Comparative analysis of foraging and habitat use by the sympatric Caribbean parrotfish Scarus vetula and Sparisoma viride (Scaridae)

On the fringing reef of Bonaire, Netherlands Antilles, a comparative study was made of habitat use, diet selection, foraging behaviour and food acquisition of the parrotfish species Scarus vetula and Sparisoma viride. The species are sympatric and live in the same reef habitats (depth zones). Both species show similar foraging selectivity, but exploit algal resources differently. Preferred food items are turf algae on substrates infested with endolithic algae, whereas crustose corallines are avoided. Foraging preferences are related to yield, i.e. the amount of AFDW (ash-free dry weight), protein and energy that can be harvested per bite. Foraging behaviour differs between the species. S. vetula takes more bites in long forays, has higher bite rates (no, of bites s(-1)), and makes fewer and smaller scars on grazing substrates than S. viride. Furthermore, S. vetula prefers flat substrate surfaces while adult S. viride graze by preference on concave surfaces. Species-specific differences in preference and utilization of grazing substrates are related to feeding mode. S. vetula employs a scraping feeding mode by which mainly epilithic algae are ingested. In contrast, S. viride is an excavating grazer that ingests large amounts of endolithic and crustose algae. Intake and assimilation of algal AFDW, protein and energy were quantified through a combination of laboratory feeding trials and field observations. S. vetula has lower food intake (mg AFDW bite(-1)) than S. viride (0.8 x 10(-3) x fish wet wt, FWW, and 2.3 x 10(-3) x FWW respectively), resulting from smaller (shallower) bites. Assimilation efficiencies of total AFDW, protein and energy by S. vetula were higher than in S. viride grazing on the same dead coral substrates, In spite of different feeding modes and different fractions of the primary production harvested, daily amounts of assimilated nutrients and energy are similar for both species, resulting from higher feeding rates (no. of bites h(-1)) and higher assimilation efficiency in S. vetula.</p

Foraging by the stoplight parrotfish Sparisoma viride. I.:Food selection in different, socially determined habitats

Food selection by the Caribbean stoplight parrotfish Sparisoma viride was investigated on a fringing coral reef of Bonaire, Netherlands Antilles. For different reef zones, the diet composition for each life phase was determined by description of randomly selected bites, and compared to the availability of food resources, as determined with the aid of chain-link transects. S. viride employs an excavating grazing mode, and feeds almost exclusively on algae associated with dead coral substrates. Preferred food types are large and sparse turfs growing on carbonate substrates inhabited by endolithic algae. Crustose corallines, with or without algal turfs, are not preferred. Feeding forays were longer on the preferred food types. Foraging preferences are related to nutritional quality of the food types and their yield, i.e. the amounts of biomass, protein and energy that can be ingested per bite, as calculated from the size of grazing scars and the biochemical composition of the algae. In spite of selective foraging, a large proportion of bites is taken on inferior food types. Endolithic algae constitute an important food resource for scraping herbivores, such as S. viride, These algae have relatively high energetic value, and allow a high yield as a result of weakening the carbonate matrix by their boring filaments. The yield of algal resources also depends on the skeletal density of the limestone substrates. On deeper reef parts (> 3.5 m depth), low-density substrates predominate, resulting in higher yields of algae per bite than are attained from high-density substrates that predominate on shallower reef parts. The increased availability of high-yield food and substrate types coincides with the occurrence of haremic territorial behaviour in S. viride males on the deeper reef parts. Territories are defended against conspecifics and have an important function as spawning sites. It is argued that the access to superior food resources on the deeper reef makes territorial defence feasible for S. viride

Foraging by the stoplight parrotfish Sparisoma viride. 11.:Intake and assimilation of food, protein and energy

Daily food intake by the herbivorous parrotfish Sparisoma viride, as well as assimilation efficiencies of algal food, protein and energy, were quantified through a combination of laboratory feeding trials and field observations. The intake of algal ash-free dry wt (AFDW) per bite increases linearly with fish wet wt (FWW, g) and algal biomass (mg AFDW CM-2), and is further determined by the skeletal density of the limestone substrate. Low-density substrates yield higher amounts of AFDW per bite than do high-density substrates. The percentage of the total food intake that is derived from endolithic and crustose coralline algae increases with the size of the fish, and can be &gt; 70 % depending on the biomass of epilithic algae. The daily pattern of foraging activity is positively correlated with diurnal changes in food quality, while seasonal daylength variations result in 13 % variation in total daily bites taken. Daily number of bites of S. viride in the field decreases with fish size, and is further dependent on life phase and foraging depth. In experiments, fish attained an assimilation efficiency of ca 20 % from a natural diet of low algal biomass and high-density dead coral substrates that predominates in the shallow reef. Assimilation efficiency was ca 70 % from a diet of high algal biomass and low-density substrates that predominates on the deeper reef parts. In spite of lower daily foraging effort, territorial fish, living in deeper parts of the reef, ingest and assimilate higher amounts of AFDW, protein and energy per day than non-territorial fish foraging on the shallow reef. The difference is caused by increased availability of high-yield food and substrate types inside territories compared to the situation on the shallow reef. Daily assimilated energy (kJ d-1) is 0.85 x FWW 0.773 for fish foraging in the shallow reef zone, and 1.22 x FWW0.854 for S. viride foraging inside territories on the deeper reef.</p

