Thesis

Brooding mothers, sibling rivals, and population structure in Boccardia proboscidea, a polychaete with alternative reproductive modes

Thesis

Brooding mothers, sibling rivals, and population structure in Boccardia proboscidea, a polychaete with alternative reproductive modes

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Abstract

The ability to produce more than one kind of offspring, a phenomenon called poecilogony, is a striking example of reproductive variability. Traditionally, larval nutrition has been classified as a dichotomy: If offspring obtain nutrition from their mothers (lecithotrophy), there is lower fecundity and greater chance of offspring survival than when they get their nutrition from plankton (planktotrophy). The polychaete Boccardia proboscidea (Spionidae) produces both types of embryos using three different reproduction strategies. This species is an ideal system to explore implications and consequences of different reproductive strategies without confounding factors encountered in interspecific comparisons. In this thesis I studied B. proboscidea at four different levels (intracapsular, maternal, population and phylogeographic) and I used a combination of laboratory techniques, field observations and computer simulations to test hypotheses of life history evolution and intrafamily conflict in the sea. Results show that larval fate was determined early in development, even when larvae were grown in isolation. Larval growth was significantly faster at high concentrations of nurse eggs and at high temperatures. Plankototrophs suffered high rates of cannibalism and grew faster when provided with phytoplankton, demonstrating the high cost of their remaining in a capsule. The mother actively brooded and opened her capsules (embryos could not liberate themselves) and their brooding period was negatively correlated with temperature and not completely explained by the effect of temperature on embryos’ developmental rates. Females from higher latitudes provided offspring with larger supplies of extra embryonic nutrition and brood their offspring for longer periods than females from southern latitudes. Two genetic mitochondrial markers, 16S rDNA and Cyt b, showed a single North American West Coast network which is structured, geographically, by the well-documented biogeographic break near Point Conception, CA. The southern group within this network covers a smaller range, but has larger haplotype diversity, than the northern group. Some individuals differing in reproductive type had the same haplotype, indicating independence of haplotypes and reproductive type. Results of this study suggested that evolutionary history, maternal influences and sibling conflict, are playing a role in the maintenance of this reproductive polymorphism that might be acting as a bet hedging strategy.

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... Benthic embryos are at risk from hazards that can strike suddenly and unexpectedly, such as predation (Shimek 1981;Rawlings 1994), dislodgement and drift (Wilson 1983), or damage from waterborne objects (Dayton 1971). Variation in duration of brooding is also expected because of parental costs that can include interrupted feeding (Menge 1975), active brood care (Baeza & Fern andez 2002;Brante et al. 2003;Oyarzun & Strathmann 2011), or reduced numbers of broods (Oyarzun 2010). Thus, a potential advantage for plasticity in hatching is mediation between risks or costs for clutches protected on the seafloor and risks for larvae in the plankton. ...
... Veliger larvae of the nudibranch Phestilla sibogae BERGH 1905 hatch at an earlier stage if egg capsules are scattered from the gelatinous egg mass (Strathmann et al. 2010). The spionid Boccardia proboscidea HARTMAN 1940 actively ventilates its brood and hatches its larvae at varying stages by tearing the egg capsules (Gibson & Gibson 2004;Oyarzun 2010); the worm hatches the egg capsules at earlier stages when brooding at higher temperatures, perhaps because of greater maternal costs of brooding at higher temperatures (Oyarzun 2010;Oyarzun & Strathmann 2011). ...
... Veliger larvae of the nudibranch Phestilla sibogae BERGH 1905 hatch at an earlier stage if egg capsules are scattered from the gelatinous egg mass (Strathmann et al. 2010). The spionid Boccardia proboscidea HARTMAN 1940 actively ventilates its brood and hatches its larvae at varying stages by tearing the egg capsules (Gibson & Gibson 2004;Oyarzun 2010); the worm hatches the egg capsules at earlier stages when brooding at higher temperatures, perhaps because of greater maternal costs of brooding at higher temperatures (Oyarzun 2010;Oyarzun & Strathmann 2011). ...
