Switching of metabolic-rate scaling between allometry and isometry in colonial ascidians

The metabolic rate and its scaling relationship to colony size were studied in the colonial ascidian Botrylloides simodensis. The colonial metabolic rate, measured by the oxygen consumption rate (V(O2) in millilitres of O(2) per hour) and the colony mass (wet weight M(w) in grams) showed the allometric relationship (V(O2) = 0.0412 M(w)(0.799). The power coefficient was statistically not different from 0.75, the value for unitary organisms. The size of the zooids and the tunic volume fraction in a colony were kept constant irrespective of the colonial size. These results, together with the two-dimensional colonial shape, excluded shape factors and colonial composition as possible causes of allometry. Botryllid ascidians show a takeover state in which all the zooids of the parent generation in a colony degenerate and zooids of a new generation develop in unison. The media for connection between zooids such as a common drainage system and connecting vessels to the common vascular system experienced reconstruction. The metabolic rate during the takeover state was halved and was directly proportional to the colonial mass. The scaling thus changed from being allometric to isometric. The alteration in the scaling that was associated with the loss of the connection between the zooids strongly support the hypothesis that the allometry was derived from mutual interaction among the zooids. The applicability of this hypothesis to unitary organisms is discussed.     

Effects of low salinity on metamorphosis in estuarine colonial ascidians

Abstract. We studied the effects of brackish water on larval attachment, events of metamorphosis, and juvenile mortality in three colonial ascidian species that live in a Florida coastal lagoon. Eudistoma olivaceum and Eudistoma hepaticum are restricted in their adult distribution to areas of relatively high and constant salinity near inlets, whereas Ecteinascidia turbinata extends more than 20 km into the Indian River, where salinity can be much more variable. In all three species, metamorphosis proceeded more quickly at 33 ppt than at lower salinities. The thresholds for successful metamorphosis differed among species in a manner that corresponded to the adult distributions, with E. turbinata being capable of completing metamorphosis at salinities as low as 22 ppt, E. hepaticum as low as 24 ppt, and E. olivaceum as low as 26 ppt. Larvae of both Eudistoma species delayed settlement in very low salinity water, whereas those of E. turbinata settled very quickly, then failed to complete metamorphosis. Juvenile mortality at salinities lower than 22 ppt was 100% for all three species. Survival in salinities higher than 22 ppt was strongly correlated with salinity in E. olivaceum and E. hepaticum , but not E. turbinata.     

The allometry of avian basal metabolic rate: good predictions need good data

Basal metabolic rate (BMR) is often predicted by allometric interpolation, but such predictions are critically dependent on the quality of the data used to derive allometric equations relating BMR to body mass (Mb). An examination of the metabolic rates used to produce conventional and phylogenetically independent allometries for avian BMR in a recent analysis revealed that only 67 of 248 data unambiguously met the criteria for BMR and had sample sizes with n>/=3. The metabolic rates that represented BMR were significantly lower than those that did not meet the criteria for BMR or were measured under unspecified conditions. Moreover, our conventional allometric estimates of BMR (W; logBMR=-1.461+0.669logMb) using a more constrained data set that met the conditions that define BMR and had n>/=3 were 10%-12% lower than those obtained in the earlier analysis. The inclusion of data that do not represent BMR results in the overestimation of predicted BMR and can potentially lead to incorrect conclusions concerning metabolic adaptation. Our analyses using a data set that included only BMR with n>/=3 were consistent with the conclusion that BMR does not differ between passerine and nonpasserine birds after taking phylogeny into account. With an increased focus on data mining and synthetic analyses, our study suggests that a thorough knowledge of how data sets are generated and the underlying constraints on their interpretation is a necessary prerequisite for such exercises.     

Unusual allometry between respiration rate and body size in Chaoborus species

The respiration rates of all four instars of Chaoborus flavicanswere measured with a flow-through respirometer at an experimentaltemperature of 20°C. The respiration rates (µg O₂larva^-1 h^-1) increased parallel to the larval stages accordingto R = 0.027 x W^0.416 (W = µg dry weight), reaching arespiration rate eight times higher for instar IV than for instarI. The slope of the increase with body weight was as low asin two tropical Chaoborus species and was considerably lowerthan usually found for other aquatic animals. Instar IV larvaecollected in the spring showed a significantly higher respirationrate than those collected in the fall. The respiration rateof the fourth instar approximately doubled with a Q₁₀ of 2.1when the experimental temperature was increased from 10 to 20°C.     

