Biology of Oysters
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Biology of Oysters

Brian Leicester Bayne

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eBook - ePub

Biology of Oysters

Brian Leicester Bayne

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About This Book

Biology of Oysters offers scientific insights into the structure and function of oysters. Written by an expert in the field of shellfish research, this book presents more than 50 years of empirical research literature. It provides an understanding of the edible oysters, in order to equip students and researchers with the background needed to undertake further investigations on this model marine invertebrate.

  • Presents empirical research findings in context with the relevant theory and its expression in computer models
  • Includes information on studies of other bivalve species such as mussels and clams
  • Offers a description of the whole organism to provide a frame of reference for further research
  • Includes research developments in the phylogeny, physiology and ecology of oysters

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Year
2017
ISBN
9780128035009
Chapter 1

Phylogeny

B.L. Bayne

Abstract

Attempts to discern the phylogeny of groups of organisms can be traced from the use of allozymes (enzymes encoded by different alleles of the same locus), thence of targeted genes in the mitochondrial morphological criteria, and nuclear genomes, and the current phase of phylogenomics in which multiple gene loci and even entire genomes can be searched for their application in phylogenetic reconstruction. At each step in this progress there has been the challenge of incongruence between different markers in the phylogenetic patterns that are resolved statistically, between morphological and molecular characters, and between genetic loci. This chapter discusses these issues with reference to both fossil and living oysters, describes the phylogenies that have been proposed, and reviews the role of phylogenetic analysis in understanding their phylogeography.

Keywords

Phylogeny; Morphological and Molecular Markers; Ostreoida; Phylogeography; Hybrid Zones
Within certain limits, defined by the fact that the shell consists of two hinged valves, oysters are among the most plastic organisms known
Gunter (1954)
The oysters are probably the most difficult single group of macrofossils to classify
Cleevely and Morris (1987)

1.1 Introduction

The view expressed by Cleevely and Morris (1987) and quoted above is widely shared by others who would welcome a coherent and enduring system of classification that includes both fossil and living oysters. Stenzel (1971) wrote of a “…protean diversity of form in the family Ostreidae, displayed even within well-defined, geographically and genetically isolated, living oyster species [and which] is not amenable to easy classification.” It is now commonplace for research papers to start with a similar remark. For example, the taxonomy of oysters “… is difficult and often inaccurately determined because of the high level of phenotypic plasticity of the shell morphology” (Xia, Wu, Xiao, & Yu, 2014); this is often followed by a proposal for a new or modified phylogeny based on molecular characters. The use of molecular sequences has expanded rapidly as more potential markers of genealogical relationships have been identified, and the technology that can recognize and analyse their distribution amongst taxa has developed in parallel. Attempts to discern the phylogeny of groups of organisms can be traced from the use of allozymes (enzymes encoded by different alleles of the same locus), thence of targeted genes in the mitochondrial and nuclear genomes, and the current phase of phylogenomics in which multiple gene loci and even entire genomes can be searched for their application in phylogenetic reconstruction (Dunn et al., 2008; Boore & Fuerstenberg, 2008). At each step in this progress there has been the challenge of incongruence between different markers in the phylogenetic patterns that are resolved statistically, between morphological and molecular characters, and between genetic loci.
In this chapter I discuss aspects of the search for clarity in the phylogeny of oysters, in which morphological and molecular characters have been employed both separately and together. The heuristic power of molecular phylogenetics is impressive; the morphological and ecological contexts in which phylogenetic resolution is set are rich and complex; and the search for the relevant evolutionary processes involves knowledge across a wide range of research endeavor—the fossil record, speciation, life history, larval dispersal, population structure, and the geography of oyster distributions. Multilocus genomic analysis is bringing new clarity to phylogenetic relationships within the Mollusca in general (Smith et al., 2011) and the Bivalvia in particular (Bieler et al., 2014; Plazzi, Ceregato, Taviani, & Passamonti, 2011; González et al., 2015) and with the promise of doing so for families of oysters. Phylogenetic analysis at any taxonomic level (among populations, species and/or higher taxa) is an exercise in hypothesis setting. Testing predictions for genealogical relations inevitably involves the underlying ecological and evolutionary processes that determine the proposed phylogenetic pattern. And resolving these relationships will continue to challenge an understanding of the biology of oysters, in all its aspects. The likely benefits include the conservation of stocks and maintenance of biodiversity, and in understanding the evolutionary trends in this important group of animals.

1.2 Origins and Classification

Identifying the earliest oysters has proved to be difficult and controversial. This is not surprising, given the paucity of fossils of the appropriate age and the limited number of definitive morphological characteristics that can be deduced from them. The most relevant geological period, the early Triassic some 250 million years ago, has been described by Fortey (1997) as a dark age of geological time when fossils are hard to come by and well-preserved oysters or their putative ancestors are rare (Hautmann, 2006). By the Jurassic 40 my later the fossil record for oysters is better. Considerable plasticity of shell shape is a common feature of oysters and their close relatives, however, and taxonomic analyses based only upon shell shape are unreliable. To quote Stenzel (1971) again: “The first prerequisite in oyster classification is the availability of ample material.” There are relatively few characters other than shape to work with when trying to interpret a fossil's taxonomic position. Of the features considered diagnostic (listed in Panel 1.1) only those associated with shell mineralogy, shell ornamentation, the form of the hinge and ligament, impressions left on the inside of the shell by muscle insertions, and evidence of whether attachment to the substrate was by the left or the right valve, are normally available. And even these few features are made difficult to categorize a priori as either ancestral or derived due to the common occurrence within the Bivalvia of homomorphy (similar morphologies in unrelated groups), convergence between unrelated taxa, and secondary loss of anatomical features.
Panel 1.1
A Diagnosis and Classification of Oysters (Based on Hautmann, 2001; Littlewood, 1994)
Diagnosis. (The terms in italics are defined in the table).
Oysters are monomyarian bivalves which are cemented by their left valve to the substrate in the postlarval stage (i.e., they are “pleurothetic”). Settling from a planktonic to a benthic habit, and the accompanying metamorphosis, mark the end of the larval stage, soon after which the foot and byssus are lost and subsequent attachment is by cementation which is mediated by the mantle. A pallial line is missing except in Recent Saccostrea in which a disjunct line of pallial muscle insertions is present. The ligament is alivincular-arcuate; the resilium is completely fibrous and continuous between valves; the ligament area has a broad resilifer, which is elevated on the right valve. Bourrelets are narrow, hinge teeth lacking. Postlarval shells of modern forms comprise an outer layer of simple prismatic calcite and a middle and inner layer of foliated calcite which may develop structural chambers. Aragonite is restricted to the myostracum and ligostracum in post-Triassic forms, but occurred as an inner shell layer in Triassic species. Gills are pseudolamellibranch.
Classification (Littlewood, 1994; Carter et al., 2011)
Phylum Mollusca
Class Bivalvia
Subclass Pteriomorphia
Order Ostreida
Superfamily Ostreoidea
Family Ostreidae
Subfamily Ostreinaea
Tribe Ostreini [Ostrea3]
Family Flemingostreidae
Subfamily Crassostreinaea
Tribe Striostreini [Saccostrea1]
Tribe Crassostreini [Crassostrea2]
Common names used here are: 1, rock oysters; 2, cupped oysters; 3, flat oysters.
a These are the “true” oysters and the main subject of th...

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