Cnidarians

Introduction

Cassiopea - Overview

Commonly known as the upside-down jellyfish, Cassiopea is a benthic scyphozoan (Rhizostomeae) found in tropical and sub-tropical shallow coastal ecosystems, as mangroves and seagrass beds. At least one species from the Red Sea, C.andromeda, is now commonly recorded in the warmer areas of the Mediterranean Sea. The species-level relationships of the genus Cassiopea are still unresolved and up to date only ten species are considered, based on molecular data. Cassiopea spp. has a unique behavior, laying down on the seafloor on the exumbrellar side, with an upside orientation of the oral arms extended into the water column towards the sea surface. Cassiopea has been recently gaining attraction in many fields of research (e.g., embryology, behavior, ecology) and presented as a model organism.

For some species, the life cycle has been described, looking at the morphology of the different life stages and studying abiotic factors triggering developmental switches. Natural and synthetic inducers were used to investigate mechanisms leading to the settlement of the planula larva and its metamorphosis into the polyp stage. At this stage, a symbiotic association is established between the polyp and microalgae (Symbiodiniaceae) commonly found in the water column. The polyp-microalgae association is required for the onset of the asexual generation of juvenile medusa stage of (monodisk strobilation) driven by specific environmental inputs. Molecular tools have also been used to study developmental patterns as the involvement of Hox genes in the oral-aboral axis formation.

Cassiopea also represents an excellent model to study mutualistic symbiosis. Jellyfish (but also polyps) provide a sheltered habitat—rich in nitrogen and phosphorus catabolites—to microalgae that, in turn, release organic carbon products (e.g., sugars) to their host. Thus, Cassiopea offers an approachable system for the study of nutritional symbiosis. In addition, the benthic behavior and bell pulsation of the upside-down jellyfish are suggested to facilitate mobilization of nutrients from the sediment pore water and bell contractions generate water currents that facilitate feeding and nutrient exchange. Thanks also to its simple neuromuscular system, Cassiopea can be used to investigate the local fluid and nutrient dynamics of a habitat. Moreover, through bioturbation, Cassiopea plays an important role in benthic-pelagic coupling of nutrient cycles.

The upside-down jellyfish has also been presented as a possible model organism for the study of behavioral biology by recent research in sleep behavior, while studies on its venom highlighted the possible utilization of this jellyfish for novel pharmacological and therapeutic agents extraction. Cassiopea spp. is considered as a biomonitor/bioindicator species of the biological water quality of coastal ecosystems, particularly in areas exposed to metal and organic loads, able to detect short pulses of contaminants/pollutants. Overall, Cassiopea’s prolific asexual reproduction together with its broad tolerance to variations in temperature, salinity and light make this jellyfish a potential successful invader, as it seems to be the case of C. andromeda in the Mediterranean Sea.

Cassiopea life cycle and ecology

Cassiopea as a model organism

To follow Marta’s current research: https://www.researchgate.net/profile/Marta-Mammone

Cold-water corals - overview

Cold-water corals are also known as deep sea corals. It is a very broad group that encompasses corals with cold water preference, including stony corals, soft corals, and black corals. They can be found in cold waters (4-12°C), which are usually found in deeper parts of the ocean (approx. below 200m). They lack symbionts (zooxanthellae) and therefore they are heterotrophs, actively feeding on marine snow coming from the euphotic zone or catching zooplankton. They are long living animals, some reaching several thousand years of age. These corals build animal forests, complex habitats that serve as nurseries, feeding grounds, mating grounds or provide good settlement for many species from bacteria to sharks.

Interest in cold-water corals has increased over the years due to new exploration cruises as well as long time series in remote ecosystems such as submarine canyons and seamounts. The knowledge on cold-water corals is expanding and bringing new questions on the table in different areas of biology and ecology.

Establishing a controlled, laboratory reared population that can be called a model organism for cold-water corals would be a major step forward, but there are several hurdles that need to be overcome. Desmophyllum pertusum (previously known as Lophelia pertusa) has been studied the most and could probably become the model organism in the future. Nevertheless, we keep a broad perspective including all cold-water corals during this module.

This module will give you basic understanding of coral anatomy and physiology, as well as how to perform growth measurements and respiration incubations. On top, you will get familiar with some cold-water corals from Blanes Canyon, how they build animal forests and what are some basic characteristics of cold-water corals.

Cold-water corals – Potential model organisms

Cold-water corals – Notes on ecology

Cold-water corals – Methods for measuring growth and respiration