Short Communication |
Corresponding author: Tamara Maggioni ( tamaramaggioni@gmail.com ) Academic editor: Matthias Glaubrecht
© 2016 Tamara Maggioni, Anabela Taverna, Marcos Tatián.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Maggioni T, Taverna A, Tatián M (2016) Redescription of the deep-sea colonial ascidian Synoicum molle (Herdman, 1886): first record since its original finding during the Challenger Expedition. Zoosystematics and Evolution 92(2): 181-185. https://doi.org/10.3897/zse.92.9521
|
The colonial ascidian Synoicum molle (Herdman, 1886) was recorded for the first time after its original description. The slope area where S. molle was discovered during the Challenger Expedition (1872-1876), located at the mouth off Río de La Plata (Argentine Sea), still remains highly underexplored. During a series of deep-sea prospections along the SW Atlantic (A.R.A. Puerto Deseado, August 2012), 7 colonies of S. molle were collected few km from the type locality. The finding permitted to perform, for the first time, a complete morphological description of this species. The proper identification of marine species, especially deep-sea organisms, has implications for the general knowledge of biodiversity, a necessary tool for the conservation and study of ecosystem benefits that marine environments provide.
Ascidiacea , continental slope, Southwestern Atlantic
Ascidians (Tunicata) are important components of marine benthic communities, even in the deep-sea. In relation to other invertebrates, ascidians reach their maximum richness at greater depths (
The colonial ascidian Synoicum molle (Herdman, 1886), originally named Polyclinum molle
We analyzed a total of 7 colonies that match the characters listed by Herdman for S. molle in a close area from the type locality of that species. These include: the form and texture of the colony; the color of the tunic; the average size of the zooids; the form of the atrial languet; the shape and characteristics of the stomach; the general form of the post-abdomen; and the elongated body of the larvae. We provide a more comprehensive and detailed description of the species including illustrations of the entire colony and of the individual zooids.
The examined specimens were collected by the research vessel A.R.A Puerto Deseado during the first Continental Slope Campaign, within the Argentinean Exclusive Economic Zone, on August 2012. The prospections comprised a bathymetric gradient between 200 and 3,000 m close to 38°S, including the Mar del Plata submarine canyon (Fig.
Onboard, the colonies were photographed, relaxed under seawater with menthol crystals for 2 hours, and then fixed in formalin-seawater 4%. In the laboratory, a minimum of 10 zooids per colony were removed and studied under a stereo microscope. Each zooid was drawn, measured and photographed. The 7 colonies were deposited in the collection of the Museo de Zoología, Universidad Nacional de Córdoba (MZUCVIO192).
The actual status of the different species was verified through the World Register of Marine Species (WoRMS) webpage (www.marinespecies.org).
Polyclinum molle Herdman, 1886: 194, pl. XXV, figs 7-9; non Rocha and Costa 2005: 59, fig 2-4.
Synoicum
molle
;
Station 2: 37°58’S, 55°12’W; 7 colonies; 308 m; Ago/17/2012.
The colonies are almost identical in shape: globular, nearly spherical (Fig.
All colonies present zooids with a marked variation in size and also of sexual maturation. When fully developed, zooids are large, with an average length of 13 mm (Fig.
The thorax bears 8 to 10 thin longitudinal muscle bands on each side, running along the entire body and joining at the end of the post-abdomen. There are between 14 and 16 simple and stout oral tentacles, alternating in size and placed in a circle. The dorsal tubercle is small and rounded.
There are from 12 to 14 rows of stigmata. Rarely, zooids may bear only 9 or 10. Nonetheless, they never exceed 14 rows. Each row contains 10 longitudinal rectangular-shaped stigmata. These vary slightly in size, being thinner and longer towards the center of the thorax. Branchial papillae are not present.
The straight and thin-walled esophagus connects with the stomach vertically. The stomach wall is smooth, although some striations -never folds- might occur randomly (Fig.
The gonads are situated in a long post-abdomen, either directly adjacent to the abdominal region or at some distance (0.3 to 4.7 mm) away from it. The ovary contains from one to 2–3 or 6–8 small oocytes. The male follicles are found just below or surrounding the oocytes, disposed in clusters or arranged in a straight line.
On average, a dozen of immature larvae are present in the atrial cavities of a few zooids of one colony. They are arranged in double rows along the entire length and half the width of the thorax. In that immature stage, larvae had only developed a small and stout tail but lacked sensory organs, papillae and vesicles.
Most characters today considered of taxonomic validity were deemed not important or could not be studied in detail in the XIX century, either because of the difficulties regarding the techniques used or due to the limited number of available specimens. Out of 14 ascidian species collected and originally described from Station 320 (Challenger Expedition), 10 are currently accepted. After taxonomical revision of the types, various specimens were placed under a different species: Styela oblonga Herdman, 1881 was synonymized with Styela squamosa Herdman, 1881 (
According to
The maximum species richness for many invertebrate zoological groups is located between 2,000 and 3,000 m. However, for ascidians, this boundary is located between 4,000 and 4,500 m (
An increase of bathymetrical gradient studies would help to test the proposed hypothesis of the establishment of a pattern of reduction of colonial forms with increasing depths as well as to elucidate the mechanisms governing this process. Moreover, the increment of the range of the bathymetrical gradient at least up to 5,000 m depth would be especially useful in consideration of the maximum species richness registered for this group of benthic animals (
We want to acknowledge Mariano Martinez for his cooperation in regard to the map figure. Special thanks to Martín Brogger and all scientists and students involved in the Argentinean BOPD Slope Campaigns. We thank an anonymous reviewer and reviewer Andreas Schmidt-Rhaesa for their constructive comments on the manuscript. We also thank Maximiliano Maggioni for his assistance and opinion in the preparation of the ascidian figures. The study was partially supported by Consejo Nacional de Investigaciones Científicas y Técnicas, CONICET (PIP CONICET Nº 20130100508) and Secretaría de Ciencia y Tecnología, Universidad Nacional de Córdoba (SECyT30720130100645CB).