The bulbus arteriosus of stenothermal and temperate teleosts: a morphological approach

The bulbus arteriosus, 'windkessel', of several species of stenothermal and temperate teleosts has been studied by conventional light microscopy and electron microscopy. The bulbus wall is divided into an endocardium, ridges, and middle and external layers. The endocardium of all species shows moderately-dense bodies, which vary widely although the significance is not known. The endocardium in Antarctic teleosts invaginates into the ridge tissue to form solid epithelial cords that show signs of active secretion related to protective substances. Cords also form in serranidic and sparidic species, but signs of active secretion are not evident. The ridges consist of cells within a filamentous meshwork. Ridge cells appear to be smooth muscle cells that undergo a phenotypic transition from the endocardium toward the middle layer. Middle layer cells are typical smooth muscle cells surrounded by a filamentous matrix. The appearance and composition of the extracellular matrix varies widely among species, with those from the Antarctic lacking collagen and elastin fibres. The external layer is a collagenous matrix that contains fibroblasts, blood vessels and nerves. In most Antarctic teleost species this layer lacks blood vessels, but contains nerve fibres. Some of these fibres could have a sensory function to control bulbus dilatation. The external layer of Trematomus bernacchii has the appearance of a germinal centre and may be involved in the immune humoral response. The epicardium is atight epithelium that may control passage of substances with the pericardial cavity.     

Changes in the fish fauna associated with a sub-Antarctic <Emphasis Type="Italic">Macrocystis pyrifera</Emphasis> kelp forest in response to canopy removal

The fish fauna associated with a Macrocystis pyrifera forest and the effects of the canopy removal on this fauna were studied in the Beagle Channel, Tierra del Fuego. Seasonal changes and differences in the fish communities that inhabit the canopy and the holdfast were also studied. Two patches of kelp forest separated by 200 m were selected. In one, the canopy was removed periodically while the other was used as a control. Samples were collected seasonally, from autumn 1999 to 2001. Fish fauna in the water column was sampled using trammel nets and holdfast fish fauna was sampled by removing the complete holdfast. Different assemblages of fish species were captured in the water column (surface and bottom) and in the holdfast. In the former there were mainly pelagic and benthopelagic species and the latter predominantly demersal species. The principal effect of the canopy removal was a drop in the abundance (and total weight) of Paranotothenia magellanica (doradito), the species with the strongest relationship with the canopy, principally at the surface of the treated patch probably due to a reduction in the availability of refuges. A high degree of seasonality was observed for fish species’ total weight, abundance and diversity, with higher values in summer and autumn. Taking into account our data and available data on kelp growth in the same locality, we suggest of cutting the kelp forest (preferably those farther away from the coast) once a year in winter or early spring, in order to minimize impact on the fish community.