Intraspecific allometry of standard metabolic rate in green iguanas, Iguana iguana

To study the allometric relationship between standard metabolic rate and body mass (mass range 16-3627 g) in green iguanas, Iguana iguana (n=32), we measured rates of oxygen consumption (V(O(2))) at 30 degrees C during scotophase. The relationship could be described as: V(O(2))(ml h(-1))=0.478W(0.734). The resulting mass exponent was similar to the 3/4 power commonly used in interspecific curves (P>0.05), but differed from a proposed intraspecific value of 2/3 (P<0.05). The mass exponents of male (n=8) and female (n=11) iguanas did not differ (P>0.05). The mass adjusted V(O(2)) was higher than predicted from generalized squamate curves. The mean mass exponent of intra-individual allometric equations of iguanas (n=7) at varying masses during ontogeny did not differ from that of the pooled equation, indicating that scaling of V(O(2)) is similar for both between and within individuals. Thermal acclimation, compensatory changes in V(O(2)) with prolonged exposure to a constant temperature, was not observed in juvenile iguanas (n=11) between 1 and 5 weeks of acclimation at 30 degrees C.     

The effect of grazer size manipulation on periphyton communities

Summary We examined the effect of grazer size on periphyton biomass, size structure, and species compostion by removing the largest invertebrate grazers on artificial macrophytes planted in the littoral of Lake Memphremagog (Que-Vt). A series of exclosures with increasingly fine mesh prevented colonization by large invertebrates but allowed in smaller grazers. Oligochaetes, chironomids, and cladocerans effectively replaced snails so that total grazer biomass in the various treatments was not significantly different from the controls. With one exception, algal biomass, measured as chlorophyll a, did not differ significantly among the various treatments. However algal size and taxonomy were affected because the dominance of large blue-green colonies was apperantly related to the presence of large grazers. The results of the size manipulations were qualitatively similar to those induced in phytoplankton communities by size selective zooplankton grazing and are consistent with models based on general allometric equations.Contribution 181 of the Lake Memphremagog Project, Limnology Research Centre     

The effect of body size on the standard metabolic rate of the lesser spotted dogfish

Standard metabolic rates were measured as the rate of oxygen consumption in 33 Scyliorhinus canicula , ranging in weight from 3–929 g. The amount of oxygen consumed per hour (Vo₂) changed predictably with body size according to the relationship Vo₂=0–104 ^W0.855, where W represents fish weight in g. On a weight specific basis, the level of standard metabolism in juvenile dogfish (5 g) was nearly double that measured in adults (500 g).     

Absence of metabolic rate allometry in an ex vivo model of mammalian skeletal muscle

Within mammalian species, standard metabolic rate (SMR) increases disproportionately with body mass (Mb), such that the mass-specific SMR correlates negatively with Mb. This phenomenon can be explained in part by reduced cellular metabolic rates in larger species. To better understand the cause(s) of this cellular metabolic rate allometry we have used an ex vivo approach to isolate and identify potential contributors. Skeletal myoblasts from mammalian species ranging inMb from 30 g to over 300,000 g were isolated and differentiated into myotubes in vitro. Oxygen consumption rates, citrate synthase (CS) activity, and lactate dehydrogenase (LDH) activity were measured in myotubes under standardized conditions. No correlation of any of these parameters was observedwith speciesMb, suggesting that there is no genetic contribution to between-species differences in cellular metabolic rates. Myotubes were incubated in serum from species ranging from 30 g to 400,000 g to determine whether between-species differences in the levels of metabolically important hormones might produce allometric trends in the cultured cells. However, there was no observed effect of serum donor Mb on any of the metabolic characteristicsmeasured. Thus, there is no evidence for a relationship between skeletal muscle oxidative metabolism and Mb in an ex vivo model.     

Microbial epibionts of the colonial ascidians Didemnum galacteum and Cystodytes sp.

