Temperature acclimation of respiration and photosynthesis in the brown algaLaminaria saccharina

Sporophytes of the brown algaLaminaria saccharina (L.) Lamour grown at 15°C contained significantly more chlorophylla (chla) than did similar plants grown at 5°C. The increase in chla in 15°C plants was due to increased numbers of photosystem II reaction centes, and possibly to increased photosynthetic unit size, compared with 5°C plants. These changes were associated with increased values (photosynthetic efficiencies) in 15°C-grownL. saccharina relative to 5°C-grown plants. The changes in together with reduced respiration rates allowed 15°C-grownL. saccharina to achieve net photosynthesis and light-saturated photosynthesis at a lower photon fluence rate (PFR) than 5°C plants when both groups were assayed at the same temperature (15°C). The photon fluence rates necessary to reach the compensation point and achieve light-saturated photosynthesis (I _c andI _k , respectively) increased with increasing incubation temperature inL. saccharina grown at both 5 and 15°C. However, acclimation responses to growth temperature compensated for the short-term effect of temperature onI _c andI _k . Consequently, plants grown at 5 and 15°C were able to achieve similar rates of light-limited photosynthesis, and similarI _c andI _k values at their respective growth temperatures. These responses are undoubtedly important for perennial seaweeds such asL. saccharina, which frequently grow in light-limited habitats and experience pronounced seasonal changes in water temperature.Please address all correspondence and requests for reprints to I.R. Davison     

Chemically mediated interactions between the red algaPlocamium hamatum (Rhodophyta) and the octocoralSinularia cruciata (Alcyonacea)

Interactions between the red algaPlocamium hamatum J. Agardh (Rhodophyta) and other benthic organisms including the alcyonacean soft coralSinularia cruciata (Tixier-Durivault) were investigated on an inshore fringing reef environment in whichP. hamatum was the dominant large fleshy alga. Field observations of sessile reef organisms including octocorals and sponges living in close proximity toP. hamatum revealed that varying degrees of tissue necrosis were suffered by the invertebrates when in physical contact with the alga. In order to establish whether the chemical constituents of the alga, especially chloromertensene, played a role in this necrosis, manipulative field experiments were carried out in the Pelorus Channel, Palm Island group (18°34S; 146°29E), North Queensland, Australia, in November and December 1988. The first experiment involved the relocation of healthy plants and soft corals into contact and non-contact situations on a mesh grid. In all cases of contact betweenP. hamatum andS. cruciata, the soft coral suffered tissue necrosis (n=6,p=0.0022). The second experiment had the same design, but involved the use of artificial plants both uncoated and coated with natural levels of chloromertensene, in contact withS. cruciata. In all cases of contact with coated treatments, necrosis was observed inS. cruciata (n=4,p=0.025). In cases where uncoated artificial fronds were placed in contact with soft corals,S. cruciata showed minor abrasion effects, but no appreciable necrosis. Coated treatments were not fouled by epiphytes during the experiment and were not consumed by predators. Uncoated treatments were rapidly reduced in size by predation and any remaining material was biofouled. These experiments thus demonstrated that the deleterious effects observed in soft corals in the field were caused by contact with the algaP. hamatum, that these effects were indeed chemically mediated by chloromertensene, and that physical contact without chemical intervention caused no such deleterious effects. This is the first experimental evidence which conclusively demonstrates allelopathy between an alga and other marine organisms and identifies the compound responsible for the observed allelopathic effects.     

