Introduction

Decapod crustaceans are dominant species in benthic communities on tropical and temperate continental shelves, however their diversity is low in polar environments, particularly in high latitude zones of the Antarctic (Gorny, 1999). Large, durophagous (crushing) predators have been absent from the Antarctic continental shelf for over 14 million years (Aronson et al., 2007; Hall & Thatje, 2010). King crabs, also known as stone crabs, belonging to the family Lithodidae, have been absent from these cold water regions due to physiological constraints as adults and larvae have lower thermal limits of 0.4-2⁰C (Aronson et al. 2007; Hall & Thatje, 2010) to enable successful metabolic processes. Anomuran lithodid crabs are constrained by low temperature however they seem to be the most cold-resistent reptant decapod (Hall & Thatje et al., 2009). The southernmost limit of lithodid distribution is on the continental shelf and slope of the western Antarctic Peninsula (WAP) located in the Bellingshausen Sea (figure 1) where the temperature does not fall below 0°C (Thatje et al., 2008). Below this temperature, crustaceans are unable to regulate magnesium in their bodily fluids which can lead to narcosis, clumsiness and paralysis of breathing (Thatje & Arntz, 2004).

Figure 1. Seas of the Southern Ocean surrounding Antarctica. (Nickpo, Wikimedia Commons, 2014)

Figure 1. Seas of the Southern Ocean surrounding Antarctica. (Nickpo, Wikimedia Commons, 2014)

Due to climate change, the West Antarctic Peninsula (WAP) surface sea water temperature has risen by 1⁰C over the past 50 years and is currently increasing by 0.01-0.02⁰C every year (Smith et al., 2012), allowing temperature-selctive taxa to colonise the area (Aronson et al. 2007). Currently, 14 species of lithodid crabs are present in the Southern Ocean north of latitudes 60⁰S and recent studies have shown their distribution to spread as far as  67.5⁰S in the Bellingshausen Sea and 75⁰S in the Ross Sea (Hall & Thatje, 2010; Thatje & Arntz, 2004). An invasion of two species of lithodid crab, Neolithodes sp. and Paralomis sp. have been been recorded on the continental slope of the WAP itself, supposedly due to adult immigration or through the desperal of demersal larve (Aronson et al., 2007; Hall & Thatje, 2010).

Due to Antarctic cooling approximately 15 million years ago, faunal communities failed and species diversity decreased, which led to impovishment of food sources and the eventual extinction of lithodids (Thatje et al., 2005). The adjustment to colder temperautres relies on the successful supply of oxygen, which in brachyurans and some lithodids, this success is contrained due to the ability to regulate magnesium in their haemolymph. Currently the soft substrates of Antarctica are dominated by epifaunal suspension-feeders and slow moving invertebrates such as echinoderms (Arntz et al., 1992) which are the top predators in the benthic food webs (Aronson & Blake, 2001). It has been suggested that due to the lack of durophagous predators in Antarctic shelf ecosystems for approximately 30-40 million years, faunal communities would be very vulnerable to the presence of such predators (Aronson et al. 2007; Thatje et al., 2008; Hall & Thatje, 2010)

This blog aims to describe the present distributions of lithodid and brachyuran crabs based on physiological constraints that prevent them from inhabiting polar regions. The potential future distributions for these crabs will be discussed and a conclusion will be given at the end to determine the effect these crabs could have on benthic food webs.

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