Rhynobrissus cuneus is a rare sea urchin species from the northern Atlantic Ocean. This echinoid was first described from 2 specimens collected in North Carolina, USA in 1956, and no other specimens have been reported since. We describe the external morphology of 7 specimens of R. cuneus collected in the state of Veracruz, Mexico in the southern Gulf of Mexico. This record extends the southern range of this species and provides further data on its habitat and bathymetric distribution. The species may eventually prove to have a wide but patchy distribution.
Rhynobrissus cuneus es una especie rara de erizo de mar del norte del océano Atlántico. Este equinoide fue descrito a partir de 2 ejemplares recolectados en Carolina del Norte, EUA en 1956; no ha habido otros registros después de su descripción. En este trabajo se describe la morfología externa de 7 ejemplares de R. cuneus recolectados en el estado de Veracruz, México, al sur del golfo de México. Este registro amplía la distribución sur de esta especie y ofrece información sobre su hábitat y distribución batimétrica. Eventualmente la especie podría mostrar una distribución más amplia e irregular.
The sea urchin genus RhynobrissusAgassiz, 1872 (Echinoidea: Brissidae) includes 4 valid extant species: R. pyramidalisAgassiz, 1872 (type species), R. hemiasteroides Agassiz, 1879, R. tumulusMcNamara, 1982 and R. cuneusCooke, 1957. The first 3 are Indo-Pacific species (McNamara, 1982), while R. cuneus has been collected on the northern Atlantic coast of the USA in North Carolina. This genus also includes 1 fossil species, R. daviesi (Jain, 2002), from the Gaj Formation in India, from the early Miocene.
The holotype and paratype of R. cuneus are deposited in the National Museum of Natural History, Smithsonian Institution, Washington D. C. (USNM E8032, E8033). These type specimens, and 3 additional ones (USNM E8234, E8235, E36599) have remained the only known specimens of this species until now. Serafy (1979) reported Rhynobrissus micrasteroides from the Yucatán Channel (USNM E20413), this species was later designated the type species of the monotypic genus NeopneustesDuncan, 1889 (Duncan, 1889).
Seven specimens of Rhynobrissus cuneus were collected during an expedition to Playa Jicacal, Los Tuxtlas, Veracruz, Mexico (18°35'15” N, 95°03'06” W) at 1m in fine sand substrate (Fig. 1). The specimens were photographed, measured and deposited in the Colección Nacional de Equinodermos, Instituto de Ciencias del Mar y Limnología (ICML), Universidad Nacional Autónoma de México (UNAM). We used Mortensen (1951) and Serafy (1979) to determine order, family and genus; and used McNamara (1982) and Cooke (1957) to identify the species.
Systematics
Order Spatangoida Claus, 1876
Family Brissidae Gray, 1855
Genus RhynobrissusAgassiz, 1872
RhynobrissusAgassiz, 1872; Clark, 1925; McNamara, 1982; Schultz, 2005.
Diagnosis (modified from Agassiz, 1872–74: 590): test thin; outline from above diamond-shaped; vertex posterior. Paired ambulacra petaloid; odd anterior ambulacrum flush with the test. Peri-petalous fasciole present; independent anal and subanal fascioles. Spines of aboral surface short and curved; on the oral surface long, curved.
Type species: Rhynobrissus pyramidalisAgassiz, 1872.
Rhynobrissus cuneusCooke, 1957
Spines of Rhynobrissus cuneus on the aboral surface: A, 3 spines from the interambulacrum in the peripetalous fasciole; B, 2 spines from the aboral surface, out of the peripetalous fasciole; C, spine from the inner side of the anal fasciole, and D, spine from the peripetalous fasciole.
