Marine Herbivorous Fish Research Articles

A fatty acyl desaturase (fads2) with dual Δ6 and Δ5 activities from the freshwater carnivorous striped snakehead Channa striata

There is a lack of understanding on how the environment and trophic niche affect the capability of long-chain polyunsaturated fatty acids (LC-PUFA) in freshwater carnivorous teleost. In this present study, we isolated and functionally characterised a fatty acyl desaturase (Fads) from the striped snakehead Channa striata. Sequence comparison and phylogenetic analysis suggested a Fads2 protein that is closely related to previously characterised Fads2 proteins from freshwater carnivorous and marine herbivorous fish species. We further demonstrated the capacity of Δ6 and Δ5 desaturation activities for this particular desaturase, with highest activities towards the conversion of omega-3 (n-3) polyunsaturated fatty acids (PUFA). Low Δ4 desaturation activity was also detected, although the significance of this at a physiological level remains to be studied. The expression of this striped snakehead Δ6/Δ5 fads2 gene was highest in brain, followed by liver and intestine. In liver, diet fortified with high LC-PUFA concentration impeded the expression of Δ6/Δ5 fads2 gene compared to vegetable oil (VO) based diets. The discovery of Δ6/Δ5 Fads2 desaturase here complements the previous discovery of a Δ4 Fads2 desaturase and an Elovl5 elongase, lending proof to the existence of all the required enzymatic machinery to biosynthesise LC-PUFA from C18 PUFA in a freshwater carnivorous species.

Temperate marine herbivorous fishes will likely do worse, not better, as waters warm up

Increased temperatures are associated with reduced body sizes, life spans, and reproductive outputs in shallow water marine fishes, reflecting the pervasive effects of temperature on metabolic rates in ectotherms. Herbivorous fishes have been seen as an exception to this trend, based on the hypothesis that physiological and demographic processes in these species are constrained by the inability to digest algae at low temperatures. It is thus argued that increased temperatures deliver a net benefit to herbivorous fishes. This study examines an alternative argument, that warming temperatures can have increasingly inimical effects on temperate piscine herbivores. We consider the hypothesis that herbivores experience greater oxidative stress at warmer temperatures, a consequence of temperature-related increases in metabolic rates. We use the age pigment lipofuscin to examine the rate of oxidative damage accumulation in populations of a temperate marine herbivorous fish, Odax pullus (Labridae), at different latitudes (temperatures) across New Zealand (175.3°E, 36.3°S–167.9°E, 47°S). We show a 55 % faster rate of oxidative damage accumulation in shorter-lived fish living at warmer latitudes. In these populations, it took 33–50 % fewer years to accumulate similar amounts of oxidative damage than in those living at colder latitudes, indicating greater oxidative stress in fish living at warmer temperatures. We conclude that at least some temperate piscine herbivores will be exposed to negative demographic impacts at their low-latitude range margins as temperatures increase.

Biotransformation of inorganic arsenic in a marine herbivorous fish Siganus fuscescens after dietborne exposure

Arsenic (As) is well known to be biodiminished along marine food chains. The marine herbivorous fish at a lower trophic level are expected to accumulate more As. However, little is known about how marine herbivorous fish biotransform the potential high As bioaccumulation. Therefore, the present study quantified the biotransformation of two inorganic As species (As(III) and As(V)) in a marine herbivorous fish Siganus fuscescens following dietborne exposure. The fish were fed on As contaminated artificial diets at nominal concentrations of 400 and 1500 μg As(III) or As(V) g−1 (dry weight) for 21 d and 42 d. After exposure, As concentrations in intestine, liver, and muscle tissues of rabbitfish increased significantly and were proportional to the inorganic As exposure concentrations. The present study demonstrated that both inorganic As(III) and As(V) in the dietborne phases were able to be biotransformed to the less toxic arsenobetaine (AsB) (63.3–91.3% in liver; 79.0%–95.2% in muscle). The processes of As biotransformation in rabbitfish could include oxidation of As(III) to As(V), reduction of As(V) to As(III), methylation to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), and subsequent conversion to AsB. These results also demonstrated that AsB synthesis processes were diverse facing different inorganic As species in different tissues. In summary, the present study elucidated that marine herbivorous fish had high ability to biotransform inorganic As to the organic forms (mainly AsB), resulting in high As bioaccumulation. Therefore, marine herbivorous fish could detoxify inorganic As in the natural environment.

