Magnetic resonance imaging as a means to assess the body growth and the gonad development of the oysterCrassostrea gigas

Abstract

-To make a preliminary exploration of the possibilities and limits of magnetic resonance imaging (MRI) for assessing body growth and gonad development of the Pacific oyster, Crassostrea gigas, individual monitoring was conducted on diploid and triploid oysters in their second year, maintained from February to October in a pond on the French Atlantic coast. Magnetic resonance imaging was done on each oyster on five dates during the study period. From these images, variations in the volumes of the flesh, digestive gland-gonad and adductor muscle, and the surface of the gills were measured over time. Apart from the well known differences between diploids and triploids related to maturation of gametes, other original observations of assessing growth were made from these MRI measurements using a non-invasive technique. The standard error of mean dry flesh variation was lower than that found by other commonly used measurement methods. A negative correlation was established between the volume of the gametes spawned and the increase in dry flesh volume after spawning. Moreover, these variations were correlated with the chemical composition of the adductor muscle. All these observations and conclusions need to be confirmed by further observations on oysters of different sizes or grown in different environments.Key words: MRI / Magnetic resonance imaging / Growth / Gonad development / Oyster / Crassostrea gigas Résumé -Pour explorer les possibilités et les limites de l'imagerie en résonance magnétique (IRM) sur la croissance corporelle et le développement des gonades de l'huître, Crassostrea gigas, des suivis individuels ont été effectués sur des huîtres diploïdes et triploïdes âgées de 2 ans, élevées en bassins de février à octobre sur les côtes Atlantiques françaises. L'IRM a été effectuée sur chaque huître, à cinq dates durant la période d'étude. Les variations de volume de la chair, de la glande digestive et du muscle adducteur, et de la surface des branchies ont ainsi été mesurées. A part les différences bien connues entre huîtres diploïdes et triploïdes liées à la maturation des gamètes, d'autres observations originales sur la croissance ont été effectuées à partir de ces mesures au moyen de l'IRM, technique non-invasive. L'erreur standard sur les variations de la chair sèche sont plus faibles que celles trouvées par les méthodes classiques. Une corrélation négative est observée entre le volume des gamètes émis et l'accroissement du volume de chair sèche après la ponte. De plus, ces variations sont corrélées avec la composition chimique du muscle adducteur. Ces observations et conclusions demandent à être confirmées par d'autres observations sur des huîtres de différentes tailles ou cultivées en conditions environnementales différentes.

Gonad volume assessment in the oyster Crassostrea gigas: Comparison between a histological method and a magnetic resonance imaging (MRI) method

“…The pixels within this polygon were considered as belonging to the gonad, and their number was multiplied by the unit surface of the pixel. Preliminary trials, done using the same method in previous years (Davenel et al, 2006, Hatt et al, 2009, concluded that the grey-level for mature gonads on MRI was between 166 and 510. On the grey-level histograms established on whole oyster body soft tissues with the methods described in subsection 2.2, all the voxels above 165 were counted as a part of the mature gonads.…”

“…Considering the large investment oysters make in reproduction (Bourles et al, 2009, Deslous-Paoli and Héral, 1988, Pouvreau et al, 2006b) and the observation of selected oyster lines with different gonad average sizes and different susceptibilities to summer mortality (Samain et al, 2007, Huvet et al, 2010, the gonad was considered a priority organ for study. Further research using the MRI method highlighted a positive relationship between the effort devoted to reproduction and subsequent growth of the tissues and showed that growth variability within a group of oysters was overestimated by the more commonly used histological method (Hatt et al, 2009).…”

Evaluation of Mud Worm (Polydora spp.) Infestation in Cupped (Crassostrea gigas) and Flat Oyster (Ostrea edulis) Broodstocks: Comparison between Magnetic Resonance Imaging and Computed Tomography

“…Previous studies on MRI reported that this technique was able to quantify the volume and mass of the whole flesh and of some organs in the cupped oyster [20][21][22] and in the flat oyster [23]. However, there are too few studies comparing different non-destructive techniques to determine the damage caused by the Polydora worm.…”