In a world with rapidly changing ecosystems and redistributions of biodiversity, it is becoming increasingly important to understand how traits and adaptions allow certain species to colonise or even dominate new habitats. The success of exotic, invasive species is often broadly ascribed to a lack of natural predators or built-in competitive advantages such as voracious eating and high reproductive output. However, such opportunistic traits also make newly established large populations of exotic species vulnerable to extinction during sudden changes in environmental conditions and food availability. If we are to predict the long-term impacts of exotic species on local ecosystems, especially for invasive species with demonstrable environmental and economic impacts, it is crucial to explore the mechanisms by which these species overcome periodic food scarcity and environmental stress. Understanding these behaviours and adaptations is important in order to model the expansion of these taxa and develop more appropriate species-specific management strategies. The invasive Comb Jelly, Mnemiopsis leidyi, is an exemplar of a widespread and prolific marine invasive predator with a variety of opportunistic traits including bloom-andbust population dynamics and rapid growth. As these Jellies compete with fish and fish larvae they can cause cascading effect on crucial planktonic food webs and disrupt commercial fisheries. A remarkable aspect of the spread of this species across Eurasian waters is that it is now distributed in habitats with much longer low-feeding periods (58°N) than in its native habitat along the Atlantic coast of North America (42°N). Yet, Mnemiopsis leidyi is holoplanktonic with no known benthic resting eggs, cysts, or specialized overwintering stages. Given that Mnemiopsis leidyi is a fast shrinking species with a low reserve capacity, it is critical that the adult population maintains its nutrient reserves during late season. Surprisingly, however, adults appear to invest massively in reproduction at this time despite no or low survival chances of their progeny. Larvae stop growing, shrink in size, and die after three weeks of full starvation. In Mnemiopsis leidyi’s northernmost invasive habitats, there is a clear absence of larvae in winter and spring. Hence, high reproductive investments during late season seems counterproductive, yet Mnemiopsis leidyi populations are able to survive most winters within their current range in northern Eurasian waters.
In a paper published in the journal Communications Biology on 7 May 2020, Jamileh Javidpour of the Department of Biology at the University of Southern Denmark, Juan-Carlos Molinero of MARine Biodiversity, Exploitation and Conservation and the University of Montpellier, Eduardo Ramírez-Romero of the Fish Ecology Group at the Instituto Mediterráneo de Estudios Avanzados, Patrick Roberts of the Max Planck Institute for the Science of Human History, and the School of Social Sciences at the University of Queensland, and Thomas Larsen, also of the Max Planck Institute for the Science of Human History, test the hypothesis that Mnemiopsis leidyi periodically resort to cannibalism to meet nutritional needs.
Cannibalism has been largely ignored as a putative strategy for nutritional supplementation in studies of gelatinous zooplankton, both in native and exotic habitats. Javidpour et al. carried out high frequency field observations of both prey and predators covering prebloom to post-bloom periods of Mnemiopsis leidyi in the south-western Baltic Sea, from August to October, to estimate ingestion rates relative to availability of prey and to understand how abiotic and biotic factors affect the population dynamics of Mnemiopsis leidyi. To investigate whether cannibalism only occurs under special conditions in the field, Javidpour et al. also incubated Mnemiopsis leidyi adults and larvae together in the laboratory using stable isotope labeling to track the ingestion of larvae. Both Javidpour et al.'s laboratory and field results support that adult Mnemiopsis leidyi cannibalise their larvae. A shift from interspecific to intraspecific predation in late summer allowed the adult population to build its nutrient reserves during a period where their basal metabolism is very high. Javidpour et al. posit that this strategy confers a fitness advantage in regions with long and cold winters, and that cannibalism may be a key trait behind Mnemiopsis leidyi’s success in establishing permanent populations in its northernmost exotic habitats.
Mnemiopsis leidyi and their prey were sampled in Kiel Fjord. which is a 17 km long inlet of the south-western Baltic Sea. Javidpour et al. (2020).
Javidpour et al. carried out a daily high frequency sampling of Mnemiopsis leidyi and their prey in Kiel Fjord in August and September 2008, the period before and after bloom collapse. Abundance and total specimen size of Mnemiopsis leidyi peaked around the 245th day of the year (1 September), seven days after the abundance peak of mesozooplankton (Copepods), the main prey for adults, but ten days before the peak of microzooplankton (Tintinnids, Ciliates and Dinoflagelates), the main prey for larval Mnemiopsis leidyi. Javidpour et al. did not observe an increase in the relative abundance of larvae to adults in response to increased microzooplankton availability. Assuming that Mnemiopsis leidyi needs more than 20–25 μg Copepod carbon per litre to support population growth, prey abundance was sufficient to support exponential growth of the Mnemiopsis leidyi population until it peaked around the 1 September. During the growth and collapse phases, the relationship between the adult and larval Mnemiopsis leidyi abundances followed two different trajectories. While the abundance relationship between Mnemiopsis leidyi adults and larvae was linear during the growth phase, adult abundances increased during the phase when both juvenile Mnemiopsis leidyi and zooplankton prey abundances collapsed.