Temperature‐related variation in growth rate, size, maturation and life span in a marine herbivorous fish over a latitudinal gradient

In ectotherms, growth rate, body size and maturation rate covary with temperature, with the direction and magnitude of variation predicted by the Temperature-Size Rule (TSR). Nutritional quality or availability of food, however, may vary over latitudinal gradients, resulting in ambiguous effects on body size and maturation rate. The Temperature-Constraint Hypothesis (TCH) predicts that marine herbivorous ectotherms are nutritionally compromised at latitudes exceeding 30°. This provides an opportunity to resolve the contrasting demographic responses of ectotherms to variation in temperature and nutritional status over latitudinal gradients. This study uses analysis of demographic rates to evaluate the predictions of the TSR in a marine herbivorous ectotherm sampled over a significant latitudinal gradient. The direction and magnitude of demographic variation was established in the marine herbivorous fish, Odax pullus (the butterfish), and compared with that of a phylogenetically related but trophically distinct species, the carnivorous Notolabrus fucicola (the banded wrasse). Both species were sampled at three locations across the length of New Zealand covering latitudes between 35°S and 49°S. Growth rate, mean size-at-age, age- and size-at-maturity, life span and abundance were estimated for each species at each location. Demographic traits of both taxa varied with latitude. Both species showed slower initial growth rates, and matured later at a larger body size at higher latitudes than populations sampled at lower latitudes. In addition, abundances increased significantly at higher latitudes in both species. These results were consistent with the TSR but not with the TCH, confirming that nutritional ecology (herbivore vs. carnivory) did not determine demographic patterns over a biologically significant latitudinal gradient. Results from this study suggest that the absence of herbivorous reef fishes from the higher latitudes of the Northern Hemisphere may not reflect a general physiological mechanism as suggested by the TCH and highlights the need to clarify the evolutionary histories of the marine biota of each hemisphere.

Herbivores in a small world: network theory highlights vulnerability in the function of herbivory on coral reefs

Summary The application of network theory within the field of ecology has predominantly been based around the examination of species interactions (e.g. food webs). Extension of networks to spatial ecology is currently limited, yet the approach has significant potential as a framework for analysing habitat connectivity and ecosystem stability. Examination of network structures characterizing the spatial dynamics of organisms has the potential to highlight the relative robustness or vulnerability of the ecosystem processes they support. On coral reefs, grazing by large herbivorous fishes is critical in maintaining the balance between coral and algae and is widely recognized as a key process in reef ecosystem functioning. However, the spatial dynamics exhibited by roving herbivores, and the implications of these dynamics for the overall stability and resilience of herbivory on reefs are not well understood. Here, we present a novel extension of network theory to the analysis of movements of marine herbivorous fishes within a reef system. Using an array of remote acoustic receivers, over a period of 12 months, we tracked the movements of fishes belonging to the three dominant species of roving herbivore within a particular coral reef community. Analysing these movements in terms of network graph theory, we found that individual fishes clustered their activities within small sections of the available linear reef habitat, and movements within and outside of these home ranges were made along predictable routes. All three species were heavily reliant on particular, well‐connected parts of the reef (nodes), mirroring an ‘ultra‐small‐world’ network. This underlying dynamic creates vulnerability in the system, making targeted removal by fishing or the degradation of individual pockets of reef habitat more likely to result in a complete collapse in the overall process of herbivory at that particular area of reef. Our results highlight an inherent vulnerability in the movement dynamics of herbivorous fishes and, by extension, in the overall process of herbivory in reef ecosystems. They suggest that maintaining high herbivore abundances across all reef habitats is critical in preserving the integrity of the grazing function on reefs. The approach of using network graph theory to analyse acoustic telemetry data is illuminating and is likely to have a range of applications across diverse ecosystems.

