numerous rivers, including the Patia and Mira (IDEAM et al. 2007). This study area is part of the research project known as the "Biological and physical baseline survey of TUM Offshore Blocks 6 and 7 subject to hydrocarbon exploration" (ANH-Invermar 2013).

Sample collection
Samples were collected from 4-22 December 2012, on board the fishing vessel Perla Verde. Collection depths ranged from 350-941 m. All the 13 trawls made during the survey were taken in soft and homogenous sea beds. Ten of the 13 sampling locations included bivalves. Each sample was collected with a benthic semi-balloon trawl net (9 × 1 m) for 10 minutes at a speed of 3 knots. Because the exact time at which the net opened was unknown, sampling was semi-quantitative. We acknowledge that this sampling technique could have missed small and microscopic species as would be taken by epibenthic sleds, but the equipment needed for this method was not available to us. Collected material was coarsely sorted on deck and later identified to lower levels at the Museo de Historia Natural Marina de Colombia (MHNMC) which is part of Instituto de Investigaciones Marinas y Costeras (INVEMAR). The empty valves were air-dried, while the softbodied organisms were preserved in 70% ethanol.
Specimen identification was based upon shell characters. Museum materials, bibliographic references and bivalve taxonomic experts were consulted to confirm the results (e.g., Dall 1896Dall , 1908Keen 1971;Coan and Valentich-Scott 2012). The identified material included many complete living organisms as well as empty shells of bivalves. The systematic order of this list corresponds to that proposed by Coan and Valentich-Scott (2012). Specimens from this study, including other mollusks not analyzed in this work (e.g., gastropods, chitons, and cephalopods), now reside at the MH-NMC's mollusk collection in Santa Marta, Colombia.
Oceanographic data were collected with an Idronaut CTDO marine profiler (yielding data for conductivity, temperature, depth, and oxygen concentration) at sites S333 and S334, both of which contained bivalves (Table 1). Sediment core sets were collected five sites (S331, S333, S334, S341, and S345) with a Gomex II Box corer that had a 32 liter storage capacity. Sediment grain analysis revealed a predominance of silt (Table 1). Grain size classification was conducted according to Folk (1974). All samples were classified as silts; 57% of samples were purely silts, while the remaining 43% also contained sand and gravel fractions (INVEMAR-ANH 2013).

EA
Trawl station; S sediment station MHNMC (Spanish acronym) Museo de Historia Natural Marina de Colombia TUM OFF Tumaco-Offshore

Results
A total of 324 bivalve specimens was collected, including 247 empty or disjointed valves and 77 live-collected organisms. The specimens were sorted into 16 species, 15 genera, and 12 families; five species were new observations in the Colombian Pacific. The known geographic range of several species has now been expanded to the Department of Nariño. Below is included a listing of the species collected, station data, live-dead status for each specimen, remarks on new verified localities, previously reported distributions for the species, plus general remarks. We have also included an illustration for all newly documented species in Colombia i.e., those other than Nucula iphigenia, Orthoyoldia panamensis and Delectopecten zacae which were previously reported for the Pacific of Colombia by Gracia and Valentich-Scott (2014).  Remarks. These records represent a new southern limit for this species. All the specimens were small (approx. 4 mm), but they were nearly identical to small specimens of Jupiteria lobula from Mexico and also the type specimens. The presence of dead valves at different stations and the distance from previous records suggest that this species is living in Colombia.
Sculpture and periostracum: Exterior sculpture of fine commarginal striae. Periostracum thin, adherent, glossy, pale yellow to light brown, often with commarginal color bands.
Anatomy: Foot large, deeply cleft medially, wide at neck; labial palp and palp proboscis anterior; labial palp large, with 2 distinct regions with finer and heavier lamellae; palp proboscis very long, coiled.
Paratypes. See Table 2  Distribution. The species is currently only known only from the type locality.
Etymology. This species is named in honor of the municipality of Tumaco, Nariño, where this study was conducted.
Differential diagnosis. Malletia tumaquensis sp. n. is similar in shape to M. alata Bernard, 1989. However, consistent differences exist in conchological features (i.e., M. tumaquensis is more elongate, while M. alata has an alate process) and anatomical characteristics (i.e., very long, thin palp proboscis in M. tumaquensis) makes it a readily distinguishable new species. Ecologically, M. tumaquensis has a shallower depth distribution (530 m) than that of M. alata (740 m, Coan and Valentich-Scott 2012). Table 3 summarizes the shell characteristics of all the Malletia species recorded in the eastern Pacific Ocean.
Remarks. Members of the family Malletiidae occur throughout the Pacific and Atlantic Oceans with most records from deep-water (Coan andValentich-Scott 2012, Kamenev 2015). Malletia tumaquensis is distinguished from the seven other species occurring in tropical west America by its more subquadrate and longer shell. Including our record, this represents the third species of the genus reported for the Colombian Pacific (i.e., M. tumaquensis, M. truncata and M. goniura).

