Determinants of territory size in the pomacentrid reef fish,Parma victoriae

Summary Factors governing the size of territories defended by the pomacentrid reef fish, Parma victoriae, were investigated, prompted by contradictory predictions in the literature concerning the effects of food supply and competitors. Observations were carried out over the non-breeding period (March–October) on a medium density population in which territories were partially contiguous. The territory size of adult fish varied between 3 and 26 m², and was inversely correlated with local densities of conspecifics. The same range in territory size was found for both males and females, which did not differ in the time they spent on territory defence and foraging activities. No correlation existed between territory size and the abundance of algal food, body size, age or time spent on territory defence. Also, there was little variation in territory size over time, despite seasonal changes in the abundance of food algae.Experimental reduction of food supplies on isolated territories of males and females had no effect on territory size. In a higher density habitat an experiment was carried out in which population density and food abundance were simultaneously manipulated. This showed that territory size was primarily determined by intraspecific interactions, as territories exhibited considerable increases in size upon removal of neighbours. No changes in the size of defended areas resulted from either artificial increases or decreases of food levels. There were also no changes in the time spent on defence of territories, foraging time or feeding rates associated with food manipulations or territory expansion, which suggested that food was not a limited resource. This conflicted with current theories proposed to explain territory defence and expansion. It is hypothesized that intraspecific interactions constrain territory size well below the optimum in terms of the abundance of preferred food algal species.     

Territorial behavior of threespot damselfish (Eupomacentrus planifrons) increases reef algal biomass and productivity

Synopsis Algal growth and damselfish (Eupomacentrus planifrons) territories were studied in two reef habitats at Discovery Bay, Jamaica. Damselfish territories were contiguous in the reef flat (0 to 2.5 m), where the algal composition and biomass varied from territory to territory. In contrast, on the lower reef terrace (22 m), damselfish territories were often spatially segregated. While the algal composition of the territories was more uniform on the reef terrace, the total algal biomass was lower than in the territories on the reef flat. Damselfish are largely herbivorous, and they defend their territories against most intruding fish, including a number of herbivorous species. Areas of the reef terrace outside of damselfish territories were heavily grazed by herbivorous fishes and contained only small quantities of non-crustose algae.The reef terrace territories were characterized by a multispecific turf of algae (greens, blue-greens, and reds) covering the Acropora cervicornis framework and by the leafy, brown alga, Lobophora variegata. A rapid reduction in the biomass of brown algae and filamentous algae was noted when damselfish were permanently removed from their territories. Only calcified, encrusting algae — plants apparently somewhat undesirable as fish food sources — would be common on the terrace zone of this reef if damselfish territories were absent. Damselfish territoriality may significantly influence the dynamics of some reefs by increasing the biomass of the algal turf thereby increasing; reef productivity. Since blue-green algae, potential nitrogen fixers, occur in these algal turfs, the fish may also be indirectly affecting reef nutrition.     

Group foraging by the herbivorous cichlid fish,Petrochromis fasciolatus,in Lake Tanganyika

Group foraging of the algae eating cichlid,Petrochromis fasciolatus, was studied along a rocky shore in Lake Tanganyika, in an area occupied by the feeding territories of several herbivorous cichlid species. SolitaryP. fasciolatus or schools comprising a few members only were nearly always repelled from such territories, although larger schools (40–250 individuals) ofP. fasciolatus were able to overwhelm the defenses of territory owners and enable subsequent foraging. The frequency and duration of individual foraging sessions on the algal mats were positively correlated with group size, probably owing to individuals in large schools suffering fewer attacks from territory owners. Almost all of the foraging sites ofP. fasciolatus were inside the territories of other herbivorous cichlids, especially those ofNeolamprologus moorii (more than 80% of total), in which the algal density was more than 10 times that in territories of the other species.N. moorii territories included only 4% rocky substrate, indicating that schools ofP. fasciolatus selectively chose territories containing rich resources.     