... Sometimes juveniles settled within the same day they were liberated (Fig. 2E). The earlier stage at hatching at lower temperature was indicated by the development rates of individual larvae grown in Eppendorf tubes from an early stage (three setigers) to metamorphosis at 118 and 208C (Oyarzun 2010) (Fig. 4). If this is the case, females are not simply adjusting the brooding period to match the developmental state of the larvae, but are changing the size at which larvae are hatched and consequently the amount of cannibalism and the potential for dispersal. ...
... Larvae were expelled from the tube by waving motions of the female's body. (Oyarzun, 2010). The upper and lower limits of the adelphophagic larvae at 208C reflected growth rates of 1 and 1.5 setigers/day, which are the growth rates at two different concentrations of nurse eggs (Oyarzun, 2010). ...
... (Oyarzun, 2010). The upper and lower limits of the adelphophagic larvae at 208C reflected growth rates of 1 and 1.5 setigers/day, which are the growth rates at two different concentrations of nurse eggs (Oyarzun, 2010). Dotted lines show the average time that females brooded at that temperature. ...
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Plasticity in hatching potentially adjusts risks of benthic and planktonic development for benthic marine invertebrates. The proportionate effect of hatching plasticity on duration of larval swimming is greatest for animals that can potentially brood or encapsulate offspring until hatching near metamorphic competence. As an example, early hatching of the nudibranch mollusk Phestilla sibogae is stimulated by scattering of encapsulated offspring, as by a predator feeding on the gelatinous egg ribbon. When egg ribbons are undisturbed, hatching is at or near metamorphic competence. Disturbance of an unguarded benthic egg mass can insert 4 or more days of obligate larval dispersal into the life history. As another example, the spionid annelid Boccardia proboscidea broods capsules, each with both cannibalistic and developmentally arrested planktivorous siblings plus nurse eggs. Early hatching produces mainly planktivorous larvae with a planktonic duration of 15 days. Late hatching produces mainly adelphophages who have eaten their planktivorous siblings and metamorphose with little or no period of swimming. Mothers actively hatch their offspring by tearing the capsules, and appeared to time hatching in response to their environment and not to the stage of development of their offspring. Higher temperature increased the variance of brooding time. Females appeared to hatch capsules at an earlier developmental stage at lower temperatures. Species that release gametes or zygotes directly into the plankton have less scope for plasticity in stage at hatching. Their embryos develop singly with little protection and hatch at early stages, often as blastulae or gastrulae. Time of hatching cannot be greatly advanced, and sensory capabilities of blastulae may be limited.
... In addition, seasonally changing levels of sand, sediment deposition in between barnacles, and availability of logs can create frequent patch extinctions and colonization events (Oyarzun unpublished, Pernet personal communication). Finally, the temperature of sediment 2-3 cm below the surface where B. proboscidea inhabits has been recorded to vary between 13 and 29 • C during a low tide series during summer time (Oyarzun 2010). Interestingly, Helmuth et al. (2006) reported that temperature stress can be higher at high latitudes where low tides coincide with mid-day high temperature, implying a latitudinal gradient in temperature stress. ...
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The ability to produce more than one kind of offspring, or poecilogony, is a striking example of reproductive variability. Traditionally, larval nutrition has been classified as a dichotomy: if offspring obtain nutrition from their mothers (lecithotrophy), there is lower fecundity and greater chance of offspring survival than when they get their nutrition from plankton (planktotrophy). The polychaete Boccardia proboscidea (Spionidae) produces both types of embryos using three different reproductive strategies. In this study, we examined the roles of genetic history and phenotypic plasticity on explaining natural variation in B. proboscidea along the Pacific coast of the United States using two genetic mitochondrial markers, 16S rDNA and Cyt b, and common garden experiments. These data show a single North American West Coast network that is structured, geographically, by the well-documented biogeographic break near Point Conception, California. The southern group within this network covers a smaller range, but has larger haplotype diversity, than the northern group. Some individuals differing in reproductive type had the same haplotype, indicating independence of these features; however, differences between laboratory and field data suggest additional geographic variation within one of the reproductive types. Females from higher latitudes provide offspring with larger supplies of extra embryonic nutrition than females from southern latitudes. Results herein suggest that both genetic history and developmental plasticity are playing a role in the maintenance of this reproductive polymorphism.