Effect of ingestion on the stable isotope signatures of marine herbivorous fish diets

Stable isotope analysis is widely used to investigate the diet–consumer relationships of free-ranging animals, including fishes. However, measuring the isotope signature of relevant diet items can be problematic when little is known about consumer diet choice, or when individual diet items are difficult to collect in the field. Here we tested the viability of using ingested material as a proxy for fresh diet items in a field study by examining the effect of ingestion on the δ15N and δ13C values of diet items commonly consumed by the marine herbivorous fish Odax pullus (Labridae). Ingestion had a significant effect on the δ15N or δ13C value of three of the five diet items considered. Two diet items showed a clear and consistent change in isotopic signature following ingestion, indicating that discrimination between light and heavy isotopes occurs during the early stages of processing in this stomachless fish. The potential for isotope signatures to change following ingestion suggests that caution should be taken when using ingested material as a proxy for diet items in studies of marine herbivory. Additionally, the predictable effect of ingestion demonstrates the potential for digestion to make a large contribution to the fractionation observed in trophic studies of marine herbivory.

Effects of replacement of dietary fish oil with soybean oil on growth performance and tissue fatty acid composition in marine herbivorous teleost Siganus canaliculatus

Fish oil (FO) substitution has been studied in many marine carnivorous fish, but seldom in marine herbivorous or omnivorous species. To evaluate the feasibility of using soybean oil (SO) as a dietary lipid and confirm its capability of converting C18 polyunsaturated fatty acid (PUFA) into long chain polyunsaturated fatty acid (LC-PUFA) in the marine herbivorous teleost Siganus canaliculatus, juvenile fish were fed with four formulated diets differing in lipid composition, with SO accounting for 0.76% (SO0), 23% (SO23), 45% (SO45) and 67% (SO67) of total dietary lipid respectively. After feeding for 8 weeks, growth performance including weight gain, specific growth rate, feed conversion ratio and protein efficiency rate were better in the SO23 and, especially, SO45 groups than in the SO0 and SO67 groups (P < 0.05). Tissue fatty acid compositions were affected by diet, with the liver contents of eicosapentaenoic (EPA), docosapentaenoic (DPA), docosahexaenoic (DHA) acids and total n-3 PUFA displaying parallel changes with the corresponding dietary fatty acids. While the muscle contents of EPA, DPA and total n-3 PUFA between SO0 and SO23 groups, and the liver contents of arachidonic acid (ARA) and 20:4n-3, as well as the muscle content of 20:3n-6 between SO0 and SO45 groups showed no difference, confirming the biosynthesis of LC-PUFA from C18 precursors in vivo as the contents of corresponding fatty acids in diets SO23/SO45 were much lower than those in diet SO0 (P < 0.05). The results indicate that SO may be a suitable dietary lipid source for S. canaliculatus, and can replace up to 67% or 45% of total dietary FO without negatively compromising growth performance or nutritional quality of fish respectively. Moreover, the study increases our knowledge of FO substitution in marine herbivorous fish.

Identification of seagrasses in the gut of a marine herbivorous fish using DNA barcoding and visual inspection techniques

Traditional visual diet analysis techniques were compared with DNA barcoding in juvenile herbivorous rabbitfish Siganus fuscescens collected in Moreton Bay, Australia, where at least six species of seagrass occur. The intergenic spacer trnH-psbA, suggested as the optimal gene for barcoding angiosperms, was used for the first time to identify the seagrass in fish guts. Four seagrass species and one alga were identified visually from gut contents; however, there was considerable uncertainty in visual identification with 38 of 40 fish having unidentifiable plant fragments in their gut. PCR and single-strand conformational polymorphism (SSCP) were able to discriminate three seagrass families from visually cryptic gut contents. While effective in identifying cryptic gut content to family level, this novel method is likely to be most efficient when paired with visual identification techniques.