Intertidal territoriality and time-budget of the surgeonfish, Acanthurus lineatus, in American Samoa

Synopsis The herbivorous surgeonfish Acanthurus lineatus aggressively maintained feeding territories in the surf zone of the outer reef flat in American Samoa. Intertidal territories were re-established each morning, as well as after displacement by low tides or rough surf. Day-to-day site fidelity of recognizable individuals was high: 99.9% return rate per day for adults (15–20 cm), 99.6% for juveniles (8–13 cm), and 97.2% for recruits (2.5–5 cm). Fish fed on turf algae primarily in the afternoon (80% of available time), and spent 10% of their time on active territorial defense and 2–13% of their time on forays from their territory. On average, a fish defended its territory 1900 times daily and took 17 000 bites (= 7400 bites m^–2 d^–1), but rough surf reduced feeding by 60% and defense by 75%. High territorial defense requirements significantly reduced feeding rates. Although the distribution and behavior of this species in Samoa was in large part similar to that reported for it elsewhere (Australia, Indian Ocean), there were notable differences: in Samoa A. lineatus densities within colonies were greater (0.4 fish m^–2), territory size was smaller (2.3 m²), and defense rate against intruders was greater (2.5 attacks min^–1). These differences in Samoa may be related to their smaller body size, greater abundance or increased food supply caused by hurricane damage to reefs which has enhanced the algal turfs that A. lineatus feeds upon.     

Comparative feeding ecology of two herbivorous damselfishes (Pomacentridae: Teleostei) from the Gulf of California,Mexico

Two species of benthic damselfishes from the Gulf of California, Mexico, use contrasting behaviors when feeding on benthic algal communities. The small (±70 g) Cortez damselfish, Eupomacentrus rectifraenum (Gill, 1862), feeds selectively from a multi-species algal mat, eats fleshy red and green algae and ignores brown and calcareous algae. The giant blue damselfish, Microspathodon dorsalis (Gill, 1862), is a large (±450 g), lethargic, nonselective feeder which grazes on a near monoculture of a fleshy red alga, Polysiphonia sp. Feeding activity for both species is low in the morning peaks during late afternoon, and drops sharply as night approaches. Based on feeding rates, gut-filling times, and weights of gut contents, Cortez damselfish process six to eight full guts of food and giant blue damselfish three full guts of food per day. The algal mat exhibits high standing crops (291–618 g dry wt · m^?2) and low productivity, but the preferred food of the Cortez damselfish (Ulva) appears to colonize the mat frequently and grow rapidly. The Polysiphonia dominated community on giant blue damselfish territories exhibits low standing crops (23 g · m^?2) and high productivity (34–47 times that of the mat per gram algae). Even though the feeding behaviors and resources used by the two damselfishes differ, both species eat similar food (delicate red and green fleshy algae, and depend on rapid colonization and/or high productivity to maintain their primary foods in the grazed algal community.     

Feeding electivity of two epiphytic chironomids in a subtropical lake

Summary Feeding electivity of two epiphytic orthoclad chironomid species, Psectrocladius sp. and Thienemanniella cf. fusca, was investigated using gut content analysis of larvae collected from Typha stems in Lake Padgett, Florida. Electivity values, computed using Strauss's Food Selection Index, were strongly positive for Cosmarium and negative for Oedogonium and Bulbochaete. Diatoms either were avoided or grazed in proportion to their abundance in the environment. Patterns of electivity were similar for both chrionomid species during all seasons studied. Pairwise food preference experiments were conducted in the laboratory using the algal species, Cosmarium impressulum, Navicula pelliculosa, Selenastrum capricornuatum, Oedogonium cardiacum, and Synedra sp. Preferences were hierarchical and consistent for both larval genera. Preference ranking for the algae offered were CosmariumSynedra>Navicula>Oedogonium>Selenastrum. Larvae seemed to be feeding electively based upon algal genus, size, position in the epiphytic community, and possibly extracellular chemistry.     