Measurements (mm) of specimens of Rhynobrissus cuneus. ICML-UNAM 4.78.0. TL: total length, TW: total width, TH: total height, FW: width of the peripetalous fasciole, FL: length of of the peripetalous fasciole, PL: plastron length, PW: plastron width, AH: height from oral surface to anus
| TL | TW | TH | FW | FL | PL | PW | AH | |
| 1 | 36.0 | 35.0 | 18.0 | 28.0 | 29.0 | 20.0 | 9.0 | 14.0 |
| 2 | 14.0 | 13.5 | 8.0 | 10.0 | 10.5 | 9.0 | 2.0 | 5.0 |
| 3 | 17.0 | 17.5 | 9.0 | 13.0 | 13.5 | 9.0 | 3.0 | 6.5 |
| 4 | 26.5 | * | 15.0 | * | * | 13.0 | 6.5 | 10.0 |
| 5 | * | * | * | 24.0 | 25.0 | * | * | * |
| 6 | * | * | 19.0 | * | * | 18.0 | 6.0 | 13.0 |
| 7 | 33.0 | 35.0 | 20.0 | 27.5 | 29.0 | 18.0 | 8.5 | 15.0 |
Rhynobrissus cuneusCooke, 1957: 9–12, pl. 1; Schultz, 2005: 401.
Description (modified from Cooke, 1957): horizontal outline diamond-shaped: rounded in front, widest at apical system, pointed at rear. Longitudinal outline wedge-shaped (cuneus [L.], wedge), oral surface flat, aboral surface slightly convex; apex at the rear of peripetalous fasciole (Fig. 2). The nearly central apical system is ethmolytic (i.e., the madreporite extends between and beyond the posterior ocular plates); adults with 4 genital pores. On aboral surface anterior ambulacrum not petaloid, inconspicuous, with small pores. Paired ambulacra long, straight, slightly sunken, with oval conjugate pores; reaching peripetalous fasciole. Interambulacra 2 and 3 approximately 5 times wider than ambulacra; interambulacrum 5 approximately 2 times wider than ambulacra. On oral surface, interambulacrum 5 with amphisternous plastron extending from labrum to subanal fasciole (Fig. 3). Peripetalous fasciole formula 5,5::6(7),7::10/11 (according to Smith and Stockley, 2005). Posterior surface with oval anal fasciole, reaching from apex to half the height (formula: 5–8), where adjoins the heart-shaped subanal fasciole with brissid pattern (Smith and Stockley, 2005); subanal fasciole covers ¼ of height. Peristome strongly reniform, because of bluntly pointed labrum, covered by many movable plates, surrounded by conspicuous phyllodes. Periproct oval with sharp ends, covered by several small movable plates on top of posterior surface (Fig. 4). Tubercles homogeneous on aboral surface, except for adapical end of ambulacrum III, where tubercles are almost 2 times bigger. Tubercles noticeably bigger on oral surface; phyllodes and posterior ambulacra bare. Aboral surface covered by slightly curved, hairlike, silky spines (Fig. 5); oral surface covered by spines 2 times longer than aboral spines, presenting spatulated ends (Fig. 6). No pedicellariae were found.
Type specimens: holotype USNM E8032 and paratype USNM E8033. Fort Macon Beach, North Carolina, USA (34°41'35” N, 76°40'59” W), January 1956, dry specimens washed onto the beach, no depth or habitat data.
Examined material: ICML-UNAM 4.78.0, 7 specimens (1 dry and 6 in ethanol 70%, Table 1), Playa Jicacal, Los Tuxtlas, Veracruz, Mexico, Gulf of Mexico (18°35'15” N, 95°03'06” W), September 17th, 2009, 1m depth, fine sand substrate.
RemarksWe present the first report of Rhynobrissus cuneus since its original description in 1957; it is also the first report of the species outside its type locality (North Carolina, USA) and the first report of the genus and species for the Gulf of Mexico.
This record extends the known distribution of R. cuneus hundreds of kilometers and serves to establish its depth and habitat information. The species may eventually prove to have a wide but patchy distribution.
We acknowledge Dr. Fernando Álvarez and Dr. José Luis Villalobos (Instituto de Biología, UNAM) for donating the specimens. We thank M.C. Berenit Mendoza for her work on the scanning electron microscope (IB-UNAM), Biol. Viridiana Tapia (ICML-UNAM) for measuring the specimens, Erika Porras for taking images, and M.S. Kelly Walsh for the English revision. Conabio Project HA018 supported AMM and FASM during their visit to the Smithsonian Institution, Washington, D. C. Comments and suggestions by two anonymous referees substantially improved the manuscript.