Growth of the tropical scallop, Euvola (Pecten) ziczac, in bottom and suspended culture in the Golfo de Cariaco, Venezuela

We compared the growth of the scallop Euuolu (Pecten) ziczuc (L.) in three situations which potentially could be used for commercial culture, in cages maintained in suspension, in cages on the bottom and in cages partly buried in a sediment bottom. The latter permitted the scallops to bury themselves as in their natural habitat. Throughout the 7-month study, growth, as measured by shell length and muscle mass, was by far superior for scallops in the partly buried cages. Possible explanations for this are ( 1) that the scallops are stressed by enclosures which prevent them from burying themselves and (2) that organic material at the sediment/water interface is an important food resource and E. ziczac has better access to this when it buries itself flush with the bottom. The timing of gonadal growth and spawning varied markedly among treatments. Some spawnings coincided with temperature increases but others did not. Differences between scallops in suspension compared to those in bottom treatments suggested that reproduction is as much controlled by conditions in the immediate environment of the scallops as by large-scale environmental factors. Survival was highest for the scallops maintained in partly buried cages

Organismal defenses versus environmentally mediated protection from herbivores: Unraveling the puzzling case of Desmarestia viridis (Phaeophyta)

Abstract The role of anti-herbivore organismal defenses in algae-herbivore interaction is frequently investigated without taking into account the potential role of environmental factors in mediating the interaction. Here we reexamine the interaction between the highly acidic, brown alga Desmarestia viridis and the green sea urchin, Strongylocentrotus droebachiensis, by incorporating a previously overlooked facet, the effect of changes in the wave environment on the ability of the urchin to establish contact with the alga. Factorial experiments in a wave tank (presence versus absence of waves; real versus mimic algae) showed that the aggregation of urchins on D. viridis was more than 2-fold greater in the absence than in the presence of waves. Similar numbers of urchins made contact with natural and mimic D. viridis plants, both with and without waves, indicating that any external release of chemicals (acid) from the alga had no perceptible repulsive effect on the urchin. The ability of the urchins to climb onto D. viridis increased markedly when urchin density attained a critical level. These results were consistent with field observations that urchins readily attack D. viridis under conditions of low wave action but do not under conditions of moderate wave action. We conclude (1) that the chemical makeup of D. viridis alone is neither necessary nor sufficient to limit contacts by the urchins and that (2) wave action is a major factor explaining the survival of D. viridis on urchin barrens, because waves limit the movements of the urchins towards the alga. We recommend that studies addressing marine algal defenses against herbivores be more comprehensive and examine interactions between algal traits, the physical environment, and the abundance and behavioral repertoire of herbivores

Some aspects of the ecology of strongylocentrotus droebachiensis in eastern Newfoundland

Strongylocentrotus droebachiensis is the dominant macroscopic benthic animal along the exposed coasts of Newfoundland. The gonads ripen during the fall, reach a peak in size and maturity in February or March, and spawnout occurs by April. In exposed areas the density is very high (up to 350 urchins/m²) and small urchins are abundant. In sheltered areas the density is much lower but most of the individuals are large. The urchin populations are concentrated in shallow water where macrophytes, their main food, grow abundantly. Urchins have variable preferences for and abilities to consume different algae. Feeding is least in the winter, when the gonads are near their peak, and greatest following spawnout. The potential grasing rate is sufficient to restrict noncalcareous macrophytes to the sublittoral fringe where the abrasion of the gonads, as they are washed back and forth by waves, keeps the urchins from advancing and devouring them. Only Agarum cribrosum and Ptilota serrata, which are undesirable urchin foods, grow at greater depths in urchin dominated areas. Sea stars, other urchins, lobsters, crabs, a number of fishes, and birds are common predators of S. droebachiensis but are not very effective in limiting their numbers