Effect of competitor abundance on feeding territoriality in a grazing fish,the ayuPlecoglossus altivelis

The grazing fish, ayu,Plecoglossus altivelis Temminck & Schlegel, establishes feeding territorialiry during the young stage. The population density fluctuates from year to year by more than a hundredfold, but the determinant of territory size is less well known. The feeding territoriality of ayu was examined under simulated habitat conditions where fish density was manipulated and food resources were renewable. Fish competed for algae attached to the substrata and were divided into residents with territories, and floaters without territories. By experimental alteration of fish density the number of residents increased with density and rerritory size decreased with density. Floaters intruded into territories in a school to feed on algae, which induced overt aggression of the resident and reduced the productivity of algae growing there. Both the intruding frequency of floaters over territorial areas and their feeding pressure on algae increased at higher floater density. Floaters functioned to shift cost-benefit relationships for various territory sizes. They acted as food competitors to restrict territory size below a maximum through competitive interference. Although the growth rate of residents was inversely related to fish density, residents grew faster than floaters in each group. Under a given set of competitor abundances, economic defensibility determined territory size.     

Effect of food conditions on the bacterial feeding of Daphnia galeata

Feeding selectivities of Daphnia galeata were measured using various density combinations of ¹⁴C-labeled Chlamydomonas and ³H-labeled natural bacteria. In all food combinations, D. galeata preferred algae. Selectivity coefficient (bacterial clearance rate: algal clearance rate) was 0.35 at the level of incipient food limitation (1.5 × 10⁶ µm³ ml^–1).However, it increased up to 0.9 with increasing total food concentration. A simple mechanical sieving model alone cannot explain such a change in the selectivity coefficient. A mechanism of clogging suggestively explain the increase of bacterial ingestion efficiency by D. galeata, especially at high food concentrations. The change of selectivity coefficient related to food condition suggests that bacterial feeding by the Daphnia is incidental event associated with the algal feeding.     

Experimental analysis of resource sharing between herbivorous damselfish and blennies on the Great Barrier Reef

Competition theory suggests that species with very similar resource requirements should not be able to coexist when resources are limiting. However, on coral reefs, territorial herbivorous fishes which use apparently very similar resources often coexist without competitive exclusion. This study investigates patterns of resource use by damselfish Pomacentrus flavicauda Whitley and blennies, primarily Salarias fasciatus Bloch, living in rubble habitats on the Great Barrier Reef. These species feed mainly on turf algae and have overlapping territories and home ranges. I ask two questions:

• 1.(1) is there any evidence of resource partitioning between these species; and
• 2.(2) do they compete for food or space? It is highly unlikely that damselfish and blenny species partition food resources by eating different species of algae, since species are intermixed at a very small scale in the algal turf. Instead, differences in food use were looked for by counting the numbers of bites fish of each species took from different parts of the rubble habitat.

Little difference in food use was found between Pomacentrus flavicauda and Salarias fasciatus except that the former fed on plankton to some extent whilst the latter did not. A manipulation experiment was performed to assess whether blennies inhabiting damselfish territories competed with damselfish for algae. Damselfish territories were used as experimental units and two manipulations performed:

• 1.(1) S. fasciatus removal, and
• 2.(2)Pomacentrus flavicauda removal.

There were four replicates of each with four undisturbed controls. Territory sizes, feeding rates and attack rates on other fishes of P. flavicauda were measured before and after blenny removal. Blenny density and biomass were measured before and after P. flavicauda removal. None of these measures provided evidence of exploitation competition between these species or of changes in levels of interference competition by damselfish after blenny removal. Blenny densities and biomass did not change significantly after damselfish removal. Damselfish appear to tolerate blennies within their territories because they cannot economically exclude them. However, the intermittent availability of an alternative source of algae outside damselfish and blenny territories and home ranges may mean that algae are not normally a limiting resource.     