... Hadfield & Strathmann (1996) describe a broad range in age at hatching for a keyhole limpet, with hatching enhanced by mechanical disturbance. Oyarzun (2010) observed variation in the stage of development at which a spionid annelid opens capsules of her brood, which suggests the possibility of hatching in response to maternally perceived risks. In this worm, time of release can affect the proportion of planktonic and benthic hatchlings. ...
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A dense infaunal polychaete assemblage was studied in Mission Bay, California. Polychaete life history features provided the framework for investigations of small-scale dispersal mechanisms, infaunal response to disturbance, and the spatial and temporal predictability of species' abundances. Rhynchospio arenincola, Streblospio benedicti, Exogone lourei, Fabricia limnicola and Capitella spp. shared many life history traits which limited the range of dispersal. These included small adult size, brood protection, small brood size, and planktonic larval stages which were reduced or absent. Pseudopolydora paucibranchiata and Polydora ligni exhibited initial brood protection but had larger brood sizes and longer lived larvae. Colonization ability at recruitment was distinct from dispersal (migratory) ability. Rates and mechanisms of colonization were governed by larval development, settlement, and mobility patterns and varied with the scale of perturbation. For R. arenincola, S. benedicti, E. lourei and F. limnicola, factors such as brood protection, reduced planktonic larval phases, and postlarval movements, particularly by brooding adults, confer small-scale dispersal abilities which permit rapid colonization of disturbed patches and result in maintenance of high infaunal densities (>200 000 individuals/m2). -from Author
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Compared 2 local demes (1.5km apart) of Streblospio benedicti (Spionidae). One was dominated by individuals exhibiting planktotrophy, where large numbers of offspring develop from small eggs as feeding larvae; the other was dominated by individuals exhibiting lecithotrophy, in which fewer numbers of larvae develop from large eggs without feeding. Over a 2.5yr period strong similarity was observed between demes in magnitude and timing of fluctuations in recruitment and total abundance, in benthic population size structure, and in secondary production, average production:biomass ratio, and annual population growth rates. Planktotrophs exhibited superior colonization ability, and showed greater seasonality and more variability in production and some demographic properties, suggesting that populations with planktotrophic development should exhibit different patch dynamics than those with lecithotrophic development. -Authors
Article
A tadpole of Scaphiopus bombifrons was seen to kill and eat eight others of its kind (in Tillman Co., Oklahoma). No others in several cultures were predaceously cannibalistic and predation could not be induced by feeding dead tadpoles to hungry individuals. Eight out of several hundreds preserved as collected at the same time and place had a buccal apparatus like that of the cannibal and were presumably cannibalistic in nature.
