The Characterization of Haliotis Rufescens (Red Abalone): Mesolayer Growth

Abstract

The structure of an abalone’s shell, as well as its self-assembly process, has been the focus of a considerable amount of research over the years due to the exceptional properties it possesses. The self-assembly process creates highly ordered hierarchal structures and has become a popular alternative in the synthesizing of materials. The abalone undergoes a self- assembly process called biomineralization, which gives the abalone’s shell unique structural properties that are fascinating to researchers, and especially intriguing to those trying to replicate the structure. In depth analyses have been performed on an important portion within the shell, called the nacre or otherwise known as “Mother of Pearl”. Investigations into the nacre portion of the shell could help researchers in imitating this structure to produce materials with specific characteristics needed for applications in industry. The nacre is made up of aragonite tablets, which is a form of calcium carbonate. These aragonite tablets are stacked in a columnar-like structure, and connected by a thin organic layer in a ‘brick’ and ‘mortar’ type structure. Within the nacre, another structure occurs periodically, and is easily distinguishable in the structure of the abalone found in the ocean. This structure is called the mesolayer, and it has its own unique structure that differs from the nacre tablet structure. The mesolayer can be divided into three distinct layers, with each having their own unique structure: the columnar-like layer, organic layer, and prismatic layer. Even though extensive research has been done on the nacre’s tablet structure, the mesolayer has been given little consideration. The nacre grows in in a confined spaced created by the outer portion of the shell and a part of the abalone’s soft-tissue, which is called the mantle. Within the space between the shell and the mantle, the abalone secretes a colloidal mucus, and the nacre grows within that in an environment that is isolated from the surrounding elements. In order to examine the biomineralization process the abalone undergoes, substrates were implanted between the shell and the mantle using the Flat Pearl Method. This technique helped to better understand how the saltwater environment affected the nacre tablet growth, as well as the mesolayer growth. The abalones used in this research were purchased from an abalone farm in California, and integrated in to tanks filled with artificial saltwater, which is made to replicate the oceanic environment. To ensure the nacre and mesolayer growth would occur, the parameters of the saltwater and feeding were meticulously monitored to guarantee the environment was suitable for the abalone. The parameters tested for each tank were: temperature, pH, salinity, concentrations of Mg2+ and Ca2+, nitrate, nitrite, and alkalinity. The temperature was controlled to allow the formation of the mesolayer. In this research, many experiments were focused on the increase of temperature, since the multiple mesolayers created in the wild abalone are thought to mainly be due to seasonal temperature changes. After the substrates were implanted into the abalone, each tank started at an initial temperature for a few weeks, then the temperature was increased for a period of time, and finally brought back down to the initial temperature for the remaining of the testing cycle. Various temperatures and periods of time were used to try to distinguish how and when the mesolayer forms within the nacre. Even though an abundant amount of research has focused on the nacre structure, there are still many qualities that have yet to be revealed, and much deliberation on the nacre growth and how its exceptional properties influence the structure of the nacre. The research completed in this thesis aspired to expand the understanding of how the outside environment affects the nacre tablet growth, as well as the mesolayer.