Contrasting life cycles of Southern Ocean pteropods alter their vulnerability to climate change

Pteropods are a key part of biogeochemical cycling and epipelagic food webs in
the Southern Ocean. However, shelled pteropods are vulnerable to climate
change, due to their aragonite shells being particularly sensitive to ocean
acidification. Currently our understanding of pteropod responses to
environmental change is hindered by uncertainties surrounding their life cycles
and population dynamics. In this study, we describe polar shelled pteropod
diversity in the north-eastern Scotia Sea, inferring life history and population
structures of the dominant pteropod species, Limacina rangii (formerly Limacina
helicina antarctica) and Limacina retroversa. An annual timeseries of Limacina
shell morphometrics was derived from individuals collected in a moored
sediment trap at 400 m depth. We found that L. rangii and L. retroversa have
contrasting life history strategies. L. rangii has a continuous spawning and
recruitment period from November to March and can overwinter as juveniles
and adults. L. retroversa has discrete spawning events from November to May,
producing non–overlapping cohorts of juveniles and adults. Their development
to the adult stage takes between two and five months, upon which they
overwinter as adults. Our findings suggest different vulnerabilities of L. rangii
and L. retroversa to a changing ocean. For example, since all life stages of L. rangii co-exist, vulnerability of one cohort is not detrimental to the stability of the overall population whereas, if one L. retroversa cohort fails to recruit, the entire population is threatened. Changes in pteropod populations could have
cascading ramifications to Antarctic ecosystems and carbon cycling.