First survey of Interstitial molluscs from Cayo Nuevo, Campeche Bank, Gulf of Mexico

Abstract Six sediment samples weighing between 224–735 g were collected in June of 2016 from Cayo Nuevo reef, located at the Campeche Bank, southern Gulf of Mexico. Samples were collected by SCUBA diving, from were two stations at depths of 7.6 and 18.2 m. Sediment was sieved and molluscs (adults and micromolluscs ≤ 10 mm) were sorted, examined, and identified to the lowest taxonomic level. A total of 1,347 specimens was found, of which 224 shells were dead and 1,123 were alive. Thirty-four families, 53 genera, and 67 species were identified. The most abundant families were Chamidae and Arcidae for the Bivalvia class, and Caecidae and Tornidae for the Gastropoda class. The vertical range of Bentharca sp. was extended.


Introduction
Frequently, species molluscan biodiversity accounts are incomplete because of the lack of some groups such as sea slugs and micromolluscs. Compared with macromolluscs, the study of micromolluscs is still in its infancy, which is probably due not to the difficulty involved in obtaining samples, but difficulties in identification of such small animals and the time-consuming process required to separate specimens from sand or other substrates (e.g., algae or rocks), and photography. In order to get a more realistic picture of the biodiversity for different habitats, micromolluscs should be incorporated into the different studies (Sasaki 2008, and pers. obs.).
The term micromollusc has been applied in arbitrary and non-standardised ways. Micromolluscs are molluscs not visible without some type of artificial assistance, such as a microscope or magnifying glass. The most restrictive definition, or sensu stricto, stated that micromollusc size should be less than 5 mm as an adult (Narciso 2005, Geiger et al. 2007. Other authors considered micromolluscs as specimens smaller than 10 mm as an adult . Finally, the wider definition of micromollusc, or sensu lato, includes molluscs whose size is typically less than 10 mm as an adult and also included juvenile representative of macromolluscs (Moore 1964;García-Cubas 1970;Tunnell 1974;Kay 1980;Vokes and Vokes 1983). Barrera (2001) stated that within Texas and Mexico, the majority of the studies involved macromolluscs. In Mexico, more than 4,643 species of marine molluscs have been recorded, and approximately 2,067 of them inhabit the Gulf of Mexico (GoMx) and the Mexican Caribbean Sea (Castillo-Rodríguez 2014). Unfortunately, Castillo-Rodríguez did not state which of them were micromolluscs. Important molluscan compilations focusing on these groups include publications by García-Cubas and Reguero (2007) and Vokes and Vokes (1983), although only a few publications have addressed the micromolluscan fauna specifically such as García-Cubas (1963, 1970and 1971 for lagoons in the Gulf of Mexico and Hicks et al. (2001) at Alacranes reef.
The Campeche Bank is located at the southern GoMx and is composed of several emergent and submerged coral reefs (Tunnell et al. 2010). One of the smaller and most remote reefs is Cayo Nuevo, located between Arenas and Triángulos reefs (190 km offshore) in the GoMx (Fig. 1). Studies on this reef are almost non-existent with the exception of polychaetes (Granados-Barba et al. 2003).
The present work focuses on the molluscs of the Cayo Nuevo sandy bottoms, Gulf of Mexico. In this substrate we could find micromolluscs sensu stricto and juveniles of macromolluscs species that inhabit the interstitial as but also empty shells that could be carried by external factors such wind and currents.