Symbiosis with microorganisms has been well documented for many marine invertebrate taxa. However, knowledge of the diversity of microorganisms associated with ascidians is still limited. This study assessed the microbial epibionts of Didemnum galacteum and Cystodytes sp., two ascidian species collected from the western coast of Ceará state (Brazil), at Dois Coqueiros beach and the port of Pecém, respectively. The microbiota were examined using optical microscopy, followed by subsequent analysis of fingerprinting profiles obtained by denaturing gradient gel electrophoresis (DGGE) and 16S rRNA clone libraries. The microscopy analysis showed for both ascidians a community comprising cyanobacteria, mainly Prochloron-like species, and diatoms. The DGGE results indicated that D. galacteum hosts a more diverse microbiota when compared to Cystodytes sp. The same analysis also suggested that the diversity of the seawater microbiota was higher than the diversity of the ascidian-associated microbiota. The analysis of the 16S rRNA clone libraries revealed the dominance of Proteobacteria symbionts associated with both ascidians, with Alphaproteobacteria as the major component in D. galacteum and Gammaproteobacteria the major component in Cystodytes sp. The analysis of the clone libraries also revealed the presence of other taxa such as Bacteroidetes, Planctomycetes, Actinobacteria, Cyanobacteria, and uncultured bacteria in D. galacteum, but not in Cystodytes sp. Among the bacteria found to be exclusively associated with the ascidians, none were shared by the two studied hosts. The combined results point to a diverse microbiota associated with the external surface of the ascidians, with a mixed composition including organisms typically found in the surrounding seawater, but also a more specific set of taxa.     

Intraspecific allometry of basal metabolic rate: relations with body size, temperature, composition, and circadian phase in the kestrel, Falco tinnunculus

The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated in the literature primarily by comparison between species of mammals or birds. This paper focuses on the intraindividual changes in BMR when body mass (W) varies with different maintenance regimens. BMR varied in individual kestrels in proportion to W1.67, which is considerably steeper than the mass exponents for homomorphic change (0.667; Heusner, 1984) for interspecific comparison among all birds (0.677) or raptors (0.678), for interindividual comparison of kestrels on ad libitum maintenance regimens (0.786), and for mass proportionality (1.00). The circadian range of telemetered core temperature also varied more strongly with intraindividual than with interspecific (Aschoff, 1981a) variation in mass. This was due to reduced nocturnal core temperature at low-maintenance regimens, which was, however, insufficient to account for the excessive reduction in BMR. kidney lean mass at Carcass analysis of eight birds sacrificed revealed a disproportionate reduction in heart and kidney lean mass at low-maintenance regimens. We surmise that variation in BMR primarily reflects variation in these metabolically highly active tissues. This may account for positive correlations found between heart, kidney, and BMR residuals relative to interspecific allometric prediction, and between alpha and rho residuals, as expected on the basis of the constant excess of BMR during alpha above BMR during rho (Aschoff & Pohl, 1970a).     

Genome size and metabolic rate in salamanders

• 1.1. The relationship between genome size (C-value) and metabolic rate in salamanders was examined by correlation analyses.
• 2.2. The C-values of 48 species of salamanders were determined by flow cytometry measurement of DNA quantity in erythrocyte nuclei; C-values of 88 species were taken from the literature.
• 3.3. Standard metabolic rates for salamanders at 5, 15, 20 and 25°C were taken from the literature.
• 4.4. Only at 25°C is there a robust, significant correlation between C-value and metabolic rate, and the hypothesis of a frugal metabolic strategy and genome size effect on metabolic rates in salamanders is discussed.

     

Comparative phylogeography of two colonial ascidians reveals contrasting invasion histories in North America

Surveys of genetic structure of introduced populations of nonindigenous species may reveal the source(s) of introduction, the number of introduction events, and total inoculum size. Here we use the mitochondrial cytochrome c oxidase subunit 1 (COI) gene to explore genetic structure and contrast invasion histories of two ecologically similar and highly invasive colonial ascidians, the golden star tunicate Botryllus schlosseri and the violet tunicate Botrylloides violaceus, in their global and introduced North American ranges. Haplotype and nucleotide diversities for B. schlosseri were significantly higher than for B. violaceus both globally (h = 0.872; ?? = 0.054 and h = 0.461; ?? = 0.007, respectively) and in their overlapping North American ranges (h = 0.874; ?? = 0.012 and h = 0.384; ?? = 0.006, respectively). Comparative population genetics and phylogenetic analyses revealed clear differences in patterns of invasion for these two species. B. schlosseri populations on the west and east coasts of North America were seeded from the Pacific and Mediterranean regions, respectively, whereas all North American B. violaceus populations were founded by one or more introduction events from Japan. Differences in genetic structure of invasive populations for these species in North America are consistent with their contrasting probable introduction vectors. B. schlosseri invasions most likely resulted from vessel hull fouling, whereas B. violaceus was likely introduced as a ??fellow traveler?? in the shellfish aquaculture trade.     

