Increasing resilience, one cannibal at a time

 

Javidpour, J., Molinero, J.C., Ramirez-Romero, E., Roberts, P., Larsen, T. 2020. Cannibalism makes invasive comb jelly, Mnemiopsis leidyi, resilient to unfavourable conditions. Comm. Biol. https://doi.org/10.1038/s42003-020-0940-2

 

A jelly by any other name

Ctenophores, also known as comb jellies, sea gooseberries, or Venus’ girdle, are a globally distributed phylum of marine invertebrates that range in size from mm’s to 1.5 m’s. Despite looking like jellyfish, they are only distantly related. Like jellyfish, comb jellies have a soft, gelatinous body that is often transparent. Unlike jellyfish, they swim mouth-forward, beating their rows of comb-like cilia. Used for feeding and locomotion, the combs scatter light creating a rainbow effect (Fig. 1).

 

 

A warty walnut

Mnemiopsis leidyi, the warty, or walnut, comb jelly, is a species native to the western Atlantic, that has invaded European and Asian regions. This slow-moving species has a lobed body, four rows of combs, and is hermaphroditic. The species also undergoes bloom-and-bust population cycles, and has a transient anus that appears as needed!

M. leidyi populations diminish quickly and have no known benthic resting stages or overwintering strategies. In its newly invaded range, M. leidyi persists >15º further north (58ºN) than its native range (42º), making retaining nutrient stores critical to winter survival. This is why scientists were surprised to discover that invasive comb jellies were using (rather than storing) precious resources to produce late-season offspring. Offspring, which were more than likely to die off in a few weeks.

 

 

 

The Study

Javidpour et al. conducted a mixture of field and laboratory studies in the Kiel Fjord, south-western Baltic Sea (the invasive range). In the field, they observed and sampled at a high frequency during pre-bloom and post-bloom periods. After each tow, samples were fixed, counted, and measured. In the lab, adults and were incubated with 15N enriched larvae to track feeding (isotopic labeling).

In the lab, adults had increased levels of 15N, relative to the control group. In the field, photographs taken while sampling show intraspecific larvae within the adults’ lobes. In other words, both study methods provided strong evidence that M. leidyi cannibalize their larvae, but only after other resources have diminished.

 

But why?

Researchers hypothesize that producing offspring – while resources are abundant – is an overwintering strategy.

1. Juveniles feed on microplankton
2. Adults feed on mesoplankton (predators of the larvae)
3. Doubled resource utilization draws down stocks, outcompeting other species
4. Resources are depleted
5. Juveniles die
6. Adults have another nutrient source (their well-fed babies)

 

Cannibalistic behaviour has not been documented in native populations of M. leidyi. It is, therefore, possible that cannibalism is an adaptation allowing the invasive population to survive at the higher latitudes they have invaded.

 

Golden Rules of Cannibalism

While there are numerous documented instances of cannibalism throughout the animal kingdom, this strange phenomenon typically follows three general rules.

1. Juveniles consumption is most frequent

2. Occurrence coincides with a decrease in other food resources

3. Fittest individuals have greater success