To confirm the potential for cannibalism under laboratory conditions, Javidpour et al. incubated adult Mnemiopsis leidyi together with nitrogen¹⁵ enriched Mnemiopsis leidyi larvae in September 2016. After 36 hours of incubation, the nitrogen¹⁵ concentrations of adults fed nitrogen¹⁵ enriched larvae were significantly higher than adults from the control treatment. In terms of biomass gain from feeding on larvae, the consumed larvae provided 4.1 % carbon and 2.5 % nitrogen of the total elemental content of each adult. The discrepancy between carbon and nitrogen fractions can be explained by the higher carbon:nitrogen ratio of the larvae than the adults. In a control treatment without adults, Javidpour et al. detected zero larval mortality during the 36 hour incubation.
To obtain evidence of cannibalistic behavior in the natural environment, Javidpour et al. photographed field sampled adults within 30 minutes after sampling. These photographs were taken in the laboratory of the GEOMAR Helmholtz Centre for Ocean Research in Kiel, during the Mnemiopsis leidyi bloom collapse in September 2008. The photographs show two larvae inside the auricles of an adult. Javidpour et al. rule out postcapture larval ingestion because the mesh size for capturing the adults was too big for larvae.
Mnemiopsis leidyi larvae (next to red arrows) located within the auricles of an adult. The specimens were captured September 29, 2008 in Kiel Fjord during population sampling of adult Mnemiopsis leidyi and photographed approximately 30 min later in the laboratory of the GEOMAR Helmholtz Centre for Ocean Research in Kiel. Javidpour et al. (2020).
According to Javidpour et al.'s estimates of ingestion rates, the shift from predominantly interspecific to intraspecific predation decreased the daily rations of adults. However, the rations during this postbloom phase remained sufficiently high, around 10–20% body carbon per day, to sustain the adult population for an additional 2–3 weeks. By comparison, predation on Copepods yielded daily rations of up to 50% of body carbon per day during the peak of the Copepod bloom, day 237, which is in line with former observations. Javidpour et al. employed Structural Equation Modeling, because it identifies both direct and indirect drivers of Mnemiopsis leidyi seasonal population growth, as well as their relative importance. The Structural Equation Modeling results showed that increased temperature and food availability, i.e., microzooplankton abundance, were the leading drivers of Mnemiopsis leidyi population growth. By contrast, the bloom decline was primarily associated with decreased temperature and cannibalism. Javidpour et al.'s observational data show that the Copepod supply was depleted soon after the Copepod population peaked and that Mnemiopsis leidyi adults then shifted to feed on their larvae.
To Javidpour .'set al knowledge, they have presented the first unequivocal evidence that adult Mnemiopsis leidyi cannibalise their own larvae. This finding fills an important gap in knowledge as to how an invasive species exhibiting boom-and-bust behavior is able to survive long periods of nutrient scarcity. By shifting to cannibalising their own larvae after emptying the prey field, adults can continue their growth. This behavior provides Mnemiopsis leidyi with the possibility of outcompeting intraguild species (e.g. Pleurobrachia pileus) by feeding on a wide size range of prey and, at the same time, enables it to build up nutrient reserves under unfavorable conditions. Mnemiopsis leidyi populations are otherwise vulnerable to local extinctions during this period because typically sized adults have energy reserves for 9 days at 20°C. Since basal metabolic rates are reduced exponentially with temperature, adults have energy reserves for up to 80 days at 3°C. Hence, provided that Mnemiopsis leidyi adults maintain their biomass well after the bloom collapse, they have sufficient reserves to survive long periods of low feeding under cold water temperatures. The fact that Mnemiopsis leidyi larvae function as nutrient and energy reserves may provide an explanation. as to why some bloom forming species are well adapted to dramatic population fluctuations in contrast to other prolific species that appear to become more vulnerable to extinction under increasingly variable conditions.
