María G. Liuzzi

Latitudinal gradients in macroalgal biodiversity in the Southwest Atlantic between 36 and 55°S

Different groups of marine benthic organisms show contrasting latitudinal patterns of biodiversity. The widely accepted paradigm of increasing biodiversity towards the tropics does not seem to be valid for macroalgal floras of the Southern Hemisphere. We compiled a database summarizing the distributional ranges of macroalgae along the coast of Argentina to test whether biodiversity decreases towards lower latitudes, as in the Pacific coast of South America, and whether breaking points in the geographical distribution can be recognized in one or more areas of the Southwest Atlantic south of 36°S. We found a clear trend of decreasing biodiversity with decreasing latitude. The interpretation of some biodiversity declines is confounded by changes in the intensity of the sampling effort. A 51% reduction in algal species richness between 42 and 41°S coincides with the boundary between the Argentine and Magellanic Zoogeographic Provinces. This sharp breaking point is related to a thermal anomaly caused by long residence times of water masses within San Matías Gulf, suggesting an upper thermal tolerance limit for most Antarctic/sub-Antarctic seaweeds. A further reduction occurs at 38–37°S. This breaking point can be explained by the disappearance of suitable hard substrata, since rocky outcrops give place to wide extensions of sandy beaches. The impoverished algal assemblage inhabiting the northern coast of Argentina is mainly related to the reduction or disappearance of the Antarctic/sub-Antarctic floristic component. This area is characterised by a predominance of widely distributed species, Chlorophytes and opportunistic filamentous or foliose algae.

A new Antarctic Osthimosia (Bryozoa, Cheilostomata, Celleporidae) with dimorphic zooids

Osthimosia chaotica sp. nov., a new celleporid cheilostome bryozoan, is described from samples collected in the South Shetland Isles and the Antarctic Peninsula. It differs from other Antarctic and Subantarctic species of the genus Osthimosia in the absence of vicarious avicularia, the characteristic development of its peristome and the reduced size of the primary orifice of the ovicelled zooids compared to those of autozooids. The occurrence of sexual polymorphism in the Bryozoa is discussed. The new species lives on laminar and filamentous algae.

A New Species of Leptochiton (Polyplacophora: Leptochitonidae) from the Southwestern Atlantic

ABSTRACT Leptochiton sanmatiensis, new species, is described from shallow-waters of Argentina. The species is characterized by having a quincuncial arrangement of tegmental granules on head valve, lateral areas of intermediate valves and postmucronal area of tail valve, and a longitudinal arrangement of these structures on the central areas of intermediate valves; each granule having a macraesthete and four micraesthetes. In addition, the new species has pectinated dorsal scales, with 14–21 ridges; a low number of ctenidia (up to six on each side in the largest specimens); and the second lateral tooth with three denticles in the cusp. The species lives on small gravel and shells.

Recent discovery of non-indigenous bryozoans in the fouling assemblage of Quequén Harbour (Argentina, Southwest Atlantic)

The non-indigenous bryozoans Aspidelectra melolontha (Landsborough) and Amathia verticillata (Delle Chiaje) (= Zoobotryon verticillatum) are recorded for the first time in Argentina. They were found on experimental substrata submerged to study the fouling assemblage of Quequén Harbour (38°34.310′S, 58°42.814′W), an estuarine environment. Biflustra puelcha (d’Orbigny) comb. nov., Conopeum reticulum (Linnaeus), Conopeum seurati (Canu), Cryptosula pallasiana (Moll), Electra monostachys (Busk) and Schizoporella cf. errata (Waters) were also found in this association. Aspidelectra melolontha, a species native to Europe with only one record for San Francisco Bay (United States) based on unpublished data, is here reported for the first time for South America, and the affinity between this species and A. zhoushanica (Wang) is discussed. The presence of C. seurati in brackish water habitats of Argentina is confirmed. B. puelcha was previously unknown as a fouler in harbours. A. verticillata is a well-known marine fouler in many harbours and marinas worldwide. The southernmost record of this species on the Atlantic coast of South America was in Brazil. Thus, its distribution is herein expanded by 14° of latitude towards the south. In view of its great invasive potential, A. verticillata seems to have the ability to continue its dispersal and to become a stable component of this and neighbouring harbours.

