John H. Himmelman

Direct Coupling of Marine Invertebrate Spawning with Phytoplankton Blooms

Spawning of green sea urchins and blue mussels may be triggered by a heat-stable metabolite released by various species of phytoplankton. Mussels require a higher phytoplankton density for a maximum response than urchins, perhaps because mussels are exposed to higher concentrations of phytoplankton as a result of their filtering activity. Phytoplankton as a spawning cue appears to integrate numerous physical and biotic factors indicating favorable conditions for larval growth and survival. Evolution of similar direct coupling of the larval phase with phytoplankton blooms may be common among marine invertebrates.

Fouling development and its effect on the growth of juvenile giant scallops (Placopecten magellanicus) in suspended culture

Abstract We examined the development of the fouling community on pearl nets and its effect on the growth of juvenile Placopecten magellanicus at Grande-Riviere and Gascons in the Baie des Chaleurs. The total biomass of fouling organisms was 2–3 times greater at Gascons than at Grande-Riviere and at both sites decreased with increasing depth. Throughout the study, at Grande-Riviere, at the mouth of the bay, the fouling community was dominated by two bivalves, Mytilus edulis and Hiatella arctica, and the hydroid Obelia gelatinosa was third in abundance. In contrast, at Gascons, 40 km inside the bay, the hydroid Tubularia larynx predominated in the summer while M. edulis and H. arctica predominated in the autumn. Factors potentially explaining the success of M. edulis and H. arctica in the fouling community are the loss of hydroids due to selective grazing by nudibranchs, the rapid growth rate of the bivalves and their ability to inhibit settlement of competitors. Changing the pearl nets regularly results in a sharp increase in the yield of muscle and other soft tissues, but only a slight increase in shell height of the cultured scallops. For example, for a 30-mm scallop suspended at 9 m in depth for 4 months, the increase in muscle mass is 68% greater in cleaned than in fouled nets whereas the difference is only 4.8% for shell height. Because fouling decreases with depth and also varies markedly between localities, judicious choosing of culture sites in term of fouling development could substantially improve meat production.

Community development following removal of urchins, Strongylocentrotus droebachiensis , from the rocky subtidal zone of the St. Lawrence Estuary, Eastern Canada

SummaryThe role of sea urchins, Strongylocentrotus droebachiensis, in structuring the rocky subtidal community was examined at Anse aux Basques on the north shore of the St. Lawrence Estuary, Québec. In an experimental area, measuring 20x20 m and extending from 0 to 10 m in depth, we greatly reduced the intensity of urchin grazing by eliminating all urchins larger than 10 mm in test diameter. This area was observed for two years and compared to an adjacent control area. In the upper portion of the experimental area during the first month after urchin removal, mid-July to mid-August 1978, a dense diatom cover developed, and during the second month the diatoms were overgrown by Ulvaria obscura. After four months (November) an Alaria esculenta overstory was present from near low water level to 3 m deep. Community development was much slower at greater depths and it took a year for the Alaria zone to extend to 4–5 m deep, and two years to extend to 6 m deep. The low light penetration at this estuarine location was probably the main factor for the slow algal development at 6–10 m deep. At the end of the experiment Agarum cribrosum was second in importance after Alaria and was most common at 3 to 6 m in depth. Laminaria spp. was found in low numbers in the first year and did not show an increase during the second year. There was a dramatic increase in the number of species and abundance of algae in the experimental area. Also, there was a marked increase in many animal species, particularly Acmaea testudinalis, Mytilus edulis and Margarites helicinus, and a decrease in Metridium senile. By contrast, in the control area, the number of algal and invertebrate species remained low. In the experimental area a sharp increase in the growth rate of a cohort of very small urchins, which was not eliminated by our removal effort, demonstrated that there is strong intraspecific competition amongst urchins when the food supply is limited. In the St. Lawrence Estuary, there are few predators of urchins and the urchin dominated community appears to be a stable situation.

