Browsing by Author "OJEDA, FP"
Now showing 1 - 14 of 14
Results Per Page
Sort Options
- ItemCEPHALIC ANATOMY OF THE HERBIVOROUS FISH GIRELLA-LAEVIFRONS (OSTEICHTHYES, KYPHOSIDAE) - MECHANICAL CONSIDERATIONS OF ITS TROPHIC FUNCTION(1990) VIAL, CI; OJEDA, FPStructural and functional restrictions hindering the development of efficient mechanism for cropping and ingestion of algal material could constitute an explanatory hypothesis for the observed low diversity of herbivorous fishes. In this study, the cephalic structures of the herbivorous kyphosid fish Girella laevifrons (Tschudi 1844) are described, including a functional analysis of its alimentary apparatus. This species can be characterized by having: (i) a great freedom of movement between the dentary and articular-angular, (ii) a double insertion of the muscle adductor mandibulae Al in both faces of the maxilla, (iii) specialized teeth, long and flexible with a peculiar articular condyle at their anterior base, (iv) a mobile articulation between ceratohyal and dorsal hypohyal, and (v) a well developed branchiostegal system. A mechanism based on a mandibular manipulation associated with a slow suction of a large volume of water is proposed. This mechanism would allow the ingestion of large amounts of algae (as long stripes) in a continuous fashion. The combined activity of both an oral manipulation and buccal suction systems are possible by the existence of a decoupling of the anterior and posterior regions of the buccal cavity involving the jaw and the hyoid arch. The adaptive value of this mechanism is discussed in relation of the feeding strategies utilized by this species.
- ItemCOMPARATIVE-ANALYSIS OF THE HEAD MORPHOLOGY OF PACIFIC TEMPERATE KYPHOSID FISHES - A MORPHOFUNCTIONAL APPROACH TO PREY-CAPTURE MECHANISMS(1992) VIAL, CI; OJEDA, FPVial & Ojeda (1990) proposed that the low diversity of herbivorous fishes observed among Percoids might be owing to the existence of morpho-functional restrictions of the feeding apparatus that hinder the acquisition of efficient mechanisms for cropping and ingesting plant material. The morphological and functional evidence presented therein for the herbivorous kyphoid Girella laevifrons indicates that its morphological specializations are related to structural decouplings of the anterior and posterior regions of the buccal cavity involving the jaw and the hyoid arch. Functionally, these decouplings allow the ingestion of algae in a continuous fashion at low energetic cost. In this study, we evaluate the general validity of the proposed model by means of a morphological and functional comparative analysis of the cephalic structures of the four most abundant Kyphosid species of the Chilean coast: the girellids Girella laevifrons (Tschudi), Girella albostriata Steindachner, and Graus nigra Philippi, and the scorpid Scorpis chilensis Guichenot. We found great osteological and myological similarities between the two strict herbivores, G. albostriata and G. laevifrons, thus suggesting that the mechanism previously described for the latter species corresponds very closely to that of the former. This mechanism provides for mandibular manipulation carried out independently by the upper jaw and for a decoupling of the lower jaw into two mechanical units. Graus and Scorpis display a somewhat different morphological design. Although both maintain the intrahyoid decoupling shared by all kyphosid species, they do not present mechanical independence between the upper and lower jaw, nor a capacity for intramandibular movements. In Graus, teeth are well developed, conical, and slightly curved toward the interior. These characteristics make possible a trophic mechanism primarily based on a strong biting especially well suited for carnivory. The structural plan of Scorpis appears more distant from species in the other two genera, althoug it shares with them characters such as the double tendon of the A1 muscle in the maxilla, the intrahyoid articulation, the type of teeth, their replacement system, and the general design of the suspensorium. The particular morphology of the adductor muscles, and the mandibular design of Scorpis suggest a trophic mechanism mainly based on a rapid suction. From a functional viewpoint, the structural differences encountered in the four species analyzed may be understood as modifications related to their different mechanisms of prey capture.
