Iheringia, Série Zoologia

1. Programa de Pós-Graduação em Oceanografia, Universidade Federal do Maranhão. Av. dos Portugueses, s/n, Cidade Universitária Dom Delgado, 65080-805 São Luís, Maranhão, Brasil. (lorenakarine.ss@gmail.com; marco.cutrim@ufma.br) 2. Departamento de Sistemática e Ecologia, Universidade Federal da Paraíba, João Pessoa, PB, Brasil. (miodeli@gmail.com) 3. Programa de Pós-Graduação em Recursos Aquáticos e Pesca, Universidade Estadual do Maranhão, Cidade Universitária Paulo VI, Tirirical, 65055-970 São Luís, MA, Brasil. (oliveira.veronica@gmail.com)

Species of Cossura are characterized by the body regionalized in thorax and abdomen, a conical prostomium tapering to a sharp or rounded tip or extended laterally as horns (Liñero-Arana & Díaz-Díaz, 2010;Jumars et al., 2015) and usually without eyes.First chaetiger uniramous and the others along the body biramous (Hilbig, 1996).A long branchial filament is present in the dorsal region.Simple chaetae, which may be smooth or have the exterior margin with small spines (spinulosa).Pygidium with anal cirri of variable shape and length (Ewing, 1984).The most used characters to distinguish the different species of the genus are the shape of the prostomium, the chaetiger which the branchiae first appear, chaetal type and anal cirri with or without intercirral processes in the pigidium (Egremy-Valdez, 2009;Fournier & Petersen, 1991).These polychaetes typically inhabit sandy or muddy marine sediments, in shallow or deep waters (Zhadan, 2015).They are non-selective surface deposit-feeders, using their oral tentacles to accomplish so (Jumars et al., 2015).
Among the 28 species currently known in the genus (Read & Fauchald, 2018; Tab.I), nine have been recorded from South America, three of them on Brazil (Amaral et al., 2013).However, most of the studies are from south and southeast Brazilian coast, close to the historically consolidated research groups (e.g.Lana et al., 2017).The North and Northeastern Brazilian tropical estuaries potentially have a very high diversity, but are largely unexplored, as exemplified here with the description of a new coastal species.

MATERIAL AND METHODS
Sampling of biological material.Benthic samples were collected in the canal of the São Marcos Bay, Maranhão, Brazil, in areas under influence of dredging activities (Fig. 1).All stations had unconsolidated muddy sediment and ca.20 m depth.On each station benthic material was sampled with a 20-L van Veen trap and the material sampled was fixed in 4% formalin.In the laboratory the material was sieved (0.5 mm) and analyzed under stereomicroscope.
Preparation and identification of material.Slides were mounted for detailed observations of the morphology of parapodia and chaetae.The specimens were identified using stereoscopic and optical microscopes.Descriptions were prepared following external body-morphology such as prostomium, branchiae position, parapodia, chaetae and anal cirri.Measures of width included only parapodia, without considering the chaetae.For improved contrast while checking for papillae, Shirlastain dye have been used.
Photographs were taken using a Zen AxioCam ICc5 of Zeiss attached to the stereoscopic and optical microscopes and the software Axion vision 4.8.The parapodia were dried and coated on a double carbon tape and examined in a scanning electron microscope (Hithachi TM-3030 from Universidade Federal do Maranhão).The type material was deposited at "Coleção de Invertebrados Paulo Young" (CIPY) from Universidade Federal da Paraíba, João Pessoa, PB.Diagnosis.Two pairs of nuchal organs at lateral margins of the peristomium.Branchial filament arising from median region of the 3 rd chaetiger.Bundles of short and long capillaries chaetae in both parapodial rami.Both chaetal types are smooth with spinulosa on distal half.

Cossura
Description.The holotype has body elongated and cylindrical, with tapered posterior end (Fig. 2).Conical prostomium, longer than wide and slightly flattened dorsoventrally, without eyes and usually with a pronounced dorsal fold (Figs 3,(5)(6)(7)(8).Division between the prostomium and peristomium not well defined (Figs 3,4).Two pairs of nuchal organs at the lateral margins of peristomium (Fig. 7).Inverted proboscis with tentacles observed through dissection in one paratype.Body divided in thorax and abdomen evidenced by a body narrowing (Fig. 4).The thoracic region reaches the 28 th segment and has glandular inflations divided by a dorsal groove perpendicular to the body axis.Thorax dorsoventrally flattened and more rigid than the abdomen.Branchial filament dorsal, annulate, arising from median region of the 3 rd chaetiger, and exceeding the body length (Figs 2, 3).Parapodia rudimentary and without great variation, with acicula absent throughout the body (Figs 9-11).First chaetiger with uniramous parapodia, and all others with biramous parapodia; all biramous parapodia alike and laterally positioned on each segment.Parapodial lobes are absent and thus chaetae emerge directly from the bodywall.Two types of capillary chaetae short and long with end curved; short chaetae exceed half the length of the long chaetae and are thinner .Both chaetal types are smooth with spinulosa (fine hairs) on the distal half.The same two chaetal types occur in the thorax and abdomen and in the neuropodia and notopodia, with predominance of long chaetae in the notopodia.The number of chaetae varies among the different regions of the body in all studied specimens: the 5 th parapodium bears seven notopodial and five neuropodial chaetae; the 20 th parapodium bears nine notopodial and seven neuropodial chaetae; the 43 th parapodium bears five notopodial and six neuropodial chaetae (Figs 9-11).Pigidium with three long anal cirri without intercirral processes, one median dorsal and two in the lateral margins of the anal opening, reaching the last four chaetigers (Fig. 15).
Habitat.Substrate with dominance of silt, clay and fine sand.
Etymology.The species name is derived from the Tupi word yacy (moon).