Reproductive cycle and seasonal feeding activity of the neogastropod Buccinum undatum

The whelk Buccinum undatum L. in the northern Gulf of St. Lawrence has a distinct annual reproductive cycle. The ovary and pallial oviduct of the female, and the seminal vesicle of the male, show parallel development, whereas the testis has an inverse cycle. Thus, at the end of winter when the ovary, pallial oviduct and seminal vesicle reach maximal size, the testis is small and undeveloped, whereas the reverse situation occurs in summer. This is the first report of a totally inverse relationship between the ovary and testis for a marine invertebrate. This unusual pattern is due to the long-term storage of sperm in the seminal vesicle of the male prior to copulation. There is no resting period at the end of oogenesis, but rather renewed ovarian development starts shortly after gamete release. By contrast, in the testis after transfer of sperm to the seminal vesicle at the end of spermatogenesis, there is a distinct period of phagocytosis. Mating begins in mid-May, reaches a peak during June and terminates prior to July. Egg laying begins in late May, reaches a peak in mid-June to mid-July and can continue until late August. Examination of the incidence of individuals with food in their stomachs showed that feeding activity is maximal in late autumn through to early spring and decreases sharply coincident with the onset of breeding in late May.

Identification of factors affecting growth and survival of the tropical scallop Euvola (Pecten) ziczac in the Golfo de Cariaco, Venezuela

We examined the effect of environmental variables on the growth and survival of the tropical scallop, Euvola ziczac (L.), in suspended culture at Turpialito in the Golfo de Cariaco, northeastern Venezuela. Four growth trials were run, three using 10-mm spat produced from hatchery spawnings in August 1992, which were grown at 8, 21 and 34 m in depth, and a fourth using 10-mm wild spat from spawning in April 1993, which were grown at 21 m in depth. Monthly measurements of somatic and reproductive growth were related to concurrent measures of numerous environmental factors, including fouling on the shells and pearl nets. Growth and survival varied markedly among treatments and the major factor explaining the differences appeared to be the negative effects of fouling (especially the effects of organisms colonizing the shells) and variations in when gonadal development began. Somatic growth was markedly greater in treatments where gonadal development was delayed. For the August 1992 cohort at 8 and 21 m in depth, low phytoplankton abundance and high temperatures, possibly coincident with physiological stress from spawning, probably caused decreased growth and survival in the period during and just after spawning. Phytoplankton abundance (number of cells and chlorophyll a concentration) was a good predictor of growth for the August 1992 cohort grown at 34 m and the April 1993 cohort at 21 m, the treatments in which the scallops attained the largest size. The growth of juveniles (prior to development of gonads) seemed to be little affected by temporal changes in environmental conditions in the Golfo do Cariaco. Suspended culture of E. ziczac is likely to be economically feasible during periods of low fouling, but not when fouling is intensive.

Gametogenesis and Spawning of the Sea Cucumber Psolus fabricii (Duben and Koren)

The reproductive cycle of the sea cucumber Psolus fabricii was studied in a population from the St. Lawrence Estuary in eastern Canada from May 1988 through August 1989. The gonad consists of numerous germinal tubules which vary greatly in size. The mean diameter of the tubules and gonadal mass follow annual cycles, increasing from early winter through spring, and dropping abruptly during spawning in the summer. Gametogenesis is generally a prolonged process and begins in small tubules in January. By summer the ovarian tubules contain oocytes with a modal diameter of 400-600 {mu}m, and the testicular tubules contain an abundance of early spermatogenic stages, but rarely spermatozoa. These small tubules of the gonad do not spawn until the following year, and there is little gametogenic activity within them until January, when oocyte growth and the production of later spermatogenic stages resumes. The latter production continues until summer and results in a marked increase in the diameter of the tubules. Then, during spawning, these now large fecund tubules are transformed into small tubules. Following spawning, the predominant activity within the spent tubules is phagocytosis of the residual gametes. The active phase of gametogenesis (January to summer) coincides with an increasing photoperiod regime, and an accelerated gametogenesis occurs in March when temperature and food availability begin to increase. Spawning was one month later in 1989 than in 1988 and did not show a consistent relationship with either temperature or light conditions. However, in both years, spawning coincided with a decrease in the freshwater run-off into the Estuary and with the predicted annual increase in phytoplankton.

Diet, behaviour and reproduction of the whelk Buccinum undatum in the northern Gulf of St. Lawrence, eastern Canada

We examine the use of prey resources and seasonal variations in behaviour and reproduction for the whelk Buccinum undatum, the most abundant subtidal carnivore in the northern Gulf of St. Lawrence. Whelks used in this study were collected in the Mingan Islands in 1987 and 1988. The proportion of whelks with food in their stomachs varies seasonally and further with habitat, being greatest on sandy bottoms. It decreases at the onset of breeding in the spring and is generally low through the summer. Fragments of certain organisms (e.g. polychaetes, bivalves, urchins) in whelk stomachs suggest that they are active predators. However, predation is virtually never seen in the field. Other organisms in the stomachs, infrequent feeding, and their high mobility and capacity to detect and locate dead animals on the bottom suggest that whelks are carrion feeders. However, the amount of carrion available is probably insufficient to meet the needs of the biomass of whelks present in these waters. An interaction with seastars may contribute to the whelks diet. They frequently approach seastars which are extracting bivalves from sediment bottoms and may benefit by feeding on prey remains left by seastars or by foraging in sediments recently disturbed by seastars.