Article
Boccardia proboscidea has poecilogonous development, i.e., it produces more than one type of offspring. Patterns of larval development within one population (La Jolla, CA) vary both among broods produced by different females, and also within a single brood, as some females produce both planktotrophic larvae and benthic juveniles within a single egg capsule. The mode of larval development is linked to the production of nurse eggs, non-viable eggs that are ingested by developing larvae (a process called adelphophagy). Not all females produce nurse eggs and most observed here produced only developing oocytes (Type 1 development). Type 1 females produce offspring that hatch at an early larval stage (3-setiger) and are planktotrophic for 30 d before metamorphosing. Type 2 females are similar to Type 1, but ~15% of the eggs per brood are non-developing nurse eggs. Type 2 offspring ingest the nurse eggs, larvae are slightly more advanced at hatching (3-5 setigers), and have a shorter planktotrophic period (19 d) than do Type 1 larvae. Type 3 females differ in that they produce broods in which most eggs (~90%) are nurse eggs. Offspring in Type 3 broods are either adelphophagic (ingest nurse eggs, have accelerated development, and hatch as juveniles) or non-adelphophagic (do not ingest nurse eggs and are similar to Type 1 planktotrophic larvae). Adults of each type are morphologically and ecologically similar and offspring of Type 1 and Type 3 females are interfertile. Although larval development varies among females, a single female will consistently produce broods of the same type. Poecilogony has been reported previously in B. proboscidea, but this is the first report of three developmental morphs within one population.
Article
In many organisms, a female's environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases, maternal effects may evolve as mechanisms for transgenerational phenotypic plasticity whereby, in response to a predictive environmental cue, a mother can change the type of eggs that she makes or can program a developmental switch in her offspring, which produces offspring prepared for the environmental conditions predicted by the cue. One potentially common mechanism by which females manipulate the phenotype of their progeny is egg size plasticity, in which females vary egg size in response to environmental cues. We describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in a seed beetle, Stator limbatus, and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid across host plants. We found that genetic variation is present within populations for the size of eggs laid on seeds of two host plants (Acacia greggii and Cercidium floridum; h2 ranged between 0.217 and 0.908), and that the heritability of egg size differed between populations and hosts (higher on A. greggii than on C. floridum). We also found that the evolution of egg size plasticity (the maternal effect) is in part constrained by a high genetic correlation across host plants (rG > 0.6). However, the cross-environment genetic correlation is less than 1.0, which indicates that the size of eggs laid on these two hosts can diverge in response to natural selection and that egg size plasticity is thus capable of evolving in response to natural selection.
Article
The majority of marine benthic invertebrates exhibit a complex life cycle that includes separate planktonic larval, and bottom-dwelling juvenile and adult phases. To understand and predict changes in the spatial and temporal distributions, abundances, population growth rate, and population structure of a species with such a complex life cycle, it is necessary to understand the relative importance of the physical, chemical and biological properties and processes that affect individuals within both the planktonic and benthic phases. To accomplish this goal, it is necessary to study both phases within a common, quantitative framework defined in terms of some common currency. This can be done efficiently through construction and evaluation of a population dynamics model that describes the complete life cycle.
Article
Two recently diverged northeastern Pacific sibling snail species, Nuccella ostrina and N. emarginata, currently inhabit adjacent zoogeographic provinces. Their distributions overlap in central California to the north of a major faunal boundary at Point Conception, California (PC). To test the hypothesis that modern sympatry is due to a recent northward range expansion by N. emarginata, I analyzed the population structures of both species with nuclear (allozyme) and mitochondrial DNA (mtDNA) markers. Populations of N. emarginata in the region of overlap exhibit significantly lower heterozygosity and allelic diversity than either populations to the south of PC or populations of N. ostrina. A single mtDNA haplotype characterizes all but one population of N. emarginata sampled in this region, but no haplotype to the south of PC is found at more than one locality. MtDNA haplotypes and allozyme allele frequencies also indicate monophyly of central California populations of N. emarginata. Sharp differences in allelic diversity over small geographic distances may reflect the action of natural selection, but because both nuclear and mtDNA markers display concordant patterns, a range expansion across PC best explains patterns of genetic variation in N. emarginata. Allozymes and mtDNA also reveal that the geologically older N. ostrina is paraphyletic with respect to N. emarginata. This pattern is consistent with, but not indicative of, a peripheral isolation model of speciation. Low genetic diversity is also expected if a significant bottleneck occurred at speciation. However, low allelic diversity is not universal throughout the geographic range of N. emarginata; high allelic diversity at the southern end of the distribution of N. emarginata suggests that in the past N, emarginata has been geographically restricted much further south than PC. A northward range expansion across PC by N. emarginata may thus represent only the most recent postglacial movement by the species. The thermal and oceanographic discontinuities found at PC may not have been directly involved in geographic isolation if N. emarginata originated much further south of this modern boundary. Despite uncertainty regarding the exact spatial distribution of populations at speciation, genetic data indicate that even though N. ostrina and N. emarginata currently exhibit a broad range of geographic overlap, speciation was likely allopatric and was initiated by physical isolation of populations in different zoogeographic provinces.
