Behavioral repertoire of high-shore littorinid snails reveals novel adaptations to an extreme environment

Transcriptome reveal gene regulation mechanisms of the barnacle Chthamalus challengeri for microhabitat adaption in the intertidal zone

BACKGROUND

Microhabitat environmental factors (e.g., temperature, oxygen concentration, nutrients, osmotic stress, and topography) are critical to the survival of intertidal organisms. Understanding how transcription responses are regulated in relation to intertidal microhabitat variation has important implications for studying adaptive evolution in these organisms. The barnacle Chthamalus challengeri, which survives in the intertidal zone and is subjected to periodic tidal changes, serves as an ideal species for detecting adaptive evolution in intertidal organisms.

RESULTS

In this study, we designed a series of in situ tidal conditions for C. challengeri and sequenced their transcriptome collected from various microhabitats. We aimed to detect the genetic adaptation mechanisms of barnacles responding to the microhabitat changes in the intertidal zone based on comparative transcriptomics. Our results indicated that different intertidal microhabitats significantly affected the gene expression models of C. challengeri, particularly for genes related to physiological and biochemical functions. Specifically, the expression of genes such as CYP450, HSP70, CYTB, and COX1 was significantly increased under low tide (air-exposed conditions), while genes like CNGA3, AK, and CP52 showed significantly increased expression under high tide (seawater-immersed conditions).

CONCLUSION

The results suggest that C. challengeri relies on cytochrome p450 enzymes to enhance oxidative capacity, counts on heat shock proteins and cell phagocytosis to resist microhabitat changes in response to different tidal conditions, and produces a hypoxic stress response to regulate energy metabolism and body temperature changes upon entering into seawater. This study provides genetic resources and clues for investigating the adaptation mechanisms of intertidal barnacles and identifies different gene expression models for C. challengeri responding to various microhabitats.

Coping with harsh heat environments: molecular adaptation of metabolic depression in the intertidal snail Echinolittorina radiata

Harsh thermal environments in the rocky intertidal zone pose serious physiological and molecular challenges to the inhabitants. Metabolic depression is regarded as an energy-conserving feature of intertidal species. To understand the molecular mechanism of metabolic depression, we investigated physiological and transcriptomic responses in the intertidal snail Echinolittorina radiata. The metabolic rate and expression of most genes were insensitive to temperatures ranging from 33 to 45 °C and then increased with further heating to 52 °C. Different from other genes, the genes involved in heat shock response (HSR) and oxidative stress response (OSR) (e.g., genes encoding heat shock protein 70 (HSP70) and cytochrome P450 protein (CYP450)) kept upregulating during metabolic depression. These high levels of HSR and OSR genes should be important for surviving the harsh thermal environments on the rocky shore. In the population experiencing more frequent moderate heat events, the depression breadth was larger, and the change in magnitude of upregulation was insensitive for HSR genes (e.g., HSP70s) but heat-sensitive for OSR genes (e.g., CYP450s) at the temperature of 37 to 45 °C. These findings indicate that both the thermal sensitivity of HSR and OSR genes and the insensitivity of metabolic genes are crucial for surviving extreme intertidal environments, and different populations of the same species rely on various physiological mechanisms to differing extents to deal with heat stress. The cellular stress response is not a "one size fits all" response across populations largely depending on local thermal regimes.

Behavioural adaptation to heat stress: Shell lifting of the hermit crab Diogenes deflectomanus

Behavioural responses to heat and desiccation stress in ectotherms are crucial for their survival in habitats where environmental temperatures are close to or even exceed their upper thermal limits. During low tide periods when pools in intertidal sediments heat up, a novel shell lifting behaviour (when hermit crabs crawl out of pools and lift up their shells) was observed in the hermit crab, Diogenes deflectomanus, on tropical sandy shores. On-shore measurements revealed that the hermit crabs left pools and lifted their shells predominantly when pool water exceeded 35.4 °C. Standing on emersed substrates above the pool water, the hermit crabs maintained their body temperatures at 26 - 29 °C, ∼ 10 °C lower than temperatures at which their physiological performances (as measured using heart rate) reached the maximum. This mismatch between preferred body temperatures and temperatures at maximal physiological performance was also observed under a laboratory controlled thermal gradient, where hermit crabs spent more time at 22 - 26 °C as compared to > 30 °C. These behaviours suggest a thermoregulatory function of the shell lifting behaviour, where the hermit crabs can avoid further increase in body temperatures when pools heat up during low tide periods. Such a behavioural decision allows the hermit crabs to be less prone to the strong temporal fluctuation in temperatures experienced during emersion periods on thermally dynamic tropical sandy shores.

The barnacle Chthamalus bisinuatus is the only sessile invertebrate colonizing oil patches on beachrocks one year after a massive oil spill on the Northeastern Brazilian coast

A large-scale oil spill has reached over 3000 km of the NE Brazilian coast since August 2019. The cause and origin of this spill remain mysterious, and the impacts on coastal ecosystems have not been clearly understood so far. Despite the efforts to remove the oil (mainly from local communities), oil stains are still present in beaches, mangroves, and beachrocks. In this short report, we describe the occurrence of the barnacle Chthamalus bisinuatus Pilsbry, 1916 colonizing oil spill stains on intertidal surfaces of beachrocks one year after the first oil records. We quickly assessed oil stains across three different reefs located at the Conde municipality, Bahia (NE Brazil), where the species was identified and its density on oil stains calculated. The occurrence of barnacles in oil stains was restricted to zones in the wake of the reefs. Their densities varied from 0 to 238 ind./dm², with an average of 34 ± 68 ind./dm². If we account for dead individuals (empty barnacle plates), they correspond to 25.9% of the sampled population. The presence of oil possibly affected barnacle survival rates but did not seem to prevent barnacle individuals from reaching adult sizes. We also found individuals of the snail Echinolittorina lineolata (d'Orbigny, 1840) crawling on these barnacles, indicating that the barnacle assemblages on oil stains are stable enough to provide refuge for these snails. It is not clear if the presence of barnacles on oil reflects the resistance of these crustaceans to the oil toxicity or is just a result of a low substrate selectivity by the cypris larvae.