Verification of Sargassum natans (Linnaeus) Gaillon (Heterokontophyta: Phaeophyceae) from the Sargasso Sea off the coast of Brazil, western Atlantic Ocean

: The Sargasso Sea, named due to the floating presence of Sargassum fluitans and S. natans , is usually reported for the tropical region of the Northern Hemisphere. On 14 July 2011, at 02°45’ N and 48°28’ W, samples of pelagic seaweed masses were collected by the Patrol Ship Bracuí of the Brazilian Navy. The seaweed was identified as S. natans , previously considered as of doubtful occurrence in Brazil.

Maria Teresa M. de Széchy 1* , Patrícia M. Guedes 1 , Maria Helena Baeta-Neves 2 and Eduardo N. Oliveira 2 Verification of Sargassum natans (Linnaeus) Gaillon (Heterokontophyta: Phaeophyceae) from the Sargasso Sea off the coast of Brazil, western Atlantic Ocean including islands in the Caribbean and elsewhere in the Atlantic (Guiry and Guiry 2011).
Sargassum natans was first reported for the Brazilian coast in the floristic list published by Taylor (1931), based upon the listing of Sargassum bacciferum (Turner) C. Agardh for Brazil by Martens (1870). Sargassum bacciferum is presently a taxonomic synonym of S. natans (Guiry and Guiry 2011). Oliveira Filho (unpubl. data), in his review of the species of macroalgae for Brazil, includes Sargassum natans as a doubtful species, but observed that it occasionally appears floating off the northernnortheastern Brazilian coast.
Other species are mentioned as floating thalli off the coast of Brazil. Oliveira Filho et al. (1979) reported the occurrence of Sargassum hystrix J. Agardh and S. platycarpum Montagne floating in waters of the Brazil Current, about 100 km from the coast, at parallel 18°15'S. These species are common in drift on the shores of the northeastern and southeastern Brazilian coast (Széchy and de Paula 2010). The occurrence of pelagic specimens of S. hystrix was explained by the occurrence of benthic populations in deep bottoms of the area (Oliveira Filho et al. 1979). Sargassum hystrix is cited by Guimarães et al. (1981), as dredged material from 90 m depth, together with calcareous algae, during the Canopus Comission between the states of Ceará and Sergipe (05°55'00" S, 34°58'00" W).
The present study describes and illustrates the morphology of the brown alga Sargassum natans found offshore on the northern Brazilian coast as floating masses. Floating masses were first noticed by the Brazilian Air Force at the coordinates 04°12" N, 47°22' W, on 9 July 2011, when they were mistakenly identified as an oil spill ( Figure 1). Subsequently, additional spills were seen at the following coordinates: 01°58' N, 48°30' W; 03°09' N, 49°17' W; 03°42' N, 48°18' W; 04°13' N, 47°17' W. On 14 July 2011, at coordinates 02°45' N, 48°28' W, samples were taken from the sea by the Patrol Ship Bracuí of the Brazilian Navy, when seaweeds were recognized as the component of those spills (Figure 2). On that date, the patch extended over about five nautical miles, in the northeast to southwest direction. The collected seaweeds were kept at low temperatures, in a freezer, and were then transferred to a 4% formaldehyde solution. Three primary lateral branches were analyzed for the description of the external and internal morphology, including qualitative and quantitative characteristics. Ten blades and ten air bladders from each branch were measured. Cross sections were made using razor blades.  (1.9-)2.3(-2.8) cm in length and (2.5-)2.8(-4.0) mm in width [length/width ratio: (4.7-)6.2(-8.6)] and (110-)148 (-180) µm in thickness at the middle part of the blade. Midribs are evident in surface view but not prominent in cross section, with a thickness of (230.0-)324.6(-400.0) µm. Margins of the blades are coarsely toothed ( Figure  5) with aculeate teeth, measuring (300.0-)450.0(-750.0) µm in length. Cryptostomata are absent on the blades ( Figure 6). Air bladders are very numerous along the axes and are present on branches of all orders, attached by pedicels, singly or in pairs (Figure 7), predominantly oval or spherical (Figure 4), measuring (1.7-)2.2(-3.0) mm in diameter. Some air bladders terminate in a small spine or laminar projection. The pedicels of air bladders are cylindrical, and (1.2-)2.1(-3.8) mm in length. Receptacles are not present on the specimens (voucher RFA 35.700).
Based on these morphological characteristics, the offshore pelagic Sargassum analyzed was identified as S. natans. The Brazilian specimens are similar to the descriptions of S. natans from the North Atlantic Ocean in the width of the blades and also in the typically dentate margins of the blades. However, the material from Brazil showed less extended blades, being shorter than those described for the Caribbean and Florida. The Brazilian specimens differ from those described by Taylor (1960), Schneider andSearles (1991), Littler andLittler (2000) and Dawes and Mathieson (2008) with respect to the presence of spines on the axes of the primary lateral branches and their branches. Spines on the axes of the primary lateral branches are mentioned in the literature for S. fluitans (Table 1).  This study is in agreement with previous reports (Taylor 1931) that mentioned the occurrence of Sargassum natans for Brazil, information that was later questioned by other investigators. Taylor (1960) himself later noted that the report of this species from Brazil was outside the natural geographical range of the species, and was followed by Oliveira Filho (unpubl. data), who likewise considered the occurrence of S. natans in Brazil as doubtful. There is no concrete information about the origin of the huge Sargassum natans masses within the territorial waters of Brazil, yet it seems likely that they branched off the pelagic community well known and studied to the north in the Sargasso Sea.