This study analyses the possible relationships between body size and length of cnidae from different tissues of the sea anemone Oulactismuscosa. We describe the cnidom, providing new qualitative and quantitative data. Our description adds spirocysts for tentacles and acrorhagi, and is more precise about the ranges and types of basitrichs, microbasic b-mastigophores, and holotrichs. We distinguish two types of holotrichs in the acrorhagi, and differentiate between microbasic b-mastigophores and basitrichs in the actinopharynx and mesenterial filaments. A relationship between cnida length and body weight was not demonstrated. The results are based on a complete account of cnida types from all tissues, and considering the great number of capsules measured (5400) and the modern statistical tools employed, we think that a normal distribution of cnida lengths is uncommon, perhaps refuted. This finding is very important when a quantitative analysis of cnidae is necessary and an adequate statistical tool must be used. We have shown that generalized linear models are an alternative and therefore analyses can be done with parametric methods despite the non-normal distribution of cnida size. The use of these statistical tools should be generalized since appropriate package for analyses (like the R package) are available from the web and the obtained results are robust and powerful.

In this study we provide an updated checklist of benthic Cnidaria from SW Atlantic Ocean that comprised the Marine Protected Areas Namuncurá I and II, located at Burdwood bank, and other neighbouring locations. A total of 88 taxa was recorded: 36 hydrozoans and 52 anthozoans from which 32 were octocorals, 10 scleractinian corals, 8 sea anemones and 2 zoanthids. Burdwood bank presented the highest richness considering that 87% of the recorded species inhabit this plateau or its slope. Besides some common species widely distributed in the studied sub-areas, at least 24 species represent new distributional records while few were exclusively recorded at Burdwood bank. The inventory here provided will help to identify key habitat-forming species in a complex habitat where marine animal forests and vulnerable marine ecosystems were previously detected. It will be also a very valuable tool for the management and monitoring of the sub-areas under protection.

We report the first record of the sea anemone Anthopleura nigrescens on the Pacific coast of Central America. Five specimens from Mata Limón (Costa Rica) were collected from rocks of a man-made breakwater at sub-tidal level. Images of live specimens are presented. A map with its worldwide distribution is included and we concluded that this actiniarian can be considered widely distributed in the Indo-Pacific.

Due to the sparse presence of algae, hydroid clumps appear as the main primary substrata for the settlement of epizoical mussels in Buenos Aires province (Argentina). The settlement and growth of Mytilus edulis platensis on Amphisbetia operculata and Plumularia setacea colonies during their first life stages was analyzed throughout the year and the double settling larvae phenomenon is discussed. Colonies of both species were frequently colonized by numerous spat, which reached a maximum density of 1370 and 1450 individuals g^-1 of colony, respectively. Settlement of Mytilus larvae on the hydroids occurred mainly in November, decreasing in December and January. The plantigrade's biomass on A. operculata increased from December until April in spite of a decrease in the number of individuals. In constrast in P. setacea both plantigrade numbers and biomass decreased from December and no individuals were found in February. The major abundance and permanency of plantigrades on A. operculata could be explained by its bigger size and more profuse branching in comparison with P. setacea. The competent Mytilus larvae also settle on mussel beds, but only the filamentous substrata seem to ensure a high probability of survival during the early benthic stages, because it provides protection from predators (sea urchins and sea stars).