Swimming Species Research Articles

Surface Sensilla on the Predaceous Fresh-Water Leech Erpobdella montezuma: Possible Importance in Feeding

Surface Sensilla on the Predaceous Fresh-Water Leech Erpobdella montezuma: Possible Importance in Feeding

A Quantitative Comparison of Two Zooplankton Sampling Methods, a Plankton Trap and a Towed Net, in the Baltic

AbstractA comparison between a large plankton trap with a capacity of 23 litres and a modern towed net was made in the course of zooplankton production studies in the Baltic. On the average, the net efficiency was 75% of that of the trap. Both methods were equally efficient in catching naupliar stages of copepods. The net efficiency was especially low during the zooplankton maximum for adult copepods and cladocerans (41 and 51%, respectively), probably due to the active avoidance of the net by fast swimming species. The net was only about 66% as efficient as the trap for catching rotifers. This difference is probably caused by the loss of small and softbodied forms through the mesh. Only about 65% of the total zooplankton biomass retained in the trap was collected by the net. Net sampling is not recommended for quantitative zooplankton studies.

Movement and Microdistribution of Sida Crystallina and Other Littoral Microcrustacea

Littoral invertebrates of Cochran Lake, Michigan were either highly plant—associated, and found infrequently away from aquatic macrophytes, or free—swimming among the plants. Plant—associated taxa were distributed according to available surface area. Numbers of periphyton and detritus feeders, chiefly chydorid Cladocera and chironomid midge larvae, were highly correlated with diatom density, whereas filter—feeding Cladocera attached to the plants (Sida, Simocephalus) were not. Cyclopoid copepods characteristic of the littoral during the day (Macrocyclops, Eucyclops, and Microcyclops) were also found primarily on aquatic plant surfaces. Abundances of the chydorid Cladocera increased slightly at night, while other taxa showed little diurnal variation. Free—swimming species in the littoral (Bosmina, Ceriodaphnia) sometimes formed large patches covering many square metres. Large species (Mesocyclops, Diaptomus, Epischura) greatly increased the numbers and diversity of this community at night. Sida crystallina, a filter feeder typically attached to plant surfaces, nonetheless swims frequently and well if disturbed. Juveniles moved between substrates more frequently than adults, and were found nearer the surface. Sites for reattachment were located visually. Frequent and often extensive episodes of swimming, especially by juveniles, provided a plausible explanation for the high concentrations of Sida observed on isolated plants, particularly those on the margins of stands of aquatic vegetation.

The ecology of Ruppia-dominated communities in Western Europe. II. Synecological classification. Structure and dynamics of the macroflora and macrofauna communities

In this paper the structure and dynamics of macroflora and macrofauna communities dominated by the genus Ruppia are treated. The aquatic plant communities were classified according to the methods of the Zürich-Montpellier School. In the mediterranean region as well as in N.W. Europe, two plant associations were distinguished, one dominated by Ruppia maritima L., the other by R. cirrhosa (Petagna) Grande. In the mediterranean region the communities in temporary ponds and marshes were classified in the Rupitum brevirostris, that could be subdivided into three subassociations occurring under different salinity conditions, whereas the communities in the permanent ponds and in the extensive lagoons were classified in the Cladophoro-Ruppietum cirrhosae, in which four subassociations were distinguished. In N.W. Europe, two subassociations of the Ruppietum maritimae and two of the Ruppietum cirrhosae have a local distribution along the coasts of the inner Baltic, whereas a number of other subassociations of both the Ruppietum maritimae and the Ruppietum cirrhosae are more generally distributed in ponds and lakes along the N.W. European coasts.The structural features of the plant communities are described in detail. The parvopotamid growth form proved to strongly dominate in nearly all Ruppia stands. Hence, horizontal and vertical spatial patterns are remarkably uniform in the communities studied. Periodicity, long-term fluctuations and succession in the plant beds are treated extensively; as these temporal patterns are strongly influenced by competition between the most important plant species, much attention was paid to such competitive relationships. The competition between Ruppia cirrhosa and Potamogeton pectinatus L. is crucial in the extensive mesohaline lagoons. R. cirrhosa completely ousts P. pectinatus at mean salinities above 9% Cl−, the opposite happens at mean salinities below 4% Cl− and mixed stands of both species occur at intermediate salinities.The species composition of the macrofauna in the Ruppia beds and their structural relation to the plants were investigated. Of the 139 species encountered, only 30 were generally abundant in the majority of the localities studied. These were mainly ubiquitous ‘true’ brackish-water species. The Ruppia-dominated communities were classified according to their macrofauna composition and the result was compared with the phytosociological classification, resulting in a combined classification of biocoenoses.The Ruppia stands generally proved to be poor in macrofauna species. This is probably related to the general poverty of macrofauna species in the N.W. European brackish waters. The macrofauna communities in the Ruppia stands in the Finnish Baltic are slightly richer in species than those in The Netherlands and in the Camargue, S. France, due to the fact that the latter are isolated. With respect to the structural relations between the macrofauna and the plants, it must be pointed out that ca. 60 species were found predominantly in the aquatic vegetation, but most of them also occurred on other (hard) substrates. Thirteen species were ‘attached epiphytes’, 29 ‘creeping epiphytes’, four egg-depositors and 15 canopy-dwellers; ca. 45 species did not show any structural relation to the vegetation (18 benthic infauna species, six creeping and 22 swimming species).A quantitative sampling programme was carried out in the Ruppia habitats studied that provided information on the numbers and biomass of the macrofauna species in the plant beds. Total fauna numbers ranged from 2000–44 000 ind./m2 and total biomass from 1.1–22.9 g ash-free dry weight/m2. Species diversity and evenness were calculated; H′ was in the range 0.13–2.12, and J′ ranged from 0.15 to 0.76. The macrofauna communities in the Ruppia beds are characterized by a strong dominance of only a few species (e.g. Nereis diversicolor O.F. Müll, Chironomus salinarius Kieffer, Tubifex costatus Claparède, Hydrobia spp.). Consequently, they have a low species diversity compared to marine and freshwater aquatic plant-dominated communities. The possible causes for this low diversity are discussed.