Why can't young fish eat plants? Neither digestive enzymes nor gut development preclude herbivory in the young of a stomachless marine herbivorous fish

Most young fishes lack the ability to function as herbivores, which has been attributed to two aspects of the digestive system: elevated nitrogen demand and a critical gut capacity. We compared the digestive morphology and biochemistry of two size classes of the marine herbivore Hyporhamphus regularis ardelio, pre-ontogenetic trophic shift (pre-OTS, <100mm) and post-ontogenetic trophic shift (post-OTS, >100mm), to determine what limits the onset of herbivory and how their digestive processes fit with current models of digestion. Two gut-somatic indices comparing gut length to body length (relative gut length) and body mass (Zihler's Index) demonstrated a significant decrease (RGL 0.59→0.49, P<0.01; ZI 3.24→2.44, P<0.01) in gut length relative to body size. There was little difference in enzyme activity between the two classes, with juveniles showing similar levels of carbohydrase and lipase and less protease compared with adults, indicating that juveniles did not preferentially target nitrogen and were as capable of digesting an herbivorous diet. These findings suggest that herbivory in this fish is not limited by the function of the post-oesophageal digestive tract, but rather the ability of the pharyngeal mill to mechanically process plants. Our findings offer partial support for the current model of stomachless digestion, indicating that further refinement may be necessary.

Utilisation of mannitol by temperate marine herbivorous fishes

The sugar alcohol mannitol is the primary photosynthate of phaeophytes, and as such is an important dietary constituent for many marine herbivorous fishes. Sugar alcohols are not thought to be efficiently digested by vertebrates. Although vertebrates lack intestinal transporters for sugar alcohols, these compounds are thought to serve as substrates for fermentation by hindgut flora. Here, we measured mannitol in twelve species of New Zealand phaeophytes and in gut contents and gut fluid from five points along the gut of eight species of New Zealand marine herbivorous fishes: Odax pullus, Kyphosus sydneyanus, Kyphosus vaigiensis, Girella tricuspidata, Girella cyanea, Parma alboscapularis, Aplodactylus arctidens and Aplodactylus etheridgii. The mannitol content of the algae varied from 6.76% dry wt. ± 0.83 SE in Cystophora scalaris to 28.81% dry wt. ± 2.03 SE in Marginariella boryana. The fishes consumed varying amounts of mannitol, correlating with their consumption of phaeophytes. In O. pullus and K. sydneyanus the amount of mannitol in gut contents decreased markedly in the hindgut. The correlation of this decrease with the density of gut microbiota and fermentation products suggests that mannitol is fermented to short-chain fatty acids in the hindgut, rather than being directly taken up by the fish. Hindgut fermentation of mannitol may thus serve as an important mechanism for energy salvage in hindgut-fermenting fishes that feed on phaeophytes.

Nutritional ecology of marine herbivorous fishes: ten years on

Summary 1. Marine herbivorous fishes are considered to be of critical importance in determining the biological structure of shallow reef environments, and by implication have key roles in carbon flux in reef ecosystems. Despite this, the nutritional processes that underpin these critical ecological roles have received comparatively little attention. 2. Here we give an overview of recent progress in the nutritional ecology of marine herbivorous fishes, and then examine two recent paradigms that we consider important in the development of the field: (i) the role of temperature in latitudinal gradients of diversity and abundance, and (ii) the impact of these fishes on coral reefs. Our aim is to illustrate how an integrative nutritional ecology approach can enrich insights gained from studies of fish herbivory, and to emphasize the distinctive differences between herbivory in marine and terrestrial systems. 3. We argue that much of the work on trophic biology in marine herbivorous fishes has focused on the ecological impacts of fishes on reefs, the deterrent properties of marine algae, and the morphological and mechanical aspects of ingestion. This has come at the expense of two of the elements necessary for an integrative understanding of feeding ecology, that is, food composition and the physiological processes involved in nutrient extraction and utilization. Together, these factors have hindered the development of the nutritional framework for analysing food resources, feeding patterns and evolutionary trends that has proved successful for terrestrial vertebrate herbivores. 4. The reef grazing and algal secondary metabolite paradigms, while both extremely productive, have failed to develop the clear predictive framework for diet choice required in broader contexts such as reef management and understanding the evolution of herbivory. The lack of a focus on nutritional factors has led to premature conclusions on the influence of temperature on algal digestion, both at the level of digestive processes and the biogeography of marine faunas. 5. Some marine herbivorous fishes appear to be bending the ‘rules’ of hindgut fermentation, especially with respect to temperature and fermentation substrates, and so the study of nutritional ecology in these animals has potential to generate novel insights for the field of vertebrate nutrition

