Show your beaks and we tell you what you eat: Different ecology in sympatric Antarctic benthic octopods under a climate change context

Abstract

Sympatry can lead to higher competition under climate change and other environmental pressures, including in South Georgia, Antarctica, where the two most common octopod species, Adelieledone polymorpha and Pareledone turqueti, occur side by side. Since cephalopods are typically elusive animals, the ecology of both species is poorly known. As beaks of cephalopods are recurrently found in top predator's stomachs, we studied the feeding ecology of both octopods through the evaluation of niche overlapping and specific beak adaptations that both species present. A multidisciplinary approach combining carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope signatures, mercury (Hg) analysis and biomaterials' engineering techniques was applied to investigate the beaks. An isotopic niche overlap of 95.6% was recorded for the juvenile stages of both octopod species, dropping to 19.2% for the adult stages. Both A. polymorpha and P. turqueti inhabit benthic ecosystems around South Georgia throughout their lifecycles (δ 13 C: −19.21 ± 1.87‰, mean ± SD for both species) but explore trophic niches partially different during adult life stages (δ 15 N: 7.01 ± 0.40‰, in A. polymorpha, and 7.84 ± 0.65‰, in P. turqueti). The beaks of A. polymorpha are less dense and significantly less stiff than in P. turqueti. Beaks showed lower mercury concentration relative to muscle (A. polymorpha-beaks: 0.052 ± 0.009 μg g −1 , muscle: 0.322 ± 0.088 μg g −1 ; P. turqueti-beaks: 0.038 ± 0.009 μg g −1 ; muscle: 0.434 ± 0.128 μg g −1). Overall, both octopods exhibit similar habitats but different trophic niches, related to morphology/function of beaks. The high Hg concentrations in both octopods can have negative consequences on their top predators and may increase under the present climate change context.

Antarctic octopod beaks as proxy for mercury concentrations in soft tissues

“…Data regarding Hg concentrations in Antarctic octopods is still scarce (Matias et al, 2019) as previous studies mainly focused on squid species and analysed few tissues (Anderson et al, 2009;Cipro et al, 2018;Seco et al, 2020;Xavier et al, 2016). However, both A. polymorpha and P. turqueti had higher T-Hg concentrations relative to the data on T-Hg levels in Antarctic squids from previous studies (Table 3).…”

“…The reason for that is beaks are capable of absorbing and creating strong links with Hg due to its high specificity for amino groups present in proteins (Barriada et al, 2008), with Hg varying greatly as differences in beak pigmentation reflect different protein content (Miserez et al, 2008). Similarly to nitrogen stable isotope signatures accompany the gradual formation of the beaks (Cherel & Hobson, 2005;Chouvelon et al, 2011), it was also found recently that Hg increases from the rostrum towards the crest which is the most recent and metabolically active region of the beak (Queirós et al, 2020) Within Antarctic cephalopods, the octopod species Adelieledone polymorpha (Robson, 1930) and Pareledone turqueti (Joubin, 1905) are good bioindicators of the health of South Georgia's benthic ecosystems, as both species are short-lived and the most abundant cephalopod benthic species in the region (Barratt et al, 2008;Collins et al, 2004;Matias et al, 2019). These two species occupy the same habitat throughout their lifecycle (i.e.…”

“…Specimens from the Antarctic octopods A. polymorpha (n = 25) and P. turqueti (n = 20) were collected on board of the Dorada and SIL vessels during bottom trawling surveys in the South Georgia region, between January and February of 2004 (A. polymorpha: n = 9; P. turqueti: n = 10; adopted from Matias et al, 2019), of 2013 (A. polymorpha: n = 6; P. turqueti: n = 5) and 2017 (A. polymorpha: n = 10; P. turqueti: n = 5). All the samples were preserved at -20°C throughout the study.…”

“…Only recently, researchers have been exploring the usage of beaks as a tool to evaluate Hg concentrations on cephalopods (Matias et al, 2019;Nguyen et al, 2018;Xavier et al, 2016). The reason for that is beaks are capable of absorbing and creating strong links with Hg due to its high specificity for amino groups present in proteins (Barriada et al, 2008), with Hg varying greatly as differences in beak pigmentation reflect different protein content (Miserez et al, 2008).…”

“…These two species occupy the same habitat throughout their lifecycle (i.e. sympatric) while exploiting different trophic niches in function of the adaptations in their digestive apparatus, such as their beak morphology (Allcock et al 2003;Daly & Rodhouse, 1994;Matias et al, 2019;Piatkowski et al, 2003). Both species are also the main prey for some top predators (e.g.…”