Article
I incubated eggs of the smooth softshell turtle (Apalone mutica) at 26°, 28°, and 30° C in 2 yr to investigate the physiological significance of temperature on developing embryos and hatchlings. In particular, I tested the hypothesis that temperature has a direct effect on physiology of developing embryos independent of the hydric environment, specifically as expressed through its effects on hatchling size and locomotor performance. Hatching success was lowest at 26°C but did not differ significantly among treatments or between years. Survivorship of turtles to 2 wk posthatching varied significantly among temperature treatments, because relatively few individuals from 26°C survived Duration of incubation was negatively related to incubation temperature. However, sex determination of hatchling turtles was independent of incubation temperature. All but one measure of body size of turtles varied positively with incubation temperature in both years, which suggests that temperature directly affected embryonic growth. Body size of hatchlings also differed significantly among clutches, even after accounting for initial egg mass, which implies maternal or genetic variation in embryonic development. Measures of locomotor performance (running and swimming speed) were significantly influenced by incubation temperature in both years, such that warmer temperatures produced faster hatchlings. In 1989, but not in 1990, larger turtles ran and swam faster than smaller individuals. Clutch exhibited a significant influence on running and swimming speed in both years, which suggests maternal or genetic effects underlying performance. Both measures of locomotor performance were positively correlated in both years, and the correlations were significant, which indicates that faster runners were also faster swimmers. Running and swimming speed were highly repeatable within the 1-d trials in both years and exhibited significant heritable variation. These findings highlight the pervasive influence of environmental and genetic factors on the physiological ecology of embryonic and hatchling turtles and indicate that growth and locomotor performance may possess the genetic variation necessary to evolve in response to selection pressures in nature. Experimental field studies of the ecological and evolutionary consequences of variation in these traits would prove valuable in elucidating the pervasive impact of environmental conditions during embryonic development.
Article
Formulated in the 1950s, the 'desperate larva' hypothesis postulated that lecithotrophic larvae become less discriminating in their settlement requirements over time, due to depletion of energy reserves. Its predictions have been confirmed for some taxa in which larvae settle gregariously or in response to biofilms, but the hypothesis has not been tested with larvae that exhibit a stringent requirement for prey- or host-derived settlement cues. Using the opisthobranch Alderia sp., we tested how age, nutritional status and prior experience affected larval response to settlement cues from the host alga Vaucheria longicaulis. Although able to metamorphose without feeding, larvae of Alderia sp. were facultatively planktotrophic if phytoplankton were available, allowing us to test the effects of energy limitation on dose-response. Unfed larvae became increasingly responsive to dissolved settlement cues with age, whereas the sensitivity of fed larvae did not change over time; larvae thus responded more strongly to habitat cues as energy reserves diminished. Rate of metamorphosis also accelerated with age; following initial settlement, 4 d old larvae completed metamorphosis 12 to 24 h faster than 1 d old larvae. Larvae may settle and then continue to evaluate their environment via perception of chemical cues, with older larvae initiating metamorphosis more rapidly. Prior exposure to a subthreshold dose of the natural cue triggered a higher response to a subsequent exposure in initial non-responders. Some families exhibited a lower threshold dose than others, suggesting that cue sensitivity may have a heritable component. Habitat choice thus depends on interactions between the genetic background, prior experience and energy content of a given larva. Our results support a modified version of the original desperate-larva hypothesis applicable to nongregarious species, in which energy-depleted larvae accept weaker cues and respond more quickly during habitat selection.
