Echinoderms and prochordates occupy a key position in vertebrate evolution. The genomes of sea urchin share 70% homology with humans. Researches on cell cycle in sea urchin and phagocytosis in asteroids have fetched Nobel Prizes. In this context, this book assumes immense importance.
Echinoderms are unique, as their symmetry is bilateral in larvae but pentamerous radial in adults. The latter has eliminated the development of an anterior head and bilateral appendages. Further, the obligate need to face the substratum for locomotion and acquisition of food has eliminated their planktonic and nektonic existence. Egg size, a decisive factor in recruitment, increases with decreasing depths up to 2,000-5,000 m in lecithotrophic asteroids and ophiuroids but remains constant in their planktotrophics. Smaller (< 18 mm) ophiuroids do not produce a lecithotrophic eggs but larger (> 110 mm) asteroids generate planktotrophic eggs only.
Publications on sex ratio of echinoderms indicate the genetic determination of sex at fertilization but those on hybridization, karyotype and ploidy induction do not provide evidence for heterogametism. But the herbivorous echinoids and larvacea with their gonads harboring both germ cells and Nutritive Phagocytes (NPs) have economized the transportation and hormonal costs on gonadal function. Despite the amazing potential just 2 and 3% of echinoderms undergo clonal reproduction and regeneration, respectively. Fission is triggered, when adequate reserve nutrients are accumulated. It is the most prevalent mode of clonal reproduction in holothuroids, asteroids and ophiuroids. However, budding is a more prevalent mode of clonal reproduction in colonial hemichordates and urochordates.
In echinoderms, fission and budding eliminate each other. Similarly, autoregulation of early development eliminates clonal reproduction in echinoids and solitary urochordates. In pterobranchs, thaliaceans and ascidians, the repeated and rapid budding leads to colonial formation. Coloniality imposes reductions in species number and body size, generation time and life span, gonad number and fecundity as well as switching from gonochorism to simultaneous hermaphorditism and oviparity to ovoviviparity/viviparity.
NON-CHORDATE-DEUTEROSOMIA
ECHINODERMATA
Introduction
Taxonomy and Structural Diversity
Distribution, Locomotion and Dispersal
Population Density and Microhabitats
Energy Budget and Reproduction
Gonad Index and Fecundity
Egg Size and its Implications
Larval Development and Thyroid Hormones
Brooding and Viviparity
Size and Life Span
Fisheries and Aquaculture
Introduction
Capture Fisheries
Aquaculture
Sexual Reproduction
Introduction
Sexuality
Gonochorism
Parthenogenesis
Hermaphroditism
Reproductive Cycle
Asexual Reproduction
Introduction
Types and Characteristics
Fission and Reproduction
Clonal and Sexual Reproduction
Induced Fission
The Trigger
Clonal Autotomy
Larval Cloning
Eggs and Embryos
Searching Stem Cells
Regeneration
Introduction
Incidence and Prevalence
Induction of Autotomy
Causes and Consequences
Growth and Differentiation
Growth Factors and Genes
Sex Determination
Introduction
Species and Fidelity
Karyotypes and Heteromorphism
Ploidy Induction
Genome and Sequencing
Sex Differentiation
Introduction
Asteroid Model
Echinoid Model
Induction of Spawning
Endocrine Disruption
Parasitic Disruption
HEMICHORDATA
Reproductive Biology
Introduction
Life Cycles
Gonads and Consequences
Regeneration in Enteropneusts
CHORDATE DEUTEROSTOMIA
Cephalochordata
Introduction
Reproductive Biology
Urochordata
Introduction
Pelagic Tunicates
Benthic Tunicates
Germline Lineage
New Findings and Highlights
Introduction
Structure and Distribution
Fecundity, Size and Depth
Aquaculture: Sea Urchins and Cucumbers
Intromittent Organ
Gonad and Hormonal Economy
Regenerative Potential
Gonads of Cloners
Clonal Reproduction
Autoregulation and Stem Cells
Cloning and Coloniality
References