- ItemDIGESTIVE MECHANISMS IN APLODACTYLUS PUNCTATUS (VALENCIENNES) - A TEMPERATE MARINE HERBIVOROUS FISH(1995) OJEDA, FP; CACERES, CWThe herbivore Aplodactylus punctatus (Valenciennes) is one of the most abundant fish species inhabiting the rocky subtidal coast of central Chile. To determine the mechanisms of algal digestion in this species, we investigated the pH pattern along the digestive tract, and its relation to feeding frequency, the occurrence of enzymes capable of hydrolizing cellulose and other related polymers of the algal cell wall, and the distribution and activity of digestive enzymes. Specimens of A. punctatus were collected in 1989 and 1992 from 2 localities of the central Chilean coast. Measurements of pH and enzymatic assays were carried out on samples of the contents of the stomach, and of the anterior, middle, and posterior portions of the intestine. Both the diet and the gastrointestinal pH pattern showed no differences between day and night. The stomach was highly acidic (pH 2.1 to 2.6), whereas the intestine was slightly alkaline (pH 6.6 to 7.8). No enzymatic activity was detected along the digestive tract for the assayed carragenans, agar, and alginates. Cellulase and amylase were found in the intestine but were not present in the stomach. Both enzymes showed the greatest activity in the anterior intestine. Protease (probably pepsin) was mainly found in the stomach. The increased acidity of the stomach, and the cellulolytic activity of the intestine - likely due to microorganisms - strongly suggests that a combination of acid hydrolysis and enzymatic digestion of algal cell wall, followed by digestion of carbohydrates and proteins, would explain the high assimilation rates of plant material reported for this species.
- ItemDISTRIBUTION PATTERNS OF TETRAPYGUS-NIGER (ECHINODERMATA, ECHINOIDEA) OFF THE CENTRAL CHILEAN COAST(1993) RODRIGUEZ, SR; OJEDA, FPWe investigated spatial distribution and temporal occurrence patterns of Tetrapygus niger in the subtidal zone off the central Chilean coast from March to November 1990. The shallowest portion of the subtidal zone and the shallowest edge of the kelp forest of Lessonia trabeculata appeared to be important recruitment zones for this species. We found a significant number of recruits along the bed border, and a marked decrease of urchin abundance toward the center of the kelp. Data obtained in September and November outside the kelp bed showed juvenile urchins [i.e. <24 mm test diameter (TD)] strongly associated with crevices. Size-frequency distributions at 2 m depth for those months also showed a large trough of intermediate-sized individuals (i.e. 15 to 30 mm TD). Temporal analysis of size-frequency distributions of individuals collected outside the kelp showed a relatively slow shift of modes between March and September and a major modal shift from September to November. Density values of urchins found in November were relatively low; however, the individuals appeared aggregated.
- ItemECOLOGICAL DOMINANCE OF LESSONIA-NIGRESCENS (PHAEOPHYTA) IN CENTRAL CHILE(1984) OJEDA, FP; SANTELICES, BExperimental removal of various types of dominant canopies has resulted in predictable patterns of competitive hierarchies among algae in intertidal and subtidal communities. Few attempts were made to distinguish functionally different groups of algae in these communities. The intertidal/subtidal boundary of exposed rocky habitats in central Chile is numerically dominated by the kelp L. nigrescens. When the kelp is absent, 2 other algal formations can be found at this intertidal level. In vegetational openings surrounded by individuals of L. nigrescens and in the absence of large grazers, large patches of Geldium chilense occur. In vegetational openings with abundance of grazers, patches of calcareous algae and bare rock are normally found. Through experimental manipulation the ecological changes of the understory algal assemblages following removal of the numerically dominant L. nigrescens were evaluated and with minor differences, the 3 functional groups of algae distinguished elsewhere (canopy forming, fugitive and obligated understory) also occur here. By experimental removal at different seasons and in presence or absence of sea urchins the occurrence of the 2 algal formations found in areas devoid of kelp is explained.
- ItemINVERTEBRATE COMMUNITIES IN HOLDFASTS OF THE KELP MACROCYSTIS-PYRIFERA FROM SOUTHERN CHILE(1984) OJEDA, FP; SANTELICES, BInvertebrates (42 taxa) and fish (1 sp.) were found in holdfasts of M. pyrifera in the giant kelp forest in Puerto Toro, Navarino Island, southern Chile. Maximum values of invertebrate density and biomass occur in late wainter and spring; higher values of species richness and diversity (H'') are found in winter. Quantitative data indicate that 4 spp. (Pseudechinus magellanicus, Pagurus forceps, Halicarcinus planatus and Anasterias antarctica) are the determinant taxa in the overall changes of invertebrate biomass and density. Data on body size and average weight of these species indicate that neither growth nor juvenile recruitment are responsible for the overall pattern of change. Migration behavior of these species is a more likely explanation. The rate of accumulation of species of invertebrates to the holdfast is fast in small holdfasts, very slow in larger holdfasts, and the pioneer species are also found in larger and older holdfasts. Thus, the dynamic of species turn over does not follow a species replacement series typical of colonization processes described for nongrowing habitats.