Factors regulating the reproductive cycles of some West Coast invertebrates

Annual reproductive cycles are found in many marine invertebrates. There is a vast literature on the subject, but the mechanisms involved have seldom been demonstrated. In the present study, 8 species of chitons and one sea urchin were studied for 3-5 years in southwestern British Columbia, at Vancouver and Porteau in the Strait of Georgia estuary, and at Botanical Beach on the outer coast of Vancouver Island. Reproductive condition was assessed mainly by the gonadal index method (percentage gonadal weight). There was a distinct annual cycle in the mean gonadal index of the urchin, Strongylocentrot-us droebachiensis Müller, and the chitons, Tonicella lineata Wood, Tonicella insignis Reeve, Mopalia hindsii Reeve, Mopalia laevior Pilsbry, Mopalia ciliata Sowerby, and Katharina tunicata Wood. In S. droebachiensis, T. lineata, T. insignis, M. laevior, and M. ciliata an abrupt spawning occurred in the spring, usually in April, and in M. hindsii there was usually an earlier spawning. K. tunicata sometimes spawned in April but the main spawning period was June. In M. laevior, K. tunicata, and probably M. hindsii, the gonads remained small during the summer and rapid gonadal growth occurred in the autumn and winter. In contrast, in S. droebachiensis, T. lineata, T, insignis, and M. ciliata gonadal growth started shortly after spawning. The data on reproduction in Mopalia lignose Gould were less clear. Mature animals were found in several seasons and drops in the mean gonadal index occurred in late winter-spring as well as in the summer. In Mopalia muscosa Gould animals in ripe and spent condition were found throughout the year. Consideration was given to the possible factors controlling gonadal growth. In a number of species, particularly species of warm water origin, it has been clearly demonstrated that gonadal development in the spring and summer is stimulated by increased temperatures. If temperature affects gonadal development in the species in the present study, it must act in several ways, since gonadal growth occurs through 2-3 periods of steadily increasing or decreasing temperature. The initiation of gonadal growth in K. tunicata and T. lineata in California and Oregon occurred at the same time as in the present study, although temperatures in the southern localities were fluctuating due to upwelling, in contrast to the regular temperature changes which occurred in British Columbia. This would suggest that temperature was not important, at least during the early stages of gonadal growth in K. tunicata and T. lineata. There are distinct annual photoperiod changes throughout the geographical ranges of the species in the present study, and in S. droebachiensis, T. lineata, T. insignis, and M. ciliata most gonadal growth occurred during the period of decreasing day length. Food conditions are known to affect the number of gametes produced in a number of species, including S. droebachiensis and K. tunicata, but there is no evidence that the timing of gonadal growth in the species in my study is controlled by a change in food conditions. The importance of temperature in stimulating spawning has been stressed by many authors, but I know of no instance where it has been demonstrated that a temperature change, sufficient to induce animals to spawn in the laboratory, actually occurred at the time of natural spawning. At First Narrows, there was usually a major spawning when the temperature reached 7-8 °C in the spring. However, in 1971, S. droebachiensis spawned when the temperature was about 6.3 °C, and temperature differences would not account for an abrupt spawning in 1973 In: "Perspectives in Marine Biology", A. A. Buzzati-Traverso (Ed.), University of California Press, Berkeley, pp. 67-36. Compared to the prolonged spawning in 1974. At Porteau, water temperatures showed a slow rise of only 0.8 °C during a two week period in which there was a complete spawning in Tonicella lineata, Tonicella insignis, and Mopalia laevior. At Botanical Beach, temperatures were a few °C warmer than at First Narrows when T. lineata, S. droebachiensis, and M. hindsii spawned, and the temperature at the time of spawning of T. lineata and K. tunicata varied several °C in different years. These observations suggest that spawning did not occur in response to a physiological threshold temperature, or to a sudden change in temperature. In 1973, S. droebachiensis and T. lineata were collected at First Narrows in late March, prior to spawning, and maintained under various temperature and light conditions: at 5.5 and 14 °C in darkness, and at 5.5 and 14 °C in light conditions similar to those in the field. These animals did not spawn when spawning occurred in the field. Similarly, S. droebachiensis, T. lineata, and T. insignis collected prior to spawning in 1974 and maintained in the laboratory did not spawn. However, animals returned to the field from the laboratory did spawn. This suggested that some condition in the field, which was not present in the laboratory, stimulated spawning, and this factor did not appear to be light or temperature. An abrupt spawning at First Narrows and Porteau in 1973 occurred at the time of the spring phytoplankton outburst, but in 1974 spawning at First Narrows was less abrupt corresponding to the slow development of the phytoplankton bloom in that year. In the laboratory, a large proportion of S. droebachiensis, T. lineata and T. insignis spawned when they were exposed to natural phytoplankton collected during the bloom with a 50 μ mesh net. This suggested that some substance bound to or released by phytoplankton stimulated spawning. For species with planktotrophic larvae the synchronization of spawning with the phytoplankton bloom increases the probability of both favourable food and temperature conditions for development or eggs, larvae, and juveniles. Gonadal growth during the coldest part of the year and spawning at the time of the spring phytoplankton bloom was found in S. droebachiensis, T. lineata, T. insignis, M. ciliata, and probably K. tunicata. This pattern is characteristic of marine invertebrates with pelagic larvae living in cold waters.Science, Faculty ofZoology, Department ofGraduat

