Aggregation

Abstract

Multicellularity evolved multiple times independently during eukaryotic diversification. Two distinct mechanisms underpin multicellularity: clonal development (serial cell division of a
single precursor cell) and aggregation (in which independent cells assemble into a multicellular entity). Clonal and aggregative development are traditionally considered to be mutually exclusive and to result from independent acquisitions of multicellularity. Here, we show that the choanoflagellate Choanoeca flexa, a close relative of animals that forms contractile monolayers of cells (or “sheet colonies”), develops by an unconventional intermediate mechanism that we name “clonal-aggregative multicellularity”. We find that C. flexa sheets can form through purely clonal processes, purely aggregative processes, or a combination of both, depending on experimental conditions. To assess the ecological relevance of these findings, we characterize the natural context of multicellular development in the native environment of C. flexa on the island of Curaçao. We show that the C. flexa life cycle is environmentally regulated by extreme salinity fluctuations in splash pools undergoing cycles of evaporation and refilling. Upon desiccation, C. flexa colonies dissociate into drought resistant quiescent cells, which resume activity and reform multicellular sheets after rehydration. We hypothesize that clonal-aggregative development reflects selection for fast transitions into and out of multicellularity in the ephemeral context of coastal splash pools. Our findings underscore the potential of the exploration of biodiversity for revealing new fundamental biological phenomena and expand the known option space for both multicellular development and for the origin of animal multicellularity.

The purpose of this research was to determine the level of defensive aggression of the threespot damselfish, Stagastes planifrons, when exposed to intruders of different species using models. Adult S. planifrons were exposed to models of a conspecific, an herbivorous fish, Sparisoma viride, and a predator, Aulostomus maculatus. Attack rates and retreat rates of S. planifrons were determined by observations during exposure to models. It was expected that aggression levels would be highest towards the conspecific model and lowest towards the predator model and that evasive behavior would be highest in the presence of the predator model and lowest with the conspecific. It was found that there is a significant difference in the level of aggression when encountering a predator versus a conspecific, showing more aggression towards the conspecific and more evasion towards the predator. No significant difference was found in the aggression levels shown between the predator and the herbivore. Exposure to the predator elicited the highest number of retreats, also showing a significant difference in the level of evasion when comparing all three models. The results suggest that S. planifrons are able to differentiate between intruder species and react depending on the level of threat posed and perhaps on what is most energy efficient.

This student research was retrieved from Physis: Journal of Marine Science III (Spring 2008)19: 13-18 from CIEE Bonaire.

Given the amount of CO2 currently being absorbed by the ocean, there is a great deal of research studying the effects of ocean acidification on a variety of species. Considering the relationship between pH and levels of calcium present in the ocean water, the healing process of Ventricaria ventricosa is hypothesized to be negatively affected by the decreased pH that is projected for the ocean. V. ventricosa is a green alga (Chlorophyta) and one of the largest unicellular organisms. When punctured, V. ventricosa forms an aggregation ring around the wound which contracts in order to heal the membrane. This research measured the healing rate of V. ventricosa in present day ocean water (pH=8.05) as well as acidified ocean water (pH less than 7.0). Individuals of V. ventricosa in present pH water conditions were able to heal the puncture wound within 120 minutes, while the individuals in acidified ocean water were not able to heal themselves within the same time frame. It is unknown whether the V. ventricosa would eventually heal themselves over a longer period of time or given a greater volume of ocean water; however it is apparent that the decreased levels of calcium in the acidified water had a negative effect on the healing process of V. ventricosa. Ocean acidification is likely to affect the basic biological functioning of a variety of marine life, which will face severe difficulties adapting to the acidified ocean water.

This student research was retrieved from Physis: Journal of Marine Science XIII (Spring 2013)19: 33-38 from CIEE Bonaire.

Abudefduf saxatilis, or sergeant major damselfish, are a common reef fish in the Caribbean and western Atlantic that form large feeding aggregations. Abudefduf saxatilis are primarily planktivorous, with zooplankton making up over 50% of their diet. Zooplankton are known to have diel movements to avoid predation, which have been shown to be triggered by the presence of ultra violet radiation. Beneath boats along the coast of Bonaire, aggregations of A. saxatilis have been observed, but why they prefer these areas over the open water column had not previously been examined. The abundance of zooplankton was estimated beneath boats as well as in the open water, up-current from the boats. Both the abundance and bite rates of A. saxatilis were also estimated beneath the boats that corresponded to the estimates of zooplankton abundance. In addition, the bite rates of A. saxatilis were estimated in the open water. It was found that the zooplankton abundance (p<0.001) and the bite rate of A. saxatilis (p<0.001) were both significantly greater beneath boats than in the open water. Also, a significant correlation was found between increasing abundances of zooplankton and A. saxatilis (p<0.01). These results demonstrate that one of the main drivers for the aggregation of A. saxatilis beneath boats is likely to feed on the zooplankton, which are in high abundance. In turn, this could alter community structure on the reef due to a decrease in the amount of algae grazing by A. saxatilis

This student research was retrieved from Physis: Journal of Marine Science XVII (Spring 2015)19: 51-57 from CIEE Bonaire.

Abstract:

Here we establish a neotype for Alatina alata (Reynaud, 1830) from the Dutch Caribbean island of Bonaire. The species was originally described one hundred and eighty three years ago as Carybdea alata in La Centurie Zoologique—a monograph published by René Primevère Lesson during the age of worldwide scientific exploration. While monitoring monthly reproductive swarms of A. alata medusae in Bonaire, we documented the ecology and sexual reproduction of this cubozoan species. Examination of forty six A. alata specimens and additional archived multimedia material in the collections of the National Museum of Natural History, Washington, DC revealed that A. alata is found at depths ranging from surface waters to 675 m. Additional studies have reported it at depths of up to 1607 m in the tropical and subtropical Atlantic Ocean. Herein, we resolve the taxonomic confusion long associated with A. alata due to a lack of detail in the original description and conflicting statements in the scientific literature. A new cubozoan character, the velarial lappet, is described for this taxon. The complete description provided here serves to stabilize the taxonomy of the second oldest box jellyfish species, and provide a thorough redescription of the species.