Nutritional ecology, functional ecology and Functional Ecology

‘Nutritional ecology’ is a bit like ‘art’: easy to recognize but difficult to define. We believe that this is, in part, because the field has progressed on a broad (even diffuse) front, and has lacked cohesion or commonality of purpose. Our aim in this special feature is to bring together leading researchers drawn from across this spectrum and have them review what they consider to be the key issues in their respective research areas. The mandate for invited authors was broad, with invitations that were no more specific than: ‘contribute a paper that provides an overview of the achievements, opportunities and limitations of nutritional ecology studies of [respective taxon/ thematic research area]’. The special feature, we hope, will provide a lens that focuses attention on similarities and highlights disparities across the field. In this introduction we will attempt to define nutritional ecology and its relationship to functional ecology, justify why it warrants a special issue, and summarise the scope of the issue.

Seaweeds preferred by herbivorous fishes

Marine macrobenthic algae (or seaweeds), epiphytic microalgae, and other aquatic plants constitute the main food items of marine herbivorous fishes. About 5% of all fish species are herbivorous; only 30% of these are marine, most of them living in coral reefs. An analysis was performed on all the seaweeds that formed part of the natural diet of these fishes, based on information contained in FishBase (http://www.fishbase.org). The results showed that many coral-reef-associated marine herbivorous fishes, such as the families Blennidae, Kyphosidae and Siganidae, fed selectively on filamentous and turf fleshy seaweeds, which they prefer over calcareous coralline and encrusting species. In particular, Chlorophyceae of the genera Cladophora, Enteromorpha and Ulva were preferred by Scartichthys viridis (Blennidae), Girella spp. (Kyphosidae), Sarpa salpa (Sparidae), and Phaeophyceae in the genera Sargassum and Dictyota were preferred by Kyphosus spp. (Kyphosidae) and Siganus spp. (Siganidae). A web-based tool was developed to provide information on plants (algae, seagrasses, terrestrial plants and fruits) preferred as food by herbivorous fishes (http://www.incofish.org/herbitool.php). The tool is intended to assist aquaculturists, conservationists and ecosystem-based fisheries managers.

Clostridia dominate 16S rRNA gene libraries prepared from the hindgut of temperate marine herbivorous fishes

Bacterial diversity in the microbial communities of posterior gut sections of three temperate marine herbivorous fish species from New Zealand was characterised using Amplified Ribosomal DNA Restriction Analysis, and 16S rRNA gene amplification and sequencing methods. The fish were collected in 1999–2000 in the Hauraki Gulf, New Zealand (35°54’–36°24’S, 174°48’–175°25’E). The gastrointestinal bacterial communities of Kyphosus sydneyanus (Gunther, 1886) (F. Kyphosidae), Odax pullus (Forster in Bloch and Schneider, 1801) (F. Labridae) and Aplodactylus arctidens Richardson, 1839 (F. Aplodactylidae) were dominated by five clades of bacteria, four of which belong to recognized clostridial clusters. The clone libraries of K. sydneyanus and O. pullus contained sequences from most of these clades, but were dominated by members of clostridial clusters XI and XIVa, respectively. The clone library of A. arctidens was dominated by members of clostridial cluster XIVb and an unassigned cluster containing Eubacterium desmolans and Papillibacter cinnaminovorans. The finding that strains of Firmicutes dominated the gastrointestinal microbial communities of all three fish species is consistent with the results of similar studies on terrestrial vertebrate herbivores. This work thus contributes to the view that gastrointestinal symbionts in some marine herbivorous fishes may play a similar role to those in terrestrial vertebrate herbivores studied to date.

Geographical gradients of marine herbivorous fishes: patterns and processes

We present new data and the first rigorous analysis of latitudinal and thermal gradients of diversity, density and biomass of marine herbivorous fishes and review proposed explanatory mechanisms. Consistently, negative relationships between latitude, and positive relationships between sea surface temperature (SST), and relative richness and relative abundance of herbivorous fishes were found worldwide. Significant differences in the strength of gradients of richness and abundance with latitude and SST between tropical and extratropical zones were found consistently across ocean basins. Standardized sampling along the western Atlantic also showed negative relationships between latitude and total density and biomass. The trends, however, are driven by different components of the fish assemblages (i.e., scarids in the Caribbean and acanthurids in Brazil). Patterns of abundance along thermal gradients, generally associated with extensive latitudinal gradients, also were found at the local scale. Feeding rate of the ocean surgeonfish Acanthurus bahianus decreases with temperature more rapidly than the mean metabolic rate of teleost fishes. This relationship suggests a temperature-related physiological constraint. From the new standardized and comparative data presented and the review of the explanatory hypotheses, we conclude that temperature-related feeding and digestive processes are most likely involved in the distribution patterns of herbivorous fishes.