Scale‐eating habits of the marine teleost Terapon jarbua (Forskål)

Terapon jarbua, a marine teleost attaining a length of 25 cm, is abundant in the estuaries of the tropical and subtropical Indian Ocean. Fish scales removed from living fish are the most important food and represent the utilization of a niche previously unrecorded in the marine environment. Laboratory experiments show that T. jarbua can digest fish scales which have a relatively high energy content of 2cal mg−1 and further that the scales are removed only from living fish and not collected off the bottom or from dead fish. The scales are taken under natural conditions from fish larger than T. jarbua and slow swimming species are favoured. T. jarbua always attacks the lateral surface of the prey species and scales are removed mainly from the posterior region of the body, particularly around the caudal peduncle. A variety of species are attacked. The black longitudinal stripes of T. jarbua fade during feeding but in the presence of conspecifics darken. This colour pattern, which is unique among south east African estuarine fish, together with the small size of T. jarbua scales, may prevent autopredation. It is suggested that the scale‐eating habit of T. jarbua arose as a modified form of predation or as an extension of ‘cleaning’ ectoparasites from the scales of larger fish. Ectoparasites are relatively common in the diet of T. jarbua.

ON THE RESPIRATION IN SCALLOPS (LAMELLIBRANCHIATA)

1. The oxygen uptake and the percentage of oxygen withdrawn from the inhaled water, were determined in resting specimens of two species of lamellibranch molluscs which have a capacity for rapid swimming locomotion, vis. in the sea scallop (Pecten grandis Sol.) and in the bay scallop (Pecten irradians Lam.).2. In accordance with the absence of prolonged ventilation pauses common in several other species of Lamellibranchia, the oxygen uptake in the bay and sea scallop is quite uniform. In both species QO2 was found to be about 70 ml./kg./hr. at 20° C. This value is about half the QO2-value found in swimming species from the mediterranean and falls well within the range of QO2-values in boreal non-swimming types of lamellibranchs recorded in the literature.3. The oxygen uptake was independent of the oxygen tension down to a concentration of oxygen of about 1-½ ml. O2/L.4. The percentage of oxygen withdrawn from the inhaled water, in most cases, was low, ranging, approximately, from ½ to 13%. It was demonstrated that in the stream of water escaping from the cloacal chamber of the bay scallop considerable gradients in the concentration of oxygen occur.

THE RESPIRATORY METABOLISM OF TISSUES OF MARINE TELEOSTS IN RELATION TO ACTIVITY AND BODY SIZE

1. Determinations were made of the Qo2 of brain, muscle and liver of three species of marine fishes representing different ecological habitats; a very active constantly swimming species, menhaden; a sluggish bottom-dweller, toadfish; and an intermediate form, scup.2. Although a direct correlation between Qo2 of brain and activity of the whole organism was noted, liver and muscle did not show any correlation with activity. The possible significance of this relationship was discussed.3. A slight increase in Qo2 of brain and muscle of toadfish and brain of scup with increasing body size was noted. The Qo2 of toadfish liver decreased with body size.