Plasticity in metabolic allometry: the role of dietary stoichiometry

Metabolism involves multiple elements. While we know much about the allometry in metabolic response of organisms to energy (carbon, C) availability, little is known about how different-sized organisms respond to the relative availability of elements. I experimentally manipulated availability of phosphorus (P) relative to C, to test whether dietary C : P affects metabolism in four species of Daphnia , spanning an order of magnitude in body mass. Results indicated that the slope of the relationship between individual respiration and body mass was M ^0.83 under a balanced diet (C : P c. 150), and M ^0.67 under an imbalanced diet (C : P c. 800). Increased respiration under dietary imbalance was not due to increased ingestion. The change in the scaling exponent was due to the greater respiratory response of smaller species to altered diets. Diet-induced metabolic plasticity contributes to variation in metabolic allometry, at least at such small scales of body size.     

Developmental perspective on size change and allometry in evolution

The evolution of changes in body size is one of the most important patterns in the history of life. Its importance arises from both the frequency of the pattern and the biological implications of size change itself, which affects myriad aspects of an organism's structure and function through well-known scaling relationships. Yet relatively little attention has been focused on the underlying genetic and developmental controls of size change or their implications with regard to other morphological changes. Here, I review the endocrine growth axis and show that variation in several key growth-control substances, particularly growth hormone (GH) and insulin-like growth factor I (IGF I), is clearly linked to intraspecific differences in postnatal growth rates and terminal body size. I intentionally review a considerable amount of literature on nonprimate mammals because this research is vital to an understanding of the general topic. Research on human pygmies, giant transgenic mice, and other models of growth disturbances indicates that shifts in GH and/or IGF I levels not only yield the expected changes in terminal body size, but also result in simple truncations or extensions of underlying allometric patterns. These data provide a possible developmental basis for the common finding of ontogenetic scaling and coordinated transformations in series of closely related fossil or living species that differ in body size. At present, however, this must be viewed as a hypothesis that requires testing through interspecific analyses. A consideration of previous interpretations of the morphological distinctions of human pygmies and some other organisms demonstrates the novel information that a developmental perspective brings to morphological comparisons. Clearly, knowledge of the genetic and developmental controls of morphogenesis will greatly enhance our understanding of a multitude of evolutionary patterns, processes and mechanisms, for it is perturbations in these these controls that ultimately produce the raw material for evolutionary transformations.     

Effects of metabolic level on the body size scaling of metabolic rate in birds and mammals

Metabolic rate is traditionally assumed to scale with body mass to the 3/4-power, but significant deviations from the '3/4-power law' have been observed for several different taxa of animals and plants, and for different physiological states. The recently proposed 'metabolic-level boundaries hypothesis' represents one of the attempts to explain this variation. It predicts that the power (log-log slope) of metabolic scaling relationships should vary between 2/3 and 1, in a systematic way with metabolic level. Here, this hypothesis is tested using data from birds and mammals. As predicted, in both of these independently evolved endothermic taxa, the scaling slope approaches 1 at the lowest and highest metabolic levels (as observed during torpor and strenuous exercise, respectively), whereas it is near 2/3 at intermediate resting and cold-induced metabolic levels. Remarkably, both taxa show similar, approximately U-shaped relationships between the scaling slope and the metabolic (activity) level. These predictable patterns strongly support the view that variation of the scaling slope is not merely noise obscuring the signal of a universal scaling law, but rather is the result of multiple physical constraints whose relative influence depends on the metabolic state of the organisms being analysed.     

Carnivore olfactory bulb size: allometry, phylogeny and ecology

Olfactory bulb size was measured in 146 species of Carnivora in order to examine whether recently observed functional patterns for overall brain size were similar for component parts of the brain. Comparative measures were analysed in relation to various allometric characters (body, brain and skull size), phylogeny, behaviour and ecology. Olfactory bulbs are significantly and positively correlated with all allometric variables, but indices of skull size correlate slightly more closely than other variables. This probably relates to functional aspects of skull size, facial proportions, and anterior elements of the brain. Phylogenetic associations were examined by two comparative methods: the method of independent contrasts and phylogenetic autoregression. Both revealed similar phylogenetic correlation at generic and familial levels. Using calculated values from either method, relative olfactory bulb size only correlates with zonation among seven behavioural and ecological variables; aquatic otters have smaller bulb sizes than carnivores of other zonal types. This agrees with discussion about the diminution of olfactory communication in aquatic environments. Also, olfactory bulb size correlates with home range size, which is consistent with a recent model on the use of olfaction for foraging in designated home ranges. Generally, comparative differences in olfactory bulb size in carnivores do not associate with functional variables found in other comparative studies. Nevertheless, future analyses of specific brain components in mammals may be more useful than overall brain size for testing evolutionary hypotheses of mammalian brain size.