Behaviour and timing of copulation and egg-laying in the neogastropod Buccinum undatum L.

The reproductive behaviour of the boreal neogastropod Buccinum undatum L. was studied in the Archipel de Mingan, northern Gulf of St. Lawrence and in the laboratory. In the field, mating begins in mid-May with the aggregation of whelks caused by the movement of males towards gravid females. There is a high degree of polygamy. During copulation, spermatozoa from the males seminal vesicle are deposited in the females bursa and shortly afterwards are transferred to the seminal receptacle where they may be stored for as long as 8 wk. This permits a prolonged period of egg-laying. A technique, based on the presence or absence of sperm in the bursa and the seminal receptacle, was developed to follow the timing of copulation and egg-laying. The quantification of breeding activities using diving observations along a permanent transect permitted us to evaluate the precision of this new technique. Beginning in mid-May, the females search for appropriate egg-laying sites, such as vertical and undercut surfaces of large boulders or the stipes of the brown alga Agarum cribrosum (Mertens) Bory. A single egg mass from one female is made up of an average of 140 egg capsules, but most egg masses are produced by many females and contain several thousand spherical capsules. Most capsules contain ≈ 2700 eggs but those covering the exterior of the mass (≈ 10% of the total capsules) contain few or no eggs. In late autumn through winter, after 5–8 months of development, ≈30 juveniles measuring ≈3 mm in shell length emerge from each capsule. Thus, not counting losses due to urchin predation and to detachment during storms, ≈ 3700 juveniles are produced by a single egg mass from one female.

Growth and survival of the tropical scallop Lyropecten (Nodipecten) nodosus maintained in suspended culture at three depths

Abstract We examined growth and survival of juvenile Lyropecten (Nodipecten) nodosus, measuring 9.4 mm in shell height, which were placed in mid-December 1993 in pearl nets at 8, 21 and 34 m in depth at Turpialito in the Golfo de Cariaco, Venezuela. The mean growth rate in shell height during the first 5 months varied inversely with depth (10.0, 8.5 and 5.0 mm mon−1 at 8, 21 and 34 m in depth, respectively). Somatic tissues also showed a progressively slower growth rate with increasing depth. The decreased growth rate with depth was not associated with the mass of total seston or its organic content but were proportional to the decrease in phytoplankton biomass with depth (chlorophyll a decreased from 4.8 to 0.7 μg l−1 between 8 and 34 m). Temperature also decreased with depth but the differences were likely too slight to account for differences in growth. Mortality varied markedly with depth. There was a sharp increase in mortality in July at 34 m and a total mortality in August at 8 m. In contrast, at 21 m survival was high throughout the study. After 5 months (in May), wet muscle mass of the scallops at 8 m attained the commercial size (5–6 g). A possible culture strategy for L. nodosus is to initially grow the scallops at 8 m in depth, where growth is greatest, and then transfer them to 21 m, where survival is greatest and where growth will continue at a moderate rate to a larger size.

Movement of whelks (Buccinum undatum) towards a baited trap

The movement of whelks, Buccinum undatum L., towards a food source was examined by releasing a large number of tagged individuals at 6 distances in each of four directions from a baited trap and recording their recovery in the trap. Eight such experiments were performed under a variety of conditions. The peak recapture of individuals released downstream at 2, 5, 10, 18, and 30 m from a baited trap was at 6 h, 6 h, 1 d, 2 d and 3 d, respectively. The area of attraction of a trap, the surface over which the whelks perceive and are drawn towards the bait, is usually irregularly shaped and determined mainly by current direction. In strong directional current it is elongated in the direction of the current and its size is much reduced. The area of attraction is also less at 20 m than at 10 m in depth. The size of the area of attraction varies seasonally due to behavioural changes probably associated with temperature and the whelks reproduction. Isopleths of 25% capture of tagged whelks usually expand and become more rounded with time, probably reflecting the entry of whelks from outside the area of attraction.