Gabriel N. Genzano

Hydroid populations from sublittoral outcrops off Mar Del Plata, Argentina: Abundance, seasonality and reproductive periodicity

Abstract An analysis is given of the frequency, abundance, seasonality, and reproductive periods of hydroids from sublittoral rocky outcrops in coastal waters off Mar del Plata, Argentina. A total of 13 hydroid species belonging to 8 families were found, 12 of them referable to the subclass Leptothecatae. Species were grouped into three categories based on their occurrence and frequency during the year. Amphisbetia operculata, Sertularella mediterranea and Plumularia setacea were the most abundant hydropolyps in the study area. They occur as distinct clumps in patches having a contagious distribution, although the exact patterns vary considerably. Colonies of A. operculata were observed throughout the year, with gonothecae present between May and January and a reproductive peak at the end of winter. The seasonality of S. mediterranea, with a peak of reproductive activity during the cold period, was similar to that of colonies of the species from the intertidal zone studied previously. Conversely, differences were noted in seasonal patterns of P. setacea between the two zones. Intertidally, this species was found only during the warm season, as small clumps (not more than 35 mm high). In contrast, sub-littoral colonies were larger (up to 130 mm high) and three overlapping generations each year were observed. Comparisons with intertidal hydroid communities are made.

Hydroid assemblages from Mar del Plata, Argentina, at depths between 0 and 500 m. Distribution and biological substrata

The first studies on hydroids of the subantarctic region (SW Atlantic Ocean) were based on samples from expeditions to the Antarctic at the end of the 19th and beginning of the 20th centuries. In spite of these works, hydroids from the northern Argentine continental shelf remained unknown until the 1960s. In coastal waters off Mar del Plata, studies on hydroids have been few in number and limited to species described from preserved material. Even rudimentary ecological information is lacking in these reports. Over the past few decades, samples from oceanographic expeditions, and specimens collected directly from the intertidal rocky shore or by means of scuba diving and snorkeling, have provided heretofore unpublished information on hydropolyps of the Mar del Plata region. The goal of this work is to analyze the hydroid fauna of the Mar del Plata coast, including bathymetric distributions, frequencies of occurrence, and biological substrata. A total of 36 species were included. Most were found at depths shallower than 8 m, where hard substrates predominated. Numbers of species decreased markedly below 80 m. The shallow-water hydroid fauna of Mar del Plata comprises a large number of cosmopolitan or widely distributed species. In deeper zones, species with subantarctic or south hemisphere distributions predominated. Hard bottom outcrops were surrounded by extensive areas of sand. Although such substrata are unfavorable for most hydroids, many species were found in soft bottom areas on polychaete tubes. Stems of hydroids provided the greatest number of epizoic species (18), followed by polychaete tubes (16 species) and sponges (15 species). Hydroids, bivalve mollusks, and sponges were the most frequent substrates. Colonies grew less frequently on bryozoans and tunicates. This general scheme changed at greater depths, where the most frequent substrates were polychaete tubes and sponges.

Hydroid colonies as primary substrata for recruits of the mussel Mytilus edulis platensis front off Mar Del Plata, Argentina

Abstract 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 plantigrades 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).

Gelatinous zooplankton (ctenophores, salps and medusae): an important food resource of fishes in the temperate SW Atlantic Ocean

ABSTRACT This study quantifies the occurrence of gelatinous zooplankton in the stomach contents of fishes from the southwest Atlantic Ocean (33°–55°S). More than 69,000 fish stomachs belonging to 107 species were examined. A total of 39 fishes were documented as consumers of gelatinous zooplankton, 23 of which were newly discovered. Three gelatinous organism consumption categories are recognized: (1) very frequent consumers (10 species, six of which were exclusive); frequent consumers (five species); and occasional consumers (26 species). Three types of gelatinous prey (ctenophores, salps and medusae) were found in the stomach contents of fishes. Ctenophores were consumed at high levels across almost the entire continental shelves of Argentina and Uruguay. Salps were frequent prey on the slope and southern shelf. In contrast, medusae were consumed in coastal areas, slopes and the southern shelf. Classification methods (group average sorting of the Bray–Curtis similarity measures based on log (X + 1)-transformed percentage data) determined six areas where fishes predated on gelatinous organisms. SIMPER (similarity percentages) analysis determined which fishes contributed more to the consumption of gelatinous organisms. Results revealed that two fish species (Stromateus brasiliensis and Squalus acanthias) had high gelatinous zooplankton predation rates throughout the entire study area, while another six species (Patagonotothen ramsayi, Helicolenus dactylopterus lahillei, Macrourus holotrachys, Merluccius hubbsi, Schroederichthys bivius, and Macruronus magellanicus), while widely distributed, seemed to have specific areas where consumption occurred. This study not only provides new knowledge about the importance of gelatinous zooplankton in the diet of numerous fishes, but might also be valuable for planning and managing local fisheries.

