Relating the acid-base compensatory strategies of brachyuran crabs to their environments and life histories

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Allen, Garett
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Brachyuran crabs are amongst the most habitat-diverse groups of animals on Earth. Different environments are associated with specific challenges that their inhabitants must overcome throughout their species’ history to maintain physiological homeostasis, including the pH of the body fluids. The frequency and magnitude of acid-base challenges are environmentally dependent with some ecosystems experiencing regular seasonal fluctuations such as inland waters or coasts, some having diurnal fluctuations as occur in intertidal zones, whereas marine environments may face irregular but relatively large shifts from coastal upwellings or permanent extreme stress near hydrothermal vents. Given that brachyurans permanently inhabit these ecosystems, their acid-base regulatory processes and stress tolerances are presumably broad making them an interesting animal group to study connections between a species’ life-history relates and physiology. Despite the diverse nature of brachyurans, their acid-base regulatory systems and stress tolerances are generally poorly understood. To date, only a few osmoregulatory-capable species are beginning to be characterized at the whole animal and mechanistic levels due to their historical use as osmoregulatory models. This dissertation serves to broaden our understanding of brachyuran physiology and identify their various means to cope with acid-base challenges. Although the antennal glands of osmoregulatory-capable species are considered to be of negligible importance to their acid-base regulation, those of a typical stenohaline osmoconforming crab, Metacarcinus magister, appear to compliment the gills by regulating systemic bicarbonate ions (HCO3-) and eliminating acid-equivalents. The shallow hydrothermal vent crab, Xenograpsus testudinatus was found to be extremely hypercapnia-tolerant being capable of immense HCO3- accumulation and excreting CO2 against inwardly-directed PCO2 gradients – both of which seem impossible in other investigated animals. The semi-terrestrial crab, Helice formosensis, relies on the antennal glands and branchial action to prevent the accumulation of nitrogenous wastes that would otherwise disrupt their acid-base homeostasis, with some indications that they use the alanine-glucose cycle which was previously believed to be absent in aquatic species. While these results create many open questions, they provide clear examples of different adaptations and even organ usage within the investigated brachyuran species, several of which are fundamentally different from popular model species such as Carcinus maenas and Callinectes sapidus.
Invertebrate, Gill physiology, Evolutionary physiology, Acid-base balance, Nitrogen excretion, Osmoregulation