Abstract
Marine organisms must be capable of adjusting to changing environmental conditions across a range of stressors and temporal and spatial scales. Attached sessile marine organisms face extra pressures that come with being incapable of moving from their habitat or avoiding unfavourable conditions. Brachiopods (Phylum Brachiopoda), a group that have remained virtually unchanged from their ancestors of 500 million years ago, have been particularly successful at adapting to major changes in ocean conditions across geological time. However, little research has been carried out on their physiological mechanisms in responses to rapid environmental changes, such as sedimentation events.
Calloria inconspicua and Liothyrella neozelanica (Subphylum Rhynchonelliformea; Order Terebratulida) are endemic to New Zealand waters and are ecologically important constituents of their environments. Populations collected from Doubtful Sound, Fiordland (L. neozelanica) and the Otago Harbour (C. inconspicua), representing two distinctly different environments, were subject to experiments with environmentally relevant levels (0 to 100 Nephelometric Turbidity Units: NTU) of water-borne sediment for up to seven days, along with a multi-month experiment sampling long-term growth of C. inconspicua. Behavioural (valve snapping), physiological (respiration and excretion), and whole-body responses (growth and survival) were investigated to determine at what level of water-borne sediment exposure becomes metabolically costly, detrimental and lethal. The research included short-term behavioural responses to sediment exposure; determination of metabolic responses to sediment exposure, through investigating respiration and excretion rates; and quantification of the effects of sediment exposure on long-term growth.
Seasonal findings for L. neozelanica showed very low activity levels during winter sampling in measures of behaviour, respiration, and excretion. Short-term behavioural responses to sediment treatments ranged between 0 and 41 valve ‘snaps’ per hour suggesting that both species of brachiopods actively adjust their behaviour to changing environmental conditions. Snapping (sudden valve closure) occurs when animals are disturbed, if light intensity is changed, or to expel mucus. The responses seen in this study indicate that both of these brachiopod species show some behavioural tolerance to short-term turbidity.
Ambient respiration rates of L. neozelanica ranged between 462.7 to 4425.9μg (O2) L-1 g (Ash Free Dry Weight: AFDW)-1 hr-1 while those of C. inconspicua ranged between 229.6 and 4690.9 μg (O2) L-1 g(AFDW)-1 hr-1. Comparisons between species responses were made during summer sampling. For both species, respiration rates decreased with increasing AFDW and size, while excretion rates showed a very different interaction, where small and medium sized L. neozelanica showed heightened excretion rates in comparison to C. inconspicua, whereas the largest brachiopods showed very similar excretion rates for both species and across all treatments. The experiments showed that sediment treatments had higher respiration rates than the control, with the 40 NTU treatment showing the highest rate. In combination, these results indicate that the largest brachiopods are most able to cope with changing sediment exposure, as their responses in all treatments were lowest and closest to the responses seen in the control.
Long-term growth rates of C. inconspicua among sediment treatments ranged between 0.021 to 0.266 mm/month, with lowest growth recorded in the 20 NTU treatment. In addition, growth rates tended to decrease with increasing shell width across all treatments.
These experimental findings provide insights into how increasing sediment exposure may affect brachiopods living in temperate oceans, as a result of anthropogenic causes. They suggest that exposure to sediment is detrimental to these brachiopod species, both in the short-term (days) and long-term (weeks to months), with a critical limit seen between 40 and 100 NTU. Sediment is becoming an increasing threat in coastal environments because of anthropogenic land use, and in combination with other environmental stressors, it is likely to become more challenging for specialised organisms such as brachiopods to survive in these changing conditions.