Family YOLDIIDAE Dall, 1908 Subfamily YOLDIINAE Dall, 1908 Genus Orthoyoldia Verrill & Bush, 1897
Orthoyoldia panamensis (Dall, 1908   Remarks. Orthoyoldia panamensis has previously been collected in depths from 120 to 475 m in Colombia (Gracia and Valentich-Scott 2014). This study extends the bathymetric range to 730 m in the Colombian Pacific.  Remarks. The single valve collected is insufficient to allow a definitive identification to species. The shape and dentition place it closest to Calyptogena gallardoi. Remarks. Prior to this study, dead shells of Pliocardia donacia were identified in Pacific Colombia at depths between 272 and 295 m (Gracia and Valentich-Scott 2014). The present collection in southern Colombia yielded one live specimen and two empty valves, suggesting that the species inhabits both the northern and southern Colombian Pacific. Further, the bathymetric limit of the species is extended to 656 m in the Colombian Pacific. Many generic uncertainties exist within the family Vesicomyidae. Thus, we follow Coan and Valentich-Scott (2012)   New location. Off Nariño, Colombian Pacific.

Distribution.
Panama (Coan & Valentich-Scott 2012). Remarks. Cuspidaria panamensis was previously known only been known from the type locality in the Gulf of Panama (Coan and Valentich-Scott 2012). Our records extend the distribution over 600 km to the south. Coan and Valentich-Scott (2012) indicate a maximum size of 41 mm for Cuspidaria panamensis. However, our material from station EA345 increases the maximum length to 44.2 mm.    New location. Off Nariño, Colombian Pacific. Distribution. Mexico to Panama (Coan and Valentich-Scott 2012). Remarks. Colombian material resembles the type material of Lyonsiella magnifica. However, our specimens have more prominent umbones, a more truncate anterior end, and a more obliquely truncate posterior end when compared to the type material. Many additional specimens would be necessary to determine if our single specimen falls within the range of intraspecific variation for Lyonsiella magnifica or it represents a new species.

Discussion
The new species of Malletiidae herein described brings to eight the number of known species for this family in the eastern Pacific Ocean (Table 3). Malletia is a widely distributed genus that is associated mainly with deep water and soft sediments (Coan and Valentich-Scott 2012). Previously reported from Colombia were M. goniura Dall, 1890 andM. truncata Dall, 1908 (type locality, Malpelo Island, at 3,334 m).
As was true of the previous study in the northern Colombian Pacific (Gracia and Valentich-Scott, 2014), these recent collections not only expand the geographic distributions of many species on the Colombian continental margin, but they also represent new collection locations. This serves as potential evidence for the species actually living in the area, rather than the transport of dead shells into the region. Further, our findings have significantly expanded the bathymetric limits of several species. One new species has been described, indicating that this region of Colombia is still relatively unexplored. Further surveys are necessary to complement this malacological inventory and to clarify the taxonomic identity of several species. These are important preliminary steps for to assist in investigating the impacts of anthropogenic practices and changes (e.g., deep-sea trawling, pollution).
Deep-sea baseline surveys seek to expand bivalve records for the Colombian Pacific Ocean. In 2014, Gracia and Valentich-Scott reported on specimens collected in the northern Colombian Pacific; 89.5% of the identified species represented new records for the region. The present survey used a similar methodology but was conducted in the southern Colombian Pacific. The number of bivalve species we encountered in the southern Colombian Pacific (16) was far lower than that for the northern Colombian Pacific. This could possibly be due to the different depths sampled in either survey, or possibly the decreasing diversity associated with increasing depth.
The transport of sediment caused by river discharge, marine currents, and other factors stimulate the resuspension of material on soft sediments (Segall et al. 1989). In the northern Colombian Pacific there is a greater influence of the equatorial countercurrent and the Panama Current, and the discharge from the Baudó River, while the southern Colombia Pacific (where Tumaco Bay is located) sees the influence of cold continental waters (CCCP 2002). All of these processes in Pacific Colombian result in a dense mixture of water and sediment that moves along the bottom of the sea and transports plant waste material. In both the northern (Choco) and the southern (Nari-ño) zones, a great abundance of sunken wood was encountered, indicating similar conditions influenced by terrestrial deposits.
Characteristics of sediments, currents, organic matter, availability of oxygen and many others factors could influence the composition, abundance, and occurrence of the benthic fauna. It should be noted that collections made in deep water in both northern and southern Pacific Colombia have yielded only a limited number of living bivalves and those that were numerically dominant were empty shells.
In conclusion, this paper serves as a contribution to our understanding of marine bivalves in deep waters of the southern Colombian Pacific. Our results reveal the importance of continued deep-sea research cruises in Colombia and subsequent taxonomic analysis of the specimens collected.