Cannibalistic behavior in Animals often adheres to a common set of rules: juveniles are eaten more frequently than adults; the behavior is concurrent with a decrease in alternative forms of nutrition; and a decrease in population density directs intraspecific prey to the fittest individuals during times of food scarcity. While cannibalistic behavior is a common ecological phenomenon in the Animal Kingdom, Javidpour et al.'s study of Mnemiopsis leidyi is rare in that cannibalism becomes detectable after the total collapse of the Copepod, i.e. the prey, population, ruling out the possibility that adults and juveniles were competing for prey. In fact, adult Mnemiopsis leidyi imay even enhance food availability for their larvae by consuming microzooplankton predators. Javidpour .et al. argue that the population dynamics between adults and their larvae necessitates the need for ecologists and conservationists to study Mnemiopsis leidyi populations as a coherent whole, an intergenerational, multicellular organism. By investing massively in reproduction during late summer Mnemiopsis leidyimay increases its ability to empty the prey-field across different size classes and built up nutrient reserves. Since larvae cannot survive winters in the species’ northernmost habitats, our study suggests that the primary purpose of Mnemiopsis leidyi larvae is to gather and store energy and nutrients for adults. This strategy is akin to autophagy within multicellular organisms during starvation periods, the process through which many insects and mammals use fat bodies as nutrient and energy reserves for overwintering. However, in order to fully understand the relative costs of adults to invest in reproduction versus the energy they gain from larval cannibalism, it would be necessary to characterize the costs of egg production for field sized adults (roughly 20 mm length), as well as the energy larvae gain by predating small size zooplankton.
The identification of this strategy has major implications for understanding the adaptive capacities of Mnemiopsis leidyi and designing appropriate conservation strategies that can control its spread. In the Black Sea, the introduction of the native predator Beroe ovate in the late 1990s decreased absolute Mnemiopsis leidyi abundance. Nevertheless, the efficiency of Mnemiopsis leidyi’s prey capture technique has meant that its impact on interspecies zooplankton continues to be substantial during late summer bloom periods. Similarly, it is likely that eutrophication and over-fishing of zooplanktivorous Fish prior to Mnemiopsis leidyi arrival in the Black Sea exacerbated the ferocity of boom-and-bust population dynamics of this invasive species. A range of measures such as decreasing
eutrophication and commercial fishing of forage Fish (to increase intraguild competition) should be considered to curb Mnemiopsis leidyi’s dominance and its ability to establish permanent populations within its current northernmost invasive range. Javidpour et al.'s dataset also has significant implications for Mnemiopsis leidyi dynamics within its native range. Many of the environmental conditions that have made Eurasian locations physiologically favorable for Mnemiopsis leidyi invasion, warming temperatures and increased eutrophication, also exist in the species’ native areas. Increasing disturbance of marine environments suggests that the negative impact of Mnemiopsis leidyi on ecosystem services may also become an increasing problem in the species’ native habitats as it is highly likely that it can turn its cannibalistic boom-and-bust strategies on and off in changing environmental circumstances. As well as further documenting the ubiquity of cannibalism in the animal kingdom and its far-reaching consequences for native and exotic ecologies, Javidpour et al.'s dataset also provides some hints regarding the evolutionary origins of cannibalism, whether it is a product of convergent evolution or a basic metazoan trait. The earliest evidence for Metazoan cannibalism can be traced to the Cambrian period where one specimen of Ottoia, a Priapulid, had a proboscis of another Ottoia preserved in its gut. Cannibalism may even have played a role in the evolutionary transition from single cell to multicellular organisms because Sponges, Porifera, have specialized cells that eat other cells in times of food scarcity. Javidpour et al. demonstrate that cannibalism is an important trait for a member of Ctenophora, but whether it is a basic or derived metazoan trait remains an open question until more conclusive evidence of cannibalism or lack hereof can be obtained from other Ctenophora taxa. Hence, more research characterising the role of cannibalism in a comparative context is essential for understanding the life history strategies of Ctenophores and for the evolutionary origins of cannibalism.
Characterising life history strategies of Ctenophores is crucial for understanding the origins of Metazoan traits, as well as for developing more accurate predictions of the future economic and ecological impacts of prolific species. Mnemiopsis leidyi is probably the most-studied Ctenophore genus in the world because of its great abundance next to densely populated areas in its native habitats, and because of its widespread impact on zooplankton and ichthyoplankton following its invasion of Eurasian waters. Javidpour et al.'s study suggests that cannibalism is vital for Mnemiopsis leidyi adults to overcome prey scarcity during a critical period when its basal metabolic rate is high. Without this strategy, Mnemiopsis leidyi would be more likely to become locally extinct during the long and cold winters in its exotic habitats. The extent to which this behavior confers a fitness advantage on Mnemiopsis leidyi adults to invest in reproductive over somatic growth under different conditions requires further parametrisation in terms of interspecific predation, exploitation of different prey fields, and costs of egg production. Given the increasing disturbance of marine environments and spread of exotic species, Javidpour et al.'s finding is important for devising more effective conservation strategies. Javidpour et al.'s data encourage ecologists and conservationists to compare individual adaptive traits across the population structure of a given species in order to explore how this might contribute to its range expansion and its impact upon other competing taxa. Furthermore, Javidpour et al.'s findings highlight cannibalism as a basal, but variably expressed, trait of the Animal Kingdom that can increase fitness and adaptability within a variety of demographic and environmental contexts.