The structure of subtidal food webs in the northern Gulf of St. Lawrence, Canada, as revealed by the analysis of stable isotopes

We analyzed stable isotopes of carbon and nitrogen to investigate the trophic structure of the subtidal food web around the Mingan Islands, northern Gulf of St. Lawrence, eastern Canada. All benthic consumers were enriched in 13C (mean δ13C of –17.1‰) compared to particulate organic matter (POM: –23.3‰). Nitrogen stable isotope ratios ranged from 6‰ to 14‰ and the organisms studied fell into three distinct trophic groups, primary producers, primary consumers (herbivores), and predators. The sea star Crossaster papposus and the sculpin Myoxocephalus scorpius, known to be top level predators, had slightly higher δ15N than other predators. Although the average isotope signature of the sea star Solaster endeca placed it among regular predators, the δ15N increased with sea star size and large individuals could be considered as top predators. The relatively small number of organisms located at intermediate trophic levels suggests a low level of omnivory in the Mingan Islands’ system, which contrasts with previously described benthic systems that exhibit a continuum between herbivores and predators. Low omnivory, in addition to low diversity, suggests that this ecosystem may be relatively unstable if exposed to natural and/or anthropogenic disturbances such as exploitation and climate change

Reproductive cycle of the bivalve Lima scabra (Pterioida: Limidae) and its association with environmental conditions

We examined the reproductive cycle of a tropical bivalve, the fire scallop Lima (Ctenoides) scabra (Born, 1778), over a 14 months period in the Golfo de Cariaco, Venezuela, and the association of gonadal growth and spawning with environmental factors. Most small individuals were males and most large individuals females, indicating the protandrous characteristic of this species. We documented three marked decreases in gonadal mass, from late October to December 1992, from early June to early July 1993 and from late October to early December 1993, and all were associated with drops in temperature related to renewed upwelling. This suggested that major synchronized spawnings were stimulated by drops in temperature or other factors related to the onset of upwelling. Multiple regression analyses indicated that the major environmental factor associated with increases in gonadal mass was phytoplanktonic food availability. Analyses of regressions of mass of somatic tissues to shell height suggested decreases in reserves in somatic tissues at the end of the November-December spawnings, but not at the end of the June-July spawning. This may be because body reserves are used for reproduction during November and December when phytoplanktonic food availability is low

Reproductive cycle of the bivalve, Lima scabra (Pterioida: Limidae):" and its association with environmental conditions

We examined the reproductive cycle of a,tropical bivalvé; the fire sdallop Lima (Ctenoides) scabra (Born, 1778), over a 14 months period in the Golfo de Cariaco, Venezuelá, and the' assooiation of gonadal gtowthWe examined the reproductive cycle of a,tropical bivalvé; the fire sdallop Lima (Ctenoides) scabra (Born, 1778), over a 14 months period in the Golfo de Cariaco, Venezuelá, and the' assooiation of gonadal gtowt