Ontogenetic Development of the Gastrointestinal Microbiota in the Marine Herbivorous Fish Kyphosus sydneyanus

Molecular techniques were used to investigate the composition and ontogenetic development of the intestinal bacterial community in the marine herbivorous fish Kyphosus sydneyanus from the north eastern coast of New Zealand. Previous work showed that K. sydneyanus maintains an exclusively algivorous diet throughout post-settlement life and passes through an ontogenetic diet shift from a juvenile diet which is readily digestible to an adult diet high in refractory algal metabolites. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to investigate the relationship between bacterial community structure and fish size. Bacterial diversity was higher in posterior gut sections than anterior gut sections, and in larger fish than in smaller fish. Partial sequencing of bacterial 16S rDNA genes PCR amplified and cloned from intestine content samples was used to identify the phylogenetic affiliation of dominant gastrointestinal bacteria. Phylogenetic analysis of clones showed that most formed a clade within the genus Clostridium, with one clone associated with the parasitic mycoplasmas. No bacteria were specific to a particular intestinal section or size class of host, though some appeared more dominant than others and were established in smaller fishes. Clones closely related to C. lituseburense were particularly dominant in most intestine content samples. All bacteria identified in the intestinal samples were phylogenetically related to those possessing fermentative type metabolism. Short-chain fatty acids in intestinal fluid samples increased from 15.6 +/- 2.1 mM in fish <100 mm to 51.6 +/- 5.5 mM in fish >300 mm. The findings of this study support the hypothesis that the ontogenetic diet shift of K. sydneyanus is accompanied by an increase in the diversity of intestinal microbial symbionts capable of degrading refractory algal metabolites into short-chain fatty acids, which can then be assimilated by the host.

The uptake of Cd, Cr, and Zn by the macroalga Enteromorpha crinita and subsequent transfer to the marine herbivorous rabbitfish, Siganus canaliculatus.

Marine macroalgae can appreciably accumulate metals out of seawater and are the principle food source for marine herbivorous fish, thus playing an important role in introducing metals into marine food webs. There have been few experimental studies on metal transfer from macroalgae up the trophic chain. In this study, we examined the assimilation efficiencies (AEs) of Cd, Cr, and Zn in the rabbitfish, Siganus canaliculatus, which feeds on the macroalga Enteromorpha crinita. The influences of metal concentration and nutrient conditions in the macroalga, and starvation on the assimilation of metals in fish were investigated. The macroalgae were radiolabeled by direct exposure to radiotracers in the dissolved phase. The uptake of metals in the macroalgae proceeded linearly and was directly proportional to the metal concentration in the ambient seawater. Ammonium and nitrate enrichment significantly increased the accumulation of Cd by the macroalgae, whereas Cr accumulation was not influenced by nutrient enrichment. The metal concentrations in the macroalgae did not have any effect on metal assimilation in the rabbitfish. The measured AEs of the metals were generally higher in starved rabbitfish than in fed fish, probably as a result of the prolonged retention of metals in the gut of fish. Rabbitfish fed macroalgae that had been previously exposed to ammonium or nitrate addition decreased their assimilation of Cd and Zn, but not Cr. Phosphate enrichment did not have any significant effect on metal AE in the rabbitfish. Our study demonstrated that marine herbivorous fish can appreciably assimilate trace metals from marine macroalgae and that trophic transfer should be considered as a source for metal accumulation in fish. Nutrient condition and starvation do have considerable effects on the AEs of Cd and Zn in rabbitfish that are fed macroalgae.

