Sex Lives of Invertebrates• Asexual Reproduction• Sexual Reproduction• Hermaphroditism• External and Internal Fertilization• Complex Life Cycles• Life Histories• Asexual reproduction involves the formation of individuals whose genes all come from one parent.• There is no fusion of sperm and egg.• Sexual reproduction is the formation of offspring by the fusion of haploid gametes.• Ovum: female gamete - usually large and nonmotile.• Spermatozooan: male gamete - usually small and motile.• Sexual reproduction increases genetic variation among offspring.Both asexual and sexual reproduction occur among the invertebrates• Fission: asexual reproduction in which a parent separates into two or more approximately equal sized individuals.• Budding: asexual reproduction in which new individuals split off from existing ones.• Gemmules of sponges are an example of a type of asexual reproduction that involves the release of specialized cells that can grow into new individuals.Diverse mechanisms of asexual reproduction enable invertebrates to produce identical offspring rapidly• Fragmentation: the breaking of the body into several pieces, some or all of which develop into complete adults.• Requires regeneration of lost body parts.• Parthenogenesis: is the process by which an unfertilized egg develops into (often) a haploid adult.• Parthenogenesis plays a role in the social organization of species of bees, wasps, and ants.Male honeybees are haploid and female honeybees are diploid.• Advantages of asexual reproduction:• Can reproduce without needing to find a mate• Can have numerous offspring in a short period of time• In stable environments, allows for the perpetuation of successful genotypes.Diverse mechanisms of sexual reproduction• Separate sexes • Simultaneous hemaphroditism• Sequential hermaphroditism• Sexual reproduction in echinoderms usually involves the release of gametes by separate males and females into the seawater.– The radial adults develop by metamorphosis from bilateral larvae.• Sexual reproduction in sessile or burrowing animals or parasites who may have difficulty encountering a member of the opposite sex.– Hermaphroditism: one individual is functional as both a male and a female.• Some self-fertilize.• Most mate with another member of the same species.• Each individual receives and donates sperm.• Results in twice as many offspring as would occur if only one set of eggs were fertilized.• Sequential hermaphroditism: an individual reverses its sex during its lifetime.• Protogynous: female first sequential