Article
Dispersal polymorphisms are known from many terrestrial organisms that inhabit fluctuating environments, but they are not well-recognized among marine invertebrates. An unusual form of variation was found in the settlement behavior of lecithotrophic larvae of the mollusc Alderia modesta (Loven, 1844) that markedly altered the dispersal potential of sibling larvae. Most clutches contained, in variable proportions, larvae that spontaneously metamorphosed in the egg mass or within 2 d of hatching, and larvae that delayed metamorphosis until encountering the obligate adult food, the yellow-green alga Vaucheria longicaulis. The proportion of larvae undergoing spontaneous metamorphosis within a clutch was highly variable, ranging from 0 to 90 %, and was unrelated to adult size. The proportion of intracapsular metamorphosis was also variable, ranging from 0 to 32%. The percentage of spontaneous metamorphosis per clutch was phenotypically plastic: when adults were starved, there was a significant tendency for each successive clutch to have a lower percentage of spontaneous metamorphosis, thus increasing the dispersal potential of the offspring. The decrease in spontaneous metamorphosis was significant after only 24 h of starvation. There was no significant increase in the cumulative level of spontaneous metamorphosis after the 2nd day post-hatching, but larval mortality increased as a linear function after the 5th day. Most larvae that did not spontaneously settle in the first 2 d delayed metamorphosis until they were exposed to the adult host alga, V; longicaulis. When 2 d old larvae were exposed to 17 species of macroalgae and sediment from the adult habitat, only V. longicaulis induced significant metamorphosis (93.3 +/- 6.7 %); all other species of algae and field sediment induced no metamorphosis or low levels (0-9 %) that were not significantly higher than sea water controls. There was no difference in the settlement specificity of 2 and 9 d old larvae when tested against 2 different algae, nor in their ability to complete metamorphosis. The high initial percentage of spontaneous metamorphosis reduces the dispersal potential of some larvae in each clutch relative to their siblings that delay metamorphosis until stimulated by V. longicaulis; this settlement dimorphism represents a novel bet-hedging strategy among marine invertebrates.
Article
Life-history parameters were used to estimate the dispersal potential of 1021 marine macroinvertebrates recorded in species lists from 91 sites comprising rocky intertidal, subtidal, kelp forest, sandy beach, and soft-bottom habitats in Washington, Oregon, and California. Mean species richness was significantly greater in the California rocky subtidal habitat. Data on development mode, planktonic larval duration, rafting potential, and adult mobility were compiled, and summaries of the dispersal potentials of taxa within each habitat type were generated and compared. In summary, development mode was known or estimated for 76% of species; larval planktonic duration for 49%; adult mobility for 76%; and rafting potential for 46%. In comparisons of species' life-history traits among habitats, sand-dominated habitats were distinct from rocky habitats. In rocky habitats, ∼42% of species had planktonic feeding larvae, 43% had planktonic nonfeeding larvae, and 15% had nonplanktonic larvae. Sandy intertidal habitats had higher proportions of taxa with nondispersing, nonplanktonic larvae and lower proportions of planktonic feeding and non-feeding larvae than all other sites. Soft-bottom subtidal communities had the highest proportion of taxa with planktonic feeding development and larvae with planktonic lifespans >30 d. Species in soft-bottom subtidal sites, therefore, have the greatest potential for extensive larval dispersal, whereas species in soft-bottom intertidal sites have the least potential for larval dispersal. In these sites with limited larval dispersal potential, there is greater potential for adult dispersal through adult movement and rafting. These differences in the dispersal potential of larvae and adults suggest that the effect of environmental changes and the effectiveness of reserves may differ between habitats. Conservation methods, including the use of marine reserves, must therefore be tailored to the habitat of interest if effective protection of community resources is to be achieved.