Epibiont community variation on two morphologically different hydroid colonies: Amphisbetia operculata and Plumularia setacea (Cnidaria, Hydrozoa)

Abstract Information on differential fouling abundance and seasonal variation of hydroid colonies is scant. These biological structures seem to be relevant for recruitment of other taxa. In this sense, the present study provides information about vagile and sessile organisms on two morphologically different hydroid colonies, Amphisbetia operculata and Plumularia setacea, during all four seasons. Both species are frequent components of the benthic rocky outcrop community of Mar del Plata (Argentina). A total of 49 epizoites on A. operculata and 44 on P. setacea, belonging to 12 divisions/phyla were identified. The main groups found on both colonies were bryozoans, crustaceans and molluscs. In both basibiont hydroids, epibiont species coverage, richness and composition changed between the studied seasons, but total colony height did not influence total coverage. Morphological differences between the two hydroid colonies determined coverage differences at two colony heights and the type of attached organisms that could be found on these colonies. Algae and filamentous invertebrates are very scarce in the studied community, and thus A. operculata and P. setacea appear as the unique and frequent arborescent structures multiplying the available substrata in the analysed outcrops. Thus, hydroid colonies might play an important role providing habitats for epibionts. Furthermore, the morphological differences between both hydroids may determine coverage difference and the kind of attached organisms found on these colonies.

A comparative study of populations of Ectopleura crocea and Ectopleura ralphi (Hydrozoa, Tubulariidae) from the Southwestern Atlantic Ocean.

Ectopleura crocea (L. Agassiz, 1862) and Ectopleura ralphi (Bale, 1884) are two of the nominal tubulariid species recorded for the Southwestern Atlantic Ocean (SWAO), presumably with wide but disjunct geographical ranges and similar morphologies. Our goal is to bring together data from morphology, histology, morphometry, cnidome, and molecules (COI and ITS1+5.8S) to assess the taxonomic identity of two populations of these nominal species in the SWAO. We have observed no significant difference or distributional patterns between the so-called Brazilian E. ralphi and Argentine E. crocea for both morphological and molecular data. Therefore, SWAO populations of Ectopleura belong to the same species. In a broader view, it is difficult to find decisive character distinguishing E. crocea from E. ralphi, and both species have indeed recently been synonymized, with the binomen E. crocea having nomenclatural priority. Geographically broader genetic analysis should be carried out in order to test the validity of this synonymy because taxonomical procedures such as studying type specimens and documenting broad phenotypic variability have not yet been conducted.

An Overview of the Medusozoa from the Southwestern Atlantic

Medusozoans are critical components of coastal and marine ecosystems. They are ubiquitous, living from the surface to the bottom layers of the world’s oceans and tolerating a wide range of environmental conditions. They modulate food webs not only by consuming large quantities of ichthyoplankton and other zooplankton and acting as predators and competitors of varied pelagic organisms including fish but also by being consumed by other predators. Population outbreaks of these gelatinous animals commonly occur over a variety of spatiotemporal scales. These population explosion events have implications for the ecosystem and, usually, for human enterprise. Despite their ecological and socioeconomical importance, there is as yet no attempt to compile existing information on the medusozoan species of the Southwestern Atlantic (SWA). We provide here an overview of the information available regarding several aspects of Medusozoa in the SWA: the characteristics of their life cycles, life histories and “blooms,” and the ecological implications for SWA ecosystems. Guidelines for future research and perspectives on the field are also provided.