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 (Barrera and Tunnell 2001). 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, 1970 and 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.

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 reef (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 (*).

The results from the analysis of the sediment from Cayo Nuevo reef revealed 67 species of molluscs, from which 50 species are gastropods, 14 species are bivalves and three are chitons. These species belong to 38 different families.

Phylum Mollusca Linnaeus, 1758

Class Polyplacophora Gray, 1821

Polyplacophora sp. 1 * (R) (Fig. 2–1)

Polyplacophora sp. 2 * (R) (Fig. 2–2)

Order Chitonina Thiele, 1909

Family Chitonoidea Rafinesque, 1815

Genus Ischnochiton Gray, 1847

Ischnochiton sp. * (UC) (Fig. 2–3)

Class Bivalvia Linnaeus, 1758

Order Arcida Stoliczka, 1871

Family Arcidae Lamarck, 1809

Arcidae sp. (UC) (Fig. 2–4)

Genus Arca Linnaeus, 1758

Arca imbricata Bruguière, 1789 * (UC) (Fig. 2–5a, b)

Genus Barbatia Gray, 1842

Barbatia domingensis * (Lamarck, 1819) (UC) (Fig. 2–6a, b)

Barbatia sp. * (C) (Fig. 2–7a, b)

Genus Bentharca Verrill & Bush, 1898

Bentharca sp. (R) (Fig. 2–8a, b)

Order Mytilida Férussac, 1822

Family Mytilidae Rafinesque, 1815

Genus Crenella T. Brown, 1827

Crenella sp. (R)

Order Pectinida Gray, 1854

Family Anomiidae Rafinesque, 1815

Genus Anomia Linnaeus, 1758

Anomia sp.* (R) (Fig. 2–9a, b)

Order Cardita Bruguière, 1792

Family Carditidae Férussac, 1822

Genus Carditopsis E. A. Smith, 1881

Carditopsis smithii (Dall, 1896) (C) (Fig. 2–10a, b)

Order Lucinida Gray, 1854

Family Lucinidae J. Fleming, 1828

Lucinidae sp. * (UC) (Fig. 2–11)

Order Venerida Gray, 1854

Family Chamidae Lamarck, 1809

Chamidae sp. (UC) (Fig. 2–12a, b)

Genus Chama Linnaeus, 1758

Chama sinuosa Broderip, 1835 * (A) (Fig. 2–13a, b)

Family Galeommatidae Gray, 1840

Galeommatidae sp. (R)

Order Carditida Dall, 1889

Family Crassatellidae Férussac, 1822

Genus Crassinella Guppy, 1874

Crassinella lunulata (Conrad, 1834) * (R) (Fig. 2–14a, b)

Order Cardiida Férussac, 1822

Family Semelidae Stoliczka, 1870 (1825)

Genus Semele Schumacher, 1817

Semele bellastriata (Conrad, 1837) * (UC) (Fig. 2–15a, b)

Class Gastropoda Cuvier, 1795

Gastropoda sp. * (R) (Fig. 2–16)

Order Lepetelloidea Dall, 1882

Family Fissurellidae Fleming, 1822

Genus Diodora Gray, 1821

Diodora minuta (Lamarck, 1822) * (UC) (Fig. 2–17)

Diodora listeri (d’Orbigny, 1847) (R) (Fig. 2–18a, b)

Family Scissurellidae Gray, 1847

Genus Scissurella d’Orbigny, 1824

Scissurella redferni (Rolán, 1996) (C) (Fig. 2–19a, b)

Order Trochida Rafinesque, 1815

Family Trochidae Rafinesque, 1815

Genus Synaptocochlea Pilsbry, 1890

Synaptocochlea picta (d’Orbigny, 1847) (A) (Fig. 2–20a, b)

Family Skeneidae W. Clark, 1851

Genus Lodderena Iredale, 1924

Lodderena ornata (Olsson & McGinty, 1958) (A) (Fig. 2–21a, b)

Family Cerithiidae Fleming, 1822

Genus Cerithium Bruguière, 1789

Cerithium sp. 1 (R) (Fig. 2–22a, 22b)

Cerithium sp. 2 (UC) (Fig. 2–23a, 23b)

Cerithium atratum (Borns, 1778) (R) (Fig. 3–1a, b)