- ItemONTOGENIC CHANGE IN THE DIET OF APLODACTYLUS-PUNCTATUS (PISCES, APLODACTYLIDAE) - AN ECOPHYSIOLOGICAL EXPLANATION(1994) BENAVIDES, AG; CANCINO, JM; OJEDA, FPAplodactylus punctatus is a temperate herbivorous fish that changes from an omnivorous to a herbivorous diet and increases its ability to assimilate algae as it grows. To investigate whether this dietary shift is related to size-specific differences in energetic demands imposed by metabolism and the amount of assimilated energy, oxygen consumption (V(O2)) was determined experimentally in 12 specimens ranging in size from 62 to 545 g. V(O2) increased allometrically with body size from 8.41 to 55.95 mg O2 individual-1 h-1. Individual energetic requirements were 2.8 to 33.7 kJ d-1. The assimilated energy was estimated, taking into consideration: (1) the energetic value of the most important alga in the diet (Lessonia trabeculata); (2) size-specific differences in assimilation rates for fish fed on this alga; (3) size-specific differences in throughput time and in the amount of food in a full gut. Comparison of the energy required and the assimilated energy revealed that fishes of < 22 to 29 cm total length had a negative energetic balance when consuming algae exclusively. This may explain the reliance of small A. punctatus on more easily-digested invertebrates. The largest individuals can meet their energetic demands by consuming algae alone, apparently because of their higher assimilation capability. In A. punctatus, changing energetic requirements and capacities for algal assimilation may be responsible for the observed ontogenetic change in diet.
- ItemONTOGENIC CHANGES IN GUT DIMENSIONS AND MACROALGAL DIGESTIBILITY IN THE MARINE HERBIVOROUS FISH, APLODACTYLUS-PUNCTATUS(1994) BENAVIDES, AG; CANCINO, JM; OJEDA, FP1. Even though herbivorous fish have longer digestive tracts than carnivorous fish, a fact which agrees with optimal digestion theory, the existence of a relation between relative length of the digestive tract and capacity to digest and assimilate algal material has not been experimentally demonstrated. 2. We tested whether an increase in gut dimensions during ontogeny was associated with an increase in capability of fish to digest macroalgae in Aplodactylus punctatus. 3. Total gut length of this temperate marine fish increased from 43 to 237 cm as body length increased from 22 to 42 cm total length (TL). 4. Macroalgae, mainly fronds of the brown kelp Lessonia trabeculata, were the principal item found in their guts. 5. Individuals smaller than 30 cm had a higher proportion of invertebrate biomass in their gut than larger fish (40.6% and 0.3% of total content, respectively). 6. Transit time for Lessonia fronds increased as a function of body size, ranging between 20 h in fish 26.6 cm TL and 52 h in 40.0 cm fish. 7. Apparent digestibility of organic matter and total nitrogen increased as a function of body size from 38% to 80% when feeding on a Lessonia diet, and from 57% to 96% on a Ulva sp. diet. 8. These results explain why small herbivorous fish need to consume animal prey to satisfy their high mass-specific protein demands. 9. This study constitutes the first experimental demonstration in fish that an increase in relative gut length is associated with an increase in the capability to digest macroalgae. This allows larger fish to meet their energetic demands by consuming algae owing to their capability to digest low-quality food.