Photoperiodic history affects the critical daylength of the short-day plant Acrosymphyton purpuriferum (Rhodophyta)

The crustose tetrasporophyte of the red alga Acrosymphyton purpuriferum is a qualitative short-day plant in the formation of its tetrasporangia. The critical daylength for the response was determined in plants precultured in various long-day regimes [20:4, 18:6, 16:8 and 14:10 (L:D, h)]. There was a strong influence of photoperiodic history. The sharper the decrease in daylength the stronger and faster the plants responded. The critical daylength (daylength inducing 50% response) increased from 9-5 h for plants precultured in 14 h days to 12.5 h for plants precultured in 20 h days. Acrosymphyton thus responds to a change tn daylength, rather than to a fixed critical value. This is of adaptive significance in synchronising the onset of reproduction throughout its broad depth range in the subtidal region

The mechanism of daylength perception in the red alga Acrosymphyton-Purpuriferum

The red alga Acrosymphton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night‐breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (< 0.05 μmol.m−2. s−1 vs. 3 μmol.m−2.s−1. The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol.m−2.s−1 after a main photoperiod at ca. 65 μmol.m−2.s−1 vs. threshold at <0.5 μmol.m−2.s−1 after a main photoperiod at ca. 35 μmol.m−2.s−1. The spectral dependence of the response was investigated in day‐extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol. m−2.s−1) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum‐dose of 2.3 mmol.m−2. However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far‐red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum‐doses of up to 20 mmol. m−

Temperature responses of disjunct temperate brown algae indicate long-distance dispersal of microthalli across the tropics

We examined the temperature tolerance of microscopic phases from geographically disjunct isolates of eight species or closely related, putatively conspecific taxa of temperate brown algae with disjunct distributions. Maximum within-taxon differences were small and ranged from 1.6° to 4.3° C. Desmarestia aculeata and Sphaerotrichia divaricata, both with northern hemisphere amphioceanic distributions, showed little or no significant intraspecific variation between the mean upper survival limits (USL) of Atlantic and Pacific strains (δUSL ≤ 1.4°C), which would agree with a relatively recent separation of the respective populations. Among the plants with bipolar distributions, there was likewise very little difference (δUSL 0–1.1°C) between northern and southern hemisphere strains in Striaria attenuata and in the species pair Desmarestia viridis/D. willii. In Desmarestia ligulata, and in the species pairs Desmarestia firma/D. munda, Dictyosiphon foeniculaceus/D. hirsutus, and Scytothamnus australis/Scytothamnus sp., significant differences occurred, which indicate longer divergence times. δUSL in these cases ranged from 1.7° to 2.7°C, without overlap between strains from the northern and southern hemispheres. All species that passed the equator during cooler epochs had a USL of 26–27°C, at least in some geographical isolates. The NE Asian kelp Undaria pinnatifida, which passed the equator in recent times, had a USL of 29.6°C. We hypothesize that the mechanism of spreading in the amphipolar species studied was migration of vegetative microthalli. The more unlikely alternative hypothesis of continuous populations through the tropics during a cooler epoch would imply a drop in seawater temperatures to approximately 20° C in summer and 15° C in winter, which is not supported by paleoclimatic evidence

Seasonal module dynamics of Turbinaria triquetra (Fucales, Phaeophyceae) in the southern Red Sea

Module dynamics in the fucoid alga Turbinaria triquetra (J. Agardh) Kutzing were studied on a shallow reef flat in the southern Red Sea. Seasonal patterns in thallus density and size were determined, and the initiation, growth, reproduction, and shedding of modules were studied using a tagging approach. The effects of module density and module/thallus size on module initiation, growth, reproduction, and shedding were analyzed, and the occurrence of intraspecific competition among modules was examined. Seasonal variation occurred mainly at the modular level. There was a restricted period of new module formation in the cooler season, followed by fast growth and reproduction, massive shedding of modules from the end of the cooler season onward, and strongly reduced biomass in summer. There was no evidence of suppressed growth in small modules due to intraspecific competition. Module density and thallus/module size had opposite effects on elongation rates. High module densities enhanced maximum elongation rates (fastest-growing module per thallus), resulting in longer thalli. On the other hand, elongation rates decreased and tissue loss increased with increasing module length. Reproduction had no clear effect on elongation rates, indicating that there was no direct trade-off between reproduction and growth. The apparent size-dependence of reproduction was due to delayed fertility in young modules. Module initiation and shedding were independent of module density. Shedding was also independent of module size and reproductive status. We conclude that seasonal changes in the environment affect module initiation, growth, reproduction, and shedding, whereas density and size-dependent processes mainly affect growth rates