Article
How and why populations fluctuate and what drives the magnitude of fluctuations are questions that have long intrigued ecologists. Dispersal may dampen population fluctuations through the effect of spreading offspring over heterogeneous habitats. The planktonic period common in many marine organisms, therefore, could dampen population fluctuations through larval dispersal. However, emphasis on the hazards of planktonic life predicts that species that have a longer planktonic period have greater fluctuations in adult populations than species with shorter or no planktonic period. I analyzed the population variation of 570 time series from the literature for intertidal and benthic subtidal marine species and found that time series of adult populations for species with no planktonic period had greater fluctuations than time series for species with a planktonic period (both short and long planktonic period). In addition, there was no difference in fluctuation of adult and recruit time series between species with long and short planktonic periods. The planktonic period did not appear to result in increased population fluctuations, as was widely assumed. Rather, the planktonic period may-be acting to decrease population fluctuations, potentially by dispersal.
Article
Develops an extension of Schaffer's (1972) model for a simple life history with constant annual adult and juvenile survival rates, distinguishing between total effort and effort per offspring. Optimal effort per offspring depends only on the minimal effort per offspring and the rate at which the juvenile survival rate increases with increasing effort per offspring. Optimal total effort depends on effort per offspring, the ratio between the realized juvenile survival rate and the maximal adult survival rate, and the threshold at which increasing total effort leads to sharp declines in the adult survival rate. -from Authors
Article
The functions of plant and insect propagule size are considered in the context of environmental heterogeneity. Qualitative phenetic characteristics associated with individuals and populations, such as development time, reproductive capacity, and survival probability, are generally correlated with propagule size. Optimal propagule size, i.e., that which confers increased fitness, varies with environmental conditions. Thus, the traditional view that progeny from large propagules are qualitatively superior or more vigorous is subject to qualification. Phenetic variation appears to result in production of progeny that are able to cope effectively with unpredictable but recurrent instabilities in their environment.
Article
Examines evolutionarily stable dispersal strategies under the assumption that larger investment in the dispersal mechanism, which facilitates seed dispersal, inevitably results in reducing the total number of seeds produced. The strategies that have been considered are the amount invested in the dispersal apparatus and the realized rate of dispersal (ie the proportion of the progeny that leaves the parental site), assuming that dispersal rate is an increasing function of the amount invested in dispersal. Thus, whenever the cost of the dispersal mechanism restricts the total number of seeds produced by the plant, the evolutionarily stable dispersal strategy implies that a smaller, yet always positive, fraction of the progeny should be dispersed. -from Authors
Article
1. It is unlikely that individual selection simply for increasing distance between juveniles or between juveniles and parents would produce the patterns of reproduction and large-scale dispersal exhibited by many benthic invertebrates with pelagic larvae. However, these patterns are compatible with the hypothesis that pelagic larval phases of several weeks are the result of individual selection for spreading sibling larvae. 2. Selection for spread of siblings as a fixed feature of the life cycle depends on survival and reproduction in the benthic phase of life and in early and late larval stages varying independently from time to time and place to place. Data on benthic populations is apparently insufficient to determine whether or not such variation occurs. However, information on horizontal diffusion rates gained from dye diffusion experiments indicates the geographic scale of independent variation required by the hypothesis. 3. Rate of diffusion in the sea increases with time (or size of diffusing patc...