Family Litiopidae Gray, 1847

Genus Alaba H. adams & A. Adams, 1853

Alaba incerta (d’Orbigny, 1841) (C) (Fig. 3–2a, b)

Family Scaliolidae Jousseaume, 1912

Finella sp. (UC) (Fig. 3–3a, 3b)

Family Pickworthiidae Iredale, 1917

Subfamily Pickworthiinae Iredale, 1917

Genus Sansonia Jousseaume, 1892

Sansonia tuberculata (Watson, 1886) (R) (Fig. 3–4a, b)

Family Hypponicidae Troschel, 1861

Hipponix sp. (C) (Fig. 3–5a, 5b)

Family Triphoridae Gray, 1847

Genus Iniforis Jousseaume, 1884

Iniforis turristhomae (Holten, 1802) (UC) (Fig. 3–6a, b)

Genus Metaxia Monterosato, 1884

Metaxia rugulosa (C. B. Adams, 1850) (R) (Fig. 3–7a, b)

Family Cerithiopsidae H. Adams & A. Adams, 1853

Genus Cerithiopsis Forbes & Hanley, 1850

Cerithiopsis sp. (R) (Fig. 3–8a, 8b)

Cerithiopsis cf. iuxtafuniculata Rolán, Espinosa & Fernández-Garcés, 2007 (R) (Fig. 3–9a, b)

Family Vermetidae Rafinesque, 1815

Vermetidaeincertae sedis irregularis d’Orbigny, 1841 (Fig. 3–10)

Genus Dendropoma Mörch, 1861

Dendropoma corrodens (d’Orbigny, 1841) (R) (Fig. 3–11)

Vermetid sp. CRedfern 2013 (A) (Fig. 3–12)

Genus Petaloconchus Lea, 1843

Petaloconchus mcgintyi (Olsson & Harbison, 1953) * (C) (Fig. 3–13)

Genus Thylacodes Guettard, 1770

Thylacodes sp. * (R) (Fig. 3–14)

Family Rissoidae Gray, 1847

Genus Simulamerelina Ponder, 1985

Simulamerelina caribaea (d’Orbigny, 1842) (UC) (Fig. 3–15a, b)

Family Zebinidae Coan, 1964

Genus Schwartziella G. Nevill, 1881

Schwartziella fischeri (Desjardin, 1949) (UC) (Fig. 3–16a, b)

Genus Zebina H. Adams & A. Adams, 1854

Zebina sp. 1 (A) (Fig. 3–17a, 17b)

Zebina sp. 2 (C) (Fig. 3–18a, 18b)

Family Truncatellidae Gray, 1840

Genus Truncatella Risso, 1826

Truncatella sp. (R) (Fig. 4–19a, 19b)

Family Caecidae Gray, 1850

Genus Caecum Fleming, 1813

Caecum circumvolutum de Folin, 1867 (C) (Fig. 3–20)

Caecum donmoorei Mitchell-Tapping, 1979 (C) (Fig. 3–21a, b)

Caecum johnsoni Winkley, 1908 (A) (Fig. 3–22)

Caecum pulchellum Stimpson, 1851 (C) (Fig. 3–23)

Caecum textile de Folin, 1867 (UC) (Fig. 3–24)

Caecum sp. B sensuRedfern 2013 (C) (Fig. 3–25)

Genus Meioceras Carpenter, 1859

Meioceras nitidum (Stimpson, 1851) (UC) (Fig. 3–26)

Family Tornidae Sacco, 1896 (1884)

Genus Parviturboides Pilsbry & McGinty, 1949

Parviturboides sp. (C) (Fig. 4–1a, b)

Genus Vitrinella C. B. Adams, 1850

Vitrinella sp. (A) (Fig. 4–2a, b)

Family Cystiscidae Stimpson, 1865

Genus Gibberula Swainson, 1840

Gibberula lavalleeana (d’Orbigny, 1824) (UC) (Fig. 4–3a, b)

Family Marginellidae Fleming, 1828

Genus Volvarina Hinds, 1844

Volvarina sp. 1 (UC) (Fig. 4–4a, 4b)

Volvarina sp. 2 (R) (Fig. 4–5a, 5b)

Family Columbellidae Swainson, 1840

Columbellidae sp. 1 (UC) (Fig. 4–6a, b)