Territorial defense of an excess food supply by an algal grazing fish, ayu

The size of the territory defended by the grazing fish ayu, Plecoglossus altivelis, often increases with decreasing intruder pressure. This territorial extension occurs even after additional growth of the fish reaches a plateau due to a surplus supply of algal food. This raises questions about the ecological advantages of defending an apparently enlarged territory. Enclosures were used to carry out observations on fish maintained as: (i) individuals with algae attached to the whole bottom area (2.4m²); (ii) individuals with algae covering 0.8m²; and (iii) with three conspecifics (competitive use) with algae attached to 2.4m². In the competitive use enclosures, one (occasionally two) fish that exclusively foraged the area appeared in each of the groups. Algal resources were sufficiently renewable that no significant difference in growth rate was detected between the dominants of the competitive groups and the two types of solitary enclosures, indicating that the defended area supplied algal resources that exceeded the dominants ability to grow. All the territorial holders realized an increase in relative body weight, suggesting the occurrence of inhibition, which corresponds to Verners concept of superterritory. However, as inferred from the pheophytin/chlorophylla ratios, algae within a territory showed less senescence, perhaps owing to cropping by the dominant, and this was considered to be an adaptive response by the fish to an unpredictable food supply. In conclusion, the ayu conditionally defends a territory for long-term benefits, and inhibition is a consequence of facultative territory size and not a cause of it.     

Effects of territorial damselfish on an algal-dominated coastal coral reef

Territorial damselfish are important herbivores on coral reefs because they can occupy a large proportion of the substratum and modify the benthic community to promote the cover of food algae. However, on coastal coral reefs damselfish occupy habitats that are often dominated by unpalatable macroalgae. The aim of this study was to examine whether damselfish can maintain distinctive algal assemblages on a coastal reef that is seasonally dominated by Sargassum (Magnetic Island, Great Barrier Reef). Here, three abundant species (Pomacentrus tripunctatus, P. wardi and Stegastes apicalis) occupied up to 60% of the reef substrata. All three species promoted the abundance of food algae in their territories. The magnitudes of the effects varied among reef zones, but patterns were relatively stable over time. Damselfish appear to readily co-exist with large unpalatable macroalgae as they can use it as a substratum for promoting the growth of palatable epiphytes. Damselfish territories represent patches of increased epiphyte load on macroalgae, decreased sediment cover, and enhanced cover of palatable algal turf.     

Depth-related differences in territory size and defense in the herbivorous cichlid,neolamprologus moorii, in lake tanganyika

Neolamprologus moorii, a small herbivorous cichlid of Lake Tanganyika, defends its territory against food competitors. Territory size and territorial defense were compared between two sites at different depths. In the shallow site, more fishes intruded into the territories than in the deep site, owing to the higher density fish population in the former. The territory holders in the shallow site attacked intruders more often, although the ratio of attacks to number of intruders was similar to that of the deep site. The territories in the deep site were larger and contained a greater amount of algae than the shallow site. The larger territory size in the deep site may have resulted from the lower cost of territory defense due to the lower competitor density and from the need to maintain a greater amount of food resources to compensate for lower algal productivity.     

Trophic relationships of the sea ice meiofauna in Frobisher Bay,Arctic Canada

Summary A distinct fauna consisting mainly of nematodes, harpacticoid and cyclopoid copepods, rotifers, turbellarians and polychaete larvae, inhabits the lower levels of the sea ice in Frobisher Bay. Similar faunas are found throughout circumpolar regions. Thirteen taxa of the Frobisher Bay ice fauna were entirely herbivorous. Their food consisted of 26 genera of algae dominated by Chlamydomonas, Nitzschia, Navicula and Chaetoceros. There was a clear tendency to feed on the most abundant ice algae, hence little evidence of selective feeding. High algal food concentrations in the ice (estimated at 5000 g C/l) were in sharp contrast with the scant nourishment available from phytoplankton under the ice (8 g C/l) from mid-winter until the start of the summer bloom. Algal stocks and estimated productivity rates indicate that ice meiofaunal food requirements may be met by the ice algae. All the major ice meiofaunal species are well adapted to feeding within the ice. All are small enough to enter brine channels and secure particulate prey from surfaces within confined spaces. The ice meiofaunal species are major consumers of the ice algae and therefore important links in the transfer of energy from the ice to pelagic and benthic predators, including fishes, birds and mammals.     