Article
StreblospiobenedictiWebster, a small tube-dwellingpolychaetecommon in Pacific, GulfofMexico, and Atlantic estuaries ofNorth America, exhibits both lecithotrophic and planktotrophic modes of larval development. In lecithotrophic forms females produce few (9-50) large ova (100â€"200@tm diam.). These develop in dorsal pouches into 9â€"12 setiger larvae, competent to settle at release. Females of planktotrophic forms produce large broods (100â€"548) of small ova (70â€"90im), brood larvae in dorsal pouches or beneath dorsal branchiae, and release 3-7 setiger larvae which bear long swimming setae and feed in the plankton for 1-5 weeks before settling. Lecitho trophy is reported for S. benedicti populations on all three coasts of N. America, planktotrophy from the Atlantic and Gulf coasts only. Reproductive differences ob served in the field are maintained by laboratory cultures reared under constant (20°C) conditions, though individuals from planktotrophic and lecithotrophic populations areinterfertile.Developmental variationsobservedin the fieldarebelievedto generate different patterns of dispersal, recruitment, population growth (r), and mortality. Poicilogony, the occurrence of multiple development modes, may account for the considerablesuccess of S. benedicti in N. America.
Article
Although a tendency for high latitude marine invertebrates to avoid pelagic larval stages was first described in the 19th century, the most detailed early study was that of Thorson in Greenland. This work also established other features of the reproduction of polar marine invertebrates that have become regarded as almost axiomatic (e.g., the release of larvae to coincide with the summer bloom) or largely ignored (a latitudinal cline in egg size within species). This short and selective review examines Thorson's conclusions in the light of recent work. It is now clear that although polar prosobranch gastropods reproduce almost entirely by direct development, for many taxa the real distinction between polar and non-polar species is in the proportion of feeding to non-feeding larvae. Some species release feeding larvae in winter and the energy source for these larvae is obscure. Growth is slow and there is little or no evidence for temperature compensation. Many crustacean species have larger eggs at higher latitudes. Egg size varies significantly within species, with larger eggs being associated with larger females and often reduced fecundity. The reasons for these within-species patterns are currently unresolved.
Article
Mitochondrial d-loop sequences were analyzed to characterize the phylogeographic and population genetic structure of the northern clingfish (Gobbiesox maeandricus Girard). Sequence analysis of 378 bp from 111 individuals sampled in 14 localities along the northeast Pacific coast and within the Strait of Georgia from 1996 to 1999 revealed marked genetic differentiation (Φct=0.247) among regional population groupings. The gene genealogy distinguished two major clades of haplotypes separated by at least 1.1% sequence divergence. One clade with very low haplotype diversity (h=0.2095, n=18) occurred only within the recently unglaciated Strait of Georgia. The other clade had high haplotype diversity (h=0.8808, n=93) and was found in all populations. High haplotype diversity was found in open coastal populations, both north and south of the maximum extant of the Wisconsin ice sheet, suggesting that the clingfish range was not pushed to a southern refugium during the last glacial maximum. A nested clade analysis also did not detect a large northward expansion from a single southern refugium. The level of sequence divergence and coalescent-based analyses suggest that the observed patterns of polymorphism are the result of Pleistocene diversification within multiple refugia, followed by population expansion and asymmetrical lineage introgression.
Article
Interest in pelagic embryos and larvae of benthic marine animals has centered on dispersal and its consequences, but longer pelagic larval durations appear to have evolved and to persist for other reasons than dispersal advantages. Here are explanatory hypotheses for three kinds of pelagic development. (1) Pelagic development prior to ability to feed or settle avoids constraints of oxygen supply to aggregated embryos and scarcity of deposition sites. Also, costs of protection may be greater and survival benefits less than indicated by present evidence. (2) Obligately feeding pelagic larvae migrate to the plankton for greater growth or lower mortality. Under this hypothesis, the observed large scale dispersal is an incidental byproduct of a migration from benthic to pelagic habitat for greater growth or survival. Given pelagic development, unevenly favorable benthic environments and varying currents might result in selection for spread of sibling larvae as a form of bet hedging, but selection for dispersal as a result of pelagic development does not explain evolution of pelagic development. (3) For larvae released near competence for settlement and metamorphosis, the function of the pelagic stage is dispersal and habitat selection. For these larvae, the observed precompetent pelagic periods of seconds to a day may indicate pelagic durations adapted for dispersal because offspring could be protected until release at competence.