- ItemOPTIMAL FEEDING STRATEGY OF THE TEMPERATE HERBIVOROUS FISH APLODACTYLUS-PUNCTATUS - THE EFFECTS OF FOOD AVAILABILITY ON DIGESTIVE AND REPRODUCTIVE PATTERNS(1994) CACERES, CW; FUENTES, LS; OJEDA, FPIn this study we examined differences in feeding behavior of populations of the marine temperate herbivorous fish Aplodactylus punctatus, in three different localities off the Chilean coast, which differ qualitatively and quantitatively in food availability. We test whether food selection follows optimal foraging strategies, whether there is any modification of the fishes' digestive tracts in relation to their diets, and whether differences in diet quality affect the allocation of energy into reproduction and maintenance in these populations. Samples of this fish and of the understore algal assemblages were taken seasonally from May 1989 to February 1990. For each population we analyzed dietary composition, weight of the digestive tract and of the food content, the condition factor (K), and the gonadosomatic index (GSI). Our results showed that the diet observed in the three populations closely resembled the differences in macroalgal abundance and composition among the three localities studied. Local differences in diet quality were inversely related to the amount of food consumption and size of the digestive tract, suggesting that under differential conditions of food availability A. punctatus is able to compensate for variations in food quality through a flexibility in its digestive strategies. The nutritional status (K) of individuals, and their reproductive pattern (GSI) were directly related to diet quality. These results indicate that although A. punctatus is able to adjust its digestive processes to different algal food regimes, the digestive modifications observed in food-poor environments are not sufficient to compensate for the lack of food and allow fish to reach the nutritional status and reproductive output reached in a food-rich environment. This study represents the first natural experiment demonstrating a direct relationship among food availability, feeding patterns, digestive processes, and reproductive effort.
- ItemPOPULATION-DYNAMICS OF COASTAL FORESTS OF MACROCYSTIS-PYRIFERA IN PUERTO TORO, ISLA NAVARINO, SOUTHERN CHILE(1984) SANTELICES, B; OJEDA, FP
- ItemRECRUITMENT, GROWTH AND SURVIVAL OF LESSONIA-NIGRESCENS (PHAEOPHYTA) AT VARIOUS TIDAL LEVELS IN EXPOSED HABITATS OF CENTRAL CHILE(1984) SANTELICES, B; OJEDA, FPMortality effects of ecological factors were not generally tested in field population studies of benthic algae. Recruitment, growth and survival of the low-intertidal/shallow-subtidal L. nigrescens were measured at 4 tidal levels in exposed rocky habitats in central Chile. Through experimental manipulation the effects of the abiotic environment, small-size grazers and mid-littoral algae on the survival of the juveniles of L. nigrescens recruited in the upper part of its vertical range were evaluated. Disturbance effects of adult plants and grazing effects of large-sized subtidal herbivores on juvenile recruitment, and effects of interspecific interference on the survival of newly settled juveniles were also evaluated. Interference by adult plants and grazing by large-sized subtidal herbivores completely inhibit recruitment. In their absence, the abiotic environment limits recruitment and growth of L. nigrescens at the uppermost tidal level while inter-specific interference owing to over-shading by the fastest growing individuals limits the lower-most extent of the belt. Interference by adult plants and grazing are patchy and often mutually exclusive in the field. Recruitment within the band of L. nigrescens is maximal in patchy vegetational openings which are large enough to experience reduced disturbance by adult plants and small enough for reduced grazing pressure. In mature, consolidated belts of L. nigrescens on vertical walls these openings correspond to roughly triangular areas with the hypotenuse formed by interholdfast distances of 1-2 m. Such patchy vegetation openings can be produced normally by dislodgement of old, eroded plants.
- ItemSCALE-DEPENDENT VARIABILITY OF DENSITY ESTIMATES AND MORPHOMETRIC RELATIONSHIPS IN SUBTIDAL STANDS OF THE KELP LESSONIA-TRABECULATA IN NORTHERN AND CENTRAL CHILE(1992) CAMUS, PA; OJEDA, FPSeveral morphological parameters, and estimates of density and evenness per depth, were analyzed for stands of the subtidal kelp Lessonia trabeculata Villouta et Santelices from 2 zones, central and northern Chile. Two sites in each zone were sampled for 2 yr, and variability patterns were statistically described using site or zone as classification factors. Bathymetric profiles of density showed a strong qualitative and quantitative variation among sites, contrasting with the high evenness in spatial distribution of plants per depth interval, despite intervals of varying length. Relationships among morphometric parameters changed markedly across spatial scales, producing a high variance in biomass predictions. In most cases, strong site- and zone-specific components were identified, which account for the variability in morphology at both spatial scales. Zone effects may mirror water motion and temperature regimes, among other factors, and site effects may be related to specific habitat configurations. These results show that extrapolation from structural features of single stands for characterizing species may result in misleading inferences. The detection of scale-dependent patterns proved to be a useful starting point for comparative studies of population structure, and necessary before attempting generalizations.