Microhabitat characteristics of <Emphasis Type="Italic">Stegastes planifrons</Emphasis> and <Emphasis Type="Italic">S. adustus</Emphasis> territories

Stegastes adustus and Stegastes planifrons are two species of damselfishes commonly found in the Caribbean. These territorial fishes have been widely studied due to their major ecological role on coral reef in controlling the growth of macroalgae that compete with corals for space and, inversely, on their deleterious role in destroying coral tissues to impulse the development of algae. However, few studies were conducted on the biotic and abiotic components of their territories. In the present study, territory size and surfaces of benthic components (macroalgae, algal turf, massive corals, branching corals, Milleporidae, sponges, sand and rubbles) were estimated for the two species at two contrasted sites. At Ilet Pigeon site (IP), the two damselfishes were found at different depth and exhibited different territory sizes. S. adustus defended a larger territory characterized by massive corals, sand and Milleporidae, while S. planifrons territories were smaller, deeper and characterized by branching corals, sponges and rubble. At Passe-à-Colas site (PC), the two fish species coexisted in the same depth range and defended territories of similar size. Their territories presented higher proportions of macroalgae, but smaller surfaces of Milleporidae than at IP. At PC, the main difference between the two species was a higher surface of massive corals inside S. planifrons territories than S. adustus territories. Differences in microhabitat characteristics between the two Stegastes seemed mostly site related. This resulted from the high plasticity of two species, allowing them to persist on Caribbean coral reefs after the decline of most branching acroporids, their former favorite habitats.     

Development of ice biota in a temperate sea area (Gulf of Bothnia)

A study of sea ice biota was carried out in the Gulf of Bothnia (northern Baltic Sea) during the winter of 1989–1990. Samples (ice cores) were taken at a coastal station at regular time intervals during the ice season. Chlorophyll a concentration, algal species distribution, bacterial numbers, and primary and bacterial production were measured. Colonization of the ice began in January when daylight was low. As the available light increased, the algae started to grow exponentially. The vertical chlorophyll a distribution changed and algal species composition and biomass changed during the season. During the initial and middle phase of colonization, ice-specific diatoms, Nitzschia frigida and Navicula pelagica, dominated the algal biomass. Nutrients (PO₄ ^3– and NO_3j) were found to be depleted during the time of algal exponential growth. The maximum algal biomass exceeded 800 g C 1^–1. The primary production supplied food for heterotrophic organisms. The presence of heterotrophic organisms of different trophic levels (bacteria, flagellates, ciliates and rotifers) indicated an active microbial food web.     

Diet disparity among sympatric herbivorous cichlids in the same ecomorphs in Lake Tanganyika: amplicon pyrosequences on algal farms and stomach contents

Background

Lake Tanganyika, an ancient lake in the Great Rift Valley, is famous for the adaptive radiation of cichlids. Five tribes of the Cichlidae family have acquired herbivory, with five ecomorphs: grazers, browsers, scrapers, biters and scoopers. Sixteen species of the herbivorous cichlids coexist on a rocky littoral slope in the lake. Seven of them individually defend feeding territories against intruding herbivores to establish algal farms. We collected epiphyton from these territories at various depths and also gathered fish specimens. Algal and cyanobacteria community structures were analysed using the amplicon-metagenomic method.

Results

Based on 454-pyrosequencing of SSU rRNA gene sequences, we identified 300 phototrophic taxa, including 197 cyanobacteria, 57 bacillariophytes, and 31 chlorophytes. Algal farms differed significantly in their composition among cichlid species, even in the same ecomorph, due in part to their habitat-depth segregation. The algal species composition of the stomach contents and algal farms of each species differed, suggesting that cichlids selectively harvest their farms. The stomach contents were highly diverse, even between species in the same tribe, in the same feeding ecomorph.

Conclusions

In this study, the amplicon-metagenomic approach revealed food niche separation based on habitat-depth segregation among coexisting herbivorous cichlids in the same ecomorphs in Lake Tanganyika.