The clam Neilonella sulculata in San Jorge Gulf (Argentina, Southwest Atlantic): spatial distribution pattern and inverse relationship between size and density

ABSTRACT Neilonella sulculata is a dominant bivalve on muddy bottoms of the continental shelf and gulfs off Argentina (Southwest Atlantic). Two benthic surveys performed in 2007 and 2009 in San Jorge Gulf, a depositional area characterized by silt/clay sediments, gave us the opportunity to test whether the population of this clam displays a significant spatial structure and to analyse if there is an inverse relationship between size and density. Density, biomass and size–frequency distribution displayed significant spatial structure, being positively autocorrelated at the smallest spatial scales (c. 1.7–14/17 km). Biomass also showed spatial contagion at scales of 25–33 km. A substantial increase in density and biomass occurred between 2007 and 2009. Empty valves in the sediment showed that the maximum size attained by the species may exhibit considerable variation at relatively short temporal scales. Regardless of temporal changes in density and biomass, the spatial structure of these variables remained stable. In densely populated areas, shell size decreased with increasing density, suggesting a density-dependent control of growth. A clear north–south increase in density and biomass was detected, which might be related to a gradient in food availability caused by a thermohaline frontal system associated with wind-related upwelling.

A new genus and species of cheilostome bryozoan associated with hermit crabs in the subantarctic Southwest Atlantic

Some bryozoan species occur on gastropod shells occupied by hermit crabs, often dominating the shell surface to the exclusion of other encrusters. Symbioses between bryozoans and pagurid crabs are known from a wide latitudinal range, including tropical, warm- and cold-temperate regions. Here we describe Burdwoodipora paguricola gen. et sp. nov., a cheilostome bryozoan associated with pagurized shells occupied by the hermit crab Pagurus comptus White in subantarctic waters of the Southwest Atlantic. The association seems to be obligate, as the bryozoan was always found enlarging the last whorl of its gastropod substrate. The new species, however, is not species specific with regard to its shell substrate, as it was found to be associated with 31 species of gastropods with helicoidal coiling. Well-developed colonies may project up to 360° from the outer lip of the shell. The chamber built by the bryozoan is usually larger than the whorl that would have been generated by the gastropod. This is one of the few known cases of an obligate association between a bryozoan and a pagurid crab outside the ascophoran family Hippoporidridae and is also the first bryozoan/hermit crab symbiosis documented in the subantarctic region. The inclusion of Burdwoodipora in the Pacificincolidae, a family currently represented by just four species from the Northern Hemisphere, is discussed.

Role of the clam Neilonella sulculata as substrate for the hydroid Bougainvillia macloviana on muddy bottoms of San Jorge Gulf (Argentina, Southwest Atlantic)

ABSTRACT The clam Neilonella sulculata is the dominant bivalve on muddy bottoms along the Argentine continental shelf (Southwest Atlantic). The medusa stage of the hydroid Bougainvillia macloviana is one of the most abundant species in the gelatinous zooplankton of the Magellan region. Two benthic surveys carried out in 2007 and 2009 in San Jorge Gulf gave us the opportunity to analyse (1) the interaction between N. sulculata and its epibiont, B. macloviana, and (2) the role of the clam as hard substrate in a depositional area characterized by silt/clay sediments. Bougainvillia macloviana has a high prevalence on the posterior region of live clams. The presence of gonophores shows that these small colonies are able to reproduce there. Hydroid colonies are very rare on dead valves of N. sulculata and also on almost all other live bivalve species in the study area. Specimens of N. sulculata with hydroids were slightly but significantly larger than those without them, both in 2007 and 2009. The high prevalence of B. macloviana on live individuals of N. sulculata and its almost total absence on other clam species suggest that this association may be critical at least for the epibiont and not the result of chance alone.