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The identification of this strategy has major implications for understanding the adaptive capacities of Mnemiopsis leidyi and designing appropriate conservation strategies that can control its spread. In the Black Sea, the introduction of the native predator Beroe ovate in the late 1990s decreased absolute Mnemiopsis leidyi abundance. Nevertheless, the efficiency of Mnemiopsis leidyi’s prey capture technique has meant that its impact on interspecies zooplankton continues to be substantial during late summer bloom periods. Similarly, it is likely that eutrophication and over-fishing of zooplanktivorous Fish prior to Mnemiopsis leidyi arrival in the Black Sea exacerbated the ferocity of boom-and-bust population dynamics of this invasive species. A range of measures such as decreasing
eutrophication and commercial fishing of forage Fish (to increase intraguild competition) should be considered to curb Mnemiopsis leidyi’s dominance and its ability to establish permanent populations within its current northernmost invasive range. Javidpour et al.'s dataset also has significant implications for Mnemiopsis leidyi dynamics within its native range. Many of the environmental conditions that have made Eurasian locations physiologically favorable for Mnemiopsis leidyi invasion, warming temperatures and increased eutrophication, also exist in the species’ native areas. Increasing disturbance of marine environments suggests that the negative impact of Mnemiopsis leidyi on ecosystem services may also become an increasing problem in the species’ native habitats as it is highly likely that it can turn its cannibalistic boom-and-bust strategies on and off in changing environmental circumstances. As well as further documenting the ubiquity of cannibalism in the animal kingdom and its far-reaching consequences for native and exotic ecologies, Javidpour et al.'s dataset also provides some hints regarding the evolutionary origins of cannibalism, whether it is a product of convergent evolution or a basic metazoan trait. The earliest evidence for Metazoan cannibalism can be traced to the Cambrian period where one specimen of Ottoia, a Priapulid, had a proboscis of another Ottoia preserved in its gut. Cannibalism may even have played a role in the evolutionary transition from single cell to multicellular organisms because Sponges, Porifera, have specialized cells that eat other cells in times of food scarcity. Javidpour et al. demonstrate that cannibalism is an important trait for a member of Ctenophora, but whether it is a basic or derived metazoan trait remains an open question until more conclusive evidence of cannibalism or lack hereof can be obtained from other Ctenophora taxa. Hence, more research characterising the role of cannibalism in a comparative context is essential for understanding the life history strategies of Ctenophores and for the evolutionary origins of cannibalism.
Characterising life history strategies of Ctenophores is crucial for understanding the origins of Metazoan traits, as well as for developing more accurate predictions of the future economic and ecological impacts of prolific species. Mnemiopsis leidyi is probably the most-studied Ctenophore genus in the world because of its great abundance next to densely populated areas in its native habitats, and because of its widespread impact on zooplankton and ichthyoplankton following its invasion of Eurasian waters. Javidpour et al.'s study suggests that cannibalism is vital for Mnemiopsis leidyi adults to overcome prey scarcity during a critical period when its basal metabolic rate is high. Without this strategy, Mnemiopsis leidyi would be more likely to become locally extinct during the long and cold winters in its exotic habitats. The extent to which this behavior confers a fitness advantage on Mnemiopsis leidyi adults to invest in reproductive over somatic growth under different conditions requires further parametrisation in terms of interspecific predation, exploitation of different prey fields, and costs of egg production. Given the increasing disturbance of marine environments and spread of exotic species, Javidpour et al.'s finding is important for devising more effective conservation strategies. Javidpour et al.'s data encourage ecologists and conservationists to compare individual adaptive traits across the population structure of a given species in order to explore how this might contribute to its range expansion and its impact upon other competing taxa. Furthermore, Javidpour et al.'s findings highlight cannibalism as a basal, but variably expressed, trait of the Animal Kingdom that can increase fitness and adaptability within a variety of demographic and environmental contexts.
See also...
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