Materials and methods
Using SCUBA gear, six sediment samples of coarse sand to fine gravel weighing 224-735 g each were collected by hand at 7.6 and 18.2 m on 19 June 2017 at Cayo Nuevo   (Table 1). Each sample was sieved by pouring water through six differently sized sieves (2 mm, 1.4 mm, 1 mm; 710 µm, 500 µm, and 250 µm) ( Table 2) and sorted dry using a dissecting Nikon SMZ800 microscope. Specimens were picked out using soft forceps and 000 fine paint brushes. Molluscs were placed into 2 mm tubes and micromolluscs were placed into PCR tubes, both with 70% ethanol for long-term storage. Identification of specimens to species level was based on Abbott (1974), Tunnell et al. (2010) and Redfern (2013). Whenever possible, at least one specimen of each species or morphotype was photographed. All the specimens were deposited at the "Colección de Moluscos de la Peninsula de Yucatán" (CMPY), Unidad Multidisciplinaria de Docencia e Investigación Campus Sisal, Universidad Nacional Autónoma de México. The nomenclature of the species listed was assigned according to Bouchet and Rocroi (2010) for Bivalvia, Bouchet et al. (2017) for Gastropoda, and Kaas and Van Belle (1985) for Polyplacophora, due to the variability of some categories we only present the Linnaean ones. Abundance categories were assigned following Hicks et al. (2001): Abundant ≥ 50 (A); Common = 6-49 (C); Uncommon = 2-5 (UC), and Rare = 1 (R). Juvenile species are denoted by an asterisk (*).
This new data becomes a taxonomic reference list for the molluscs that inhabit Cayo Nuevo, GoMx, including micromolluscs as well as juvenile macromolluscs. To place it within a useful context we mention other inventories made in this area  recorded some 5,517 species of invertebrates in the GoMx, of which 2,455 were marine molluscs (Moretzsohn et al. 2009). González et al. (1991) recorded 298 species of molluscs and included 33 localities distributed around the coasts of the Yucatan Peninsula and adjacent coral reefs but did not mentioned Cayo Nuevo. García-Cubas et al. (1999) recorded 110 species of gastropods in the northern and northeastern regions of the Yucatan Peninsula. Rice and Kornicker (1962) recorded 130 species for Alacranes reef in the Campeche Bank and later, Hicks et al. (2001) recorded 215 species of molluscs on the same reef. Although earlier articles include reefs or sampling locations within the Bank of Campeche (e.g., Rehder and Abbott 1951, Springer and Bullis 1956, Kornicker et al. 1959, no mention of molluscs from Cayo Nuevo were found. Only Barrera (2001) study focuses on reef micromolluscs, recording 131 species from the East and West Flower Garden Banks (FGB).
Regarding bivalves, the most abundant species reported by Barrera (2001) for the FGB were Gregariella coralliophaga (145 individuals) (summing nine identified as Barbatia domingensis (102 individuals) and eleven identified as Barbatia cancellaria (68 individuals, currently a synonym), and Carditopsis smithii (51 individuals). In comparison, 37 individuals of Barbatia domingensis and 12 Carditopsis smithii were collected at Cayo Nuevo and these were not the most abundant species.
It should come as no surprise that many organisms were not identified to species level (e.g., Leptochiton sp., Bentharca sp., Crenella sp., Anomia sp., Lottia sp., Diodora sp., Cerithium sp., Cerithiopsis sp., Thylacodes sp., Zebina sp. 1, Zebina sp. 2, and Phrontis sp.). We relied on regional and local literature that in fact was scarce. In the case of juveniles, shells within a genus are similar because they share many characters and the differential characters are difficult to discern even as adults and almost impossible in juveniles. Our specimens identified under the name of Gibberula lavalleeana could be considerate as a species complex, due to the evidence and description of new species in Cuban waters (Espinosa and Ortea 2007).
These faunistic results from Cayo Nuevo represent the first inventory of molluscs from this remote reef. These findings contribute to record expansions for the southern GoMx of Bentharca sp. This contribution highlights the importance of conserving small areas that can harbour a considerable diversity of organisms. Seasonal changes on the mollusc community assemblages were not evaluated but would be an interesting future project, as would collecting growth series of species to assist in confirming identifications. In 2004, González and Torruco stated the importance of the Campeche Bank's reefs and proposed a marine reserve for the reefs located within this area, Cayo Nuevo included. However, this proposal never materialized and, up to now, only Alacranes reef has governmental protection under the status of marine reserve. Species checklists of micromolluscs, as well as other faunal groups, are of vital importance to serve as a baseline data set, due to the proximity to Mexico's offshore oil production area within the GoMx. The soft benthic interstitial mollusc communities are diverse, and their monitoring could well represent ecological indicators of ecosystem health, especially in the light of potential future oil-spills. base map. Financial support provided by the Harte Charitable Foundation, through the Harte Research Institute (Biodiversity of the southern Gulf of Mexico) and by the CONABIO NE018 Project (Update on the knowledge of the biodiversity of shallow water benthic marine invertebrate species (<50m) from the southern GoMx). First author received a postdoctoral grant from DGAPA-UNAM 2018-2019. We are grateful to Editor Nathalie Yonow for comments in the manuscript. We also thank the reviewer Dr. Henk Dekker and two other anonymous reviewers for their constructive comments and suggestions. We dedicate this work to our malacologist colleague, Dr. Wes Tunnell, who just left us. Dr. Tunnell was always curious for the Gulf of Mexico invertebrates and tireless promoter of marine biodiversity knowledge in the region.