Specific features and origin of the Caspian Sea diatom flora

The systematic composition of the diatom flora of the Caspian Sea is original, but poor, especially in marine species. At present, 286 species, varieties and forms, belonging to 46 genera and 4 orders are known to exist in the Caspian Sea. Of this number, 77 species and varieties (27%) are planktonic and 209 (63%) benthonic. The phytoplankton of the Caspian Sea is wholly neritic. In regard to salinity, a characteristic of the Caspian diatom flora is that it is composed of three different groups: marine, brackish, and fresh-water species. The brackish water group is of great interest, as it includes many diatoms which are known only from this water body (endemisms). The poor species-composition of the diatom flora is accounted for by variations in the hydrogical conditions of the Caspian Sea in the geological past. The formation of the flora is based on such sources as the ancient flora remains of the Upper-Tertiary Caspian-Black Sea, the younger elements of the Pliocene Sea during its isolation period, and the immigrants entering the Caspian Sea during the short period of its communication with the Black Sea.     

Under-ice amphipods in the Greenland Sea and Fram Strait (Arctic): environmental controls and seasonal patterns below the pack ice

During three "Polarstern" cruises to the ice-covered Greenland Sea (spring 1997, summer 1994, autumn 1995) studies on the under-ice habitat (morphology, hydrography, ice-algal biomass) and on the macrofaunal, autochthonous under-ice amphipods (species diversity, abundance) were carried out in order to describe environmental controls and seasonal patterns in this community. In spring, the ice underside was rather smooth and whitish, while in summer melting structures and sloughed-off ice-algal threads were observed, in autumn detritus clumps accumulated in depressions at the ice underside. Only in summer, a thin layer of warm (up to -0.6°C) and less saline (as low as S=6.3) water was found at the ice-water interface above Polar Water. Integrated ice-algal biomass was highest during autumn (2.6 mg chl a m^-2) and lowest during summer (1.2 mg chl a m^-2). Four species of under-ice amphipods occurred in spring and summer (Apherusa glacialis, Onisimus glacialis, O. nanseni, Gammarus wilkitzkii), but only the last species was observed at the ice underside in autumn. A. glacialis and G. wilkitzkii were equally abundant in spring; A. glacialis dominated in summer. The highest total abundance of amphipods occurred during summer (31.9 ind. m^-2), compared to lower abundances in spring and autumn (5.3 and 1.1 ind. m^-2, respectively). A factor analysis revealed seasonal patterns in the data set, which mainly influenced A. glacialis, and species-specific relations between several environmental factors and the distribution of under-ice amphipods. Abundance of A. glacialis was closely related to the under-ice hydrography and ice-algal biomass, whereas the other amphipod species were more influenced by the under-ice morphology. It is therefore stated that the observed thinning of the Arctic sea ice and the resulting increased meltwater input and change in morphology of floes will have a profoundly adverse effect on the under-ice amphipods.     

Siliceous sponge spicules in coral reef sediments

Experimental etching with hydrofluoric acid indicated that silica deposition occurs in a recognizable pattern in common sponge microscleres. The postdepositional alteration of these spicules has previously been generally unrecognized or misinterpreted in the literature. Early stages of postdepositional etching of sponge spicules were observed in the acid insoluble fraction of sediments from the West Atlantic barrier reef near Carrie Bow Cay, Belize. Preliminary data on silica distribution in the Belize barrier reef show that concentrations in fine sediment (<0.25 mm) increase landward of the main reef tract. Sponge spicules are the main component of particulate silica in sediments of the reef and fore-reef where sponge populations abound, whereas grains prevail in the back-reef lagoon deposits. Recycling of locally dissolved silica appears to be important for the growth of many off-shore reef sponges.     

Nutrients and biomass distribution in the Gulf of Aqaba (Elat), Red Sea

The distribution of phosphate, nitrate and silicate values obtained from 300 samples and of biomass determined by displacement volume in about 1,900 vertical plankton hauls (65, 200 and 330 m) collected from 8 oceanographic station during 1974–1975 in the Gulf of Aqaba (Elat) are summarized and illustrated by crosssections. Generally, the Gulf is poor in nutrients and the data indicate that it is filled with upper (150 m) Red Sea waters, flowing in over the sill of Tiran. Nutrient content of the upper and deep waters immediately outside the sill are well within the known range of those in the northern Red Sea. Biomass values are relatively low in the Gulf are generally similar to those reported from the northern Red Sea. Both nutrients and biomass values display seasonal and bathymetric variations in the Gulf and outside the sill.     