Columbellidae sp. 2 (R) (Fig. 4–7a, 7b)

Genus Steironepion Pilsbry & H. N. Lowe, 1932

Steironepion moniliferum (G. B. Sowerby I, 1844) (UC) (Fig. 4–8a, b)

Family Nassariidae Iredale, 1916 (1835)

Genus Phrontis H. Adams & A. Adams, 1853

Phrontis sp. (UC) (Fig. 4–9a, b)

Family Muricidae Rafinesque, 1815

Genus Trachypollia Woodring, 1928

Trachypollia sp. (R) (Fig. 4–10a, b)

Family “Turridae” H. Adams & A. Adams, 1853 (1838)

Turridae sp. 1 (R) (Fig. 4–11a, 11b)

Family Omalogyridae G.O. Sars, 1878

Genus Ammonicera Vayssière, 1893

Ammonicera lineofuscata Rolán, 1992 (A) (Fig. 4–12)

Ammonicera minortalis Rolán, 1992 (A) (Fig. 4–13a, b)

Family Rissoellidae Gray, 1850

Genus Rissoella Gray, 1847

Rissoella galba Robertson, 1961 (R) (Fig. 4–14a, b)

Order Aplysiida

Family Aplysiidae Lamarck, 1809

Genus Aplysia Linnaeus, 1767

Aplysia sp. (R)

Order Siphonarimorpha

Family Pyramidellidae Gray, 1840

Genus Pseudoscilla Boettger, 1901

Pseudoscilla aff. babylonia (C. B. Adams, 1845) (R) (Fig. 4–15a, 185b)

The most abundant families of gastropods were the Caecidae (456 specimens, seven species), Tornidae (221 specimens, two species), and Omalogyridae (132 specimens, two species). The most abundant families of bivalves were Arcidae (40 specimens, five species) and Chamidae (59 specimens, two species). The most abundant gastropod species were Caecum johnsoni (310 specimens), Vitrinella sp. (208 specimens), Vermetid sp. C (91 specimens), Lodderena ornata (71 specimens), and Caecum donmoorei (147 specimens). For the Bivalvia the most abundant species were Chama sinuosa (57 specimens), Barbatia domingensis (57 specimens) and Carditopsis smithii (12 specimens).

From the six sediment samples, the most commonly found molluscs were Lodderena ornata, Caecum johnsoni and Ammonicera lineofuscata, while other species appeared only once: Leptochiton sp., Arcidae sp., Bentharca sp., Crenella sp., Anomia sp., Chama sp., Galeommatidae sp., Chione elevata, Semele bellastriata, Cerithium sp. 1, Gastropoda sp., Diodora listeri, Cerithium atratum, Sansonia tuberculata, Iniforis turristhomae, Metaxia rugulosa, Cerithiopsis cf. iuxtafuniculata, Cerithiopsis sp., Vermetidaeincertae sedis irregularis, Dendropoma corrondens, Thylacodes sp., Finella sp., Caecum textile, Hipponix sp., Volvarina sp. 2, Columbellidae sp. 2, Aplysia sp., and Pseudoscilla aff. babylonia.

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: Felder and Camp (2009) 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).

The molluscan assemblage at Cayo Nuevo shares many species also present at the FGB (Barrera 2001) and Alacranes Reef (Hicks et al. 2001) (19 families/21 genera and 21 families/22 genera, respectively) (Table 3). The most diverse families recorded by Barrera (2001) and Hicks et al. (2001) were Caecidae (six genera and ten species) and Rissoidae (five genera and seven species) for Gastropoda and Arcidae (four genera and seven species) for Bivalvia.

Barrera (2001) most abundant gastropod species were Amphithalamus vallei (672 individuals) and cf. Vitrinella sp. (534 individuals); however, at Cayo Nuevo, 208 individuals of sf. Vitrinella sp., were found. Differences in the numbers of collected individuals can be explained by geography, but also by differences in the quantity of sediment collected and processed, fifteen sites with 300 ml sediment sampled at FGB and six samples of 224–735 g at Cayo Nuevo. Vitrinella sp. could not be identified to specific level due to the low similarity of characters shown with other described western Atlantic species. Barrera (2001) previously suggested that it could be an undescribed new species, but further detailed studies are required to establish its identity.

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.