     

The feeding ecology of three species of Caribbean angelfishes (family Pomacanthidae)

Synopsis The foraging behavior and associated morphology of the feeding apparatus of three sympatric species of angelfishes, Holacanthus tricolor, Pomacanthus arcuatus and Pomacanthus paru were studied at St. Croix, U.S. Virgin Islands. All three had overlapping diets, consisting of algae and numerous species of sponges. The two Pomacanthus species also fed on gorgonians. The morphology of the dentition, jaws and gill rakers was similar in all three species. Male Holacanthus tricolor defended territories overlapping the foraging areas of two to four females. Within the male's territory, females defended smaller territories against other females of the same size, but tolerated smaller females. In contrast, both Pomacanthus spp. formed pairs which defended intraspecific feeding territories.     

Effects of UV-B irradiated algae on zooplankton grazing

We tested the effects of UV-B stressed algae on grazing rates of zooplankton. Four algal species (Chlamydomonas reinhardtii, Cryptomonas sp., Scenedesmus obliquus and Microcystis aeruginosa) were used as food and fed to three zooplankton species (Daphnia galeata, Bosmina longirostris and Brachionus calyciflorus), representing different taxonomic groups. The phytoplankton species were cultured under PAR conditions, and under PAR supplemented with UV-B radiation at two intensities (0.3 W m^–2 and 0.7 W m^–2, 6 hours per day). Ingestion and incorporation experiments were performed at two food levels (0.1 and 1.0 mg C l^–1) using radiotracer techniques. The effect of food concentration on ingestion and incorporation rate was significant for all three zooplankton species, but the effect of UV-B radiation was more complex. The reactions of the zooplankton species to UV-B stressed algae were different. UV-B stressed algae did not affect Daphnia grazing rates. For Bosmina the rates increased when feeding on UV-B stressed Microcystis and decreased when feeding on UV-B stressed Chlamydomonas, compared with non-stressed algae. Brachionus grazing rates were increased when feeding on UV-B stressed Cryptomonas and UV-B stressed Scenedesmus, and decreased when feeding on UV-B stressed Microcystis, compared with non-stressed algae. These results suggest that on a short time scale UV-B radiation may result in increased grazing rates of zooplankton, but also in decreased grazing rates. Long term effects of UV-B radiation on phytoplankton and zooplankton communities are therefore difficult to predict.     

Importance of algae in the diet of the oligochaetes Lumbriculus variegatus (MÜller) and Rhyacodrilus sodalis (Eisen)

Summary The importance of algae in the diet of the oligochaetes Lumbriculus variegatus and Rhyacodrilus sodalis was determined from cellections made in a eutrophic bay from April 1977 to April 1978. During the summer, algae represented 70–85% of the gut contents of both species. The most frequently ingested algae were Cymatopleura elliptica, Cymbella spp., Epithemia turgida, Pinnularia spp., and Synedra ulna. Almost all species were consumed in proportion to their abundance in the environment. However, Nostoc pruniforme and Nostoc verrucosum were not eaten due to size selection. The high level of algal consumption exhibited by the oligochaetes was primarily related to the large standing crop of algae on the sediments. During the winter, when algal densities were low in the environment, detritus and associated bacteria were the major food source.     

Spatial and Temporal Variability in Territoriality of a Tropical Benthic Damselfish on a Coral Reef (Réunion Island)

The relationship between length of individual Stegastes nigricans and size of the territory that they defend, and both the spatial and temporal variation in territoriality were investigated monthly on a fringing coral reef of Réunion Island over the course of one year. Length of individual S. nigricans was correlated positively with size of the territory. Most small individuals were found separate from adults, in areas with low algal turf cover. This is probably due to strong intra-specific competition for access to algal food resources and shelter availability, and/or differences in habitat preference. Both abundance of individuals and surface defended showed seasonal fluctuations. Densities were slightly higher in austral summer than in winter, and the surface defended by individuals larger than 8cm increased in winter. A permanent redistribution of territories also occured between individuals over time.