Age-and sex-dependent patterns of aggregation in the tropical gastropod Strombus luhuanus

The spatial dispersion of the tropical coral sand-dwelling mesogastropod Strombus luhuanus Linné, 1758 was studied at several sites in two localities (Australian Great Barrier Reef and southern Papua New Guinea), from September 1980 to February 1983. S. luhuanus usually occurred in local aggregations, many of which were relatively dense with discrete boundaries: Four types of aggregation are described: mixed age-class, juvenile, mating, and clusters. The former two are termed colonies, because they persisted over time and exhibited some coordinated movements. Mixed age-class colonies contain individuals of all ages, but within them the different ageclasses are frequently spatially segregated from each other. This was sometimes due to the younger age-classes' preference for shallower areas, but also occurred in areas of constant depth. Juvenile colonies are relatively smaller in area but higher in density, and are dominated by a single size-class. Mating aggregations are concentrations of copulating individuals, in which competition between males for access to females occurs. Clusters are concentrations of inactive individuals (juvenile and adult) piled together in close contact. All types of aggregation were concentrated in only a part of what appeared to be the suitable habitat. Most aggregations' boundaries shifted over time within the habitat. We conclude that intraspecific attraction must be an important factor which maintains the spatial structure of all aggregations, although habitat specificity sets the larger area in which they move. Processes which may underlie this pattern, and some of its implications, are discussed.     

Morphological correlates of intraspecific grafting reactions in the marine demosponge Callyspongia diffusa

There are profound differences in the ways that the sponge Callyspongia diffusa reacts to self versus non-self tissue contact. Syngeneic or isogeneic contacts result in rapid tissue fusion and persistent maintenance of normal sponge structure. However, allogeneic contacts result in immune-type rejection reactions leading to tissue death at the interface. Using grafted sponge tissues, both types of response were subject to fine-structural analysis by scanning electron microscopy. Isografts showed no overt cellular reactions other than compatible fusion, whereas sponge tissues at allograft interfaces showed incompatibility reactions. The major steps in the histopathologic sequence of events in allograft reactions may be summarized as follows: (1) immediate allorecognition occurs accompanied by microscopic cellular rearrangements; (2) macroscopic tissue bridges form across the graft interface; (3) massive cellular infiltration towards graft sites, with cells often passing into a graft partner through the tissue bridges; (4) cellular hyperplasia and then cell death occurs at the interface; and (5) sloughing of dead tissue leaves areas of vacant skeleton between the two grafted individuals. Allograft rejection reactions in sponges have many of the hallmarks of histoincompatibility reactions in higher animals. Although there could be different alloimmune mechanisms operating in different animals, the salient events of allorecognition and antagonistic rejection appear strikingly similar among diverse multicellular animals.     

Modes of water manipulation by the lobate ctenophore Leucothea sp.

Examination of the lobate ctenophore Leucothea sp. has revealed new patterns of swimming and water manipulation in addition to the typical ctenophore mode of slow swimming with ctene plate (comb) ciliary propulsion. We distinguish between slow ctene propulsion and rapid ctene propulsion; the latter is accomplished by an increased ciliary beat that produces a coupled vortex wake, resulting in jet propulsion. The large oral lobes both capture prey and provide undulatory muscular propulsive power. The auricles exhibit distinct phasic synchrony and aphasic or sculling motions that generate small vortices in the water which facilitate prey capture. Distinctive papillae covering the exterior of Leucothea sp. may be chemo- or mechano-sensory structures. The integration of all of these structures results in an organism that is more complex behaviorally than might be expected on the basis of its superficially simple and delicate body plan. Field work involved blue-water diving in the waters of the California Bight during June and July, 1985.