A re-evaluation of the diel feeding hypothesis for marine herbivorous fishes

We re-evaluated the "diel feeding hypothesis" by measuring diel variation in starch, protein, and floridoside in three algal "types" collected from a fringing coral reef at Lizard Island, Great Barrier Reef, Australia. Samples of two species of rhodophyte algae, Gracilaria arcuata and Acanthophora spicifera, and the turf assemblage from the territories of the herbivorous pomacentrid Stegastes nigricans were collected at four time periods through the day: 0630–0730, 1000–1100, 1330–1430, and 1630–1730 hours. We also measured the ability of several species of marine fish (the herbivores Acanthurus nigricans, A. lineatus, A. olivaceus, and Parma alboscapularis and the detritivore Ctenochaetus striatus) to hydrolyse floridoside by estimating α-galactosidase activity in tissue from the anterior intestine. We detected no diel pattern in protein content of the algae but found a significant steady increase in starch content throughout the day. Floridoside content increased in the morning and decreased in the afternoon, a pattern that may be driven by midday photoinhibition of the algae. All the fishes tested could utilise floridoside. Our results support the diel feeding hypothesis. Although floridoside content decreased in the afternoon, our results suggest floridoside was used during the day by the algae to synthesise starch. Thus the algae increased in nutritional value until photoinhibition occurred at midday then subsequently maintained their nutritional value during the afternoon. This pattern of algal nutrients increasing to a midday peak and remaining relatively constant throughout the afternoon correlates well with the diel feeding pattern in many species of marine herbivorous fish.

Diet and endogenous carbohydrases in the temperate marine herbivorous fish Kyphosus sydneyanus

Stomach content analysis showed that Kyphosus sydneyanus collected by spearing from coastal reefs near Leigh, north‐east New Zealand, is herbivorous throughout its post‐settlement life. All size classes collected consumed a wide range of macroalgae, predominantly the phaeophytes Carpophyllum maschalocarpum and Ecklonia radiata, and the rhodophytes Gigartina macrocarpa and Caulacanthus ustulatus. Multivariate analysis of variance and discriminant function analysis showed an ontogenetic diet shift, with gut contents dominated by rhodophytes and chlorophytes in juveniles and by phaeophytes in adults. No significant seasonal variation in diet was apparent for any size class. Amylase and amylopectinase activity was highest in the smaller size classes. Laminarinase activity was relatively low throughout all size classes, and did not correlate with the ontogenetic increase in phaeophyte consumption. The activity of endogenous enzymes hydrolysing starch broadly correlated with rhodophyte consumption in adult specimens of three other species of marine herbivorous fishes: Odax pullus, Girella tricuspidata and Parma alboscapularis; laminarin hydrolysis, however, was negatively correlated with phaeophyte consumption. Overall, the diet and enzyme results suggest that juvenile K. sydneyanus rely mainly on the endogenous digestion of rhodophytes and chlorophytes, while exogenous (microbial) digestion is used to digest the phaeophyte‐dominated diet of adults.

Diet and endogenous carbohydrases in the temperate marine herbivorous fish Kyphosus sydneyanus

Stomach content analysis showed that Kyphosus sydneyanus collected by spearing from coastal reefs near Leigh, north-east New Zealand, is herbivorous throughout its post-settlement life. All size classes collected consumed a wide range of macroalgae, predominantly the phaeophytes Carpophyllum maschalocarpum and Ecklonia radiata, and the rhodophytes Gigartina macrocarpa and Caulacanthus ustulatus. Multivariate analysis of variance and discriminant function analysis showed an ontogenetic diet shift, with gut contents dominated by rhodophytes and chlorophytes in juveniles and by phaeophytes in adults. No significant seasonal variation in diet was apparent for any size class. Amylase and amylopectinase activity was highest in the smaller size classes. Laminarinase activity was relatively low throughout all size classes, and did not correlate with the ontogenetic increase in phaeophyte consumption. The activity of endogenous enzymes hydrolysing starch broadly correlated with rhodophyte consumption in adult specimens of three other species of marine herbivorous fishes: Odax pullus, Girella tricuspidata and Parma alboscapularis; laminarin hydrolysis, however, was negatively correlated with phaeophyte consumption. Overall, the diet and enzyme results suggest that juvenile K. sydneyanus rely mainly on the endogenous digestion of rhodophytes and chlorophytes, while exogenous (microbial) digestion is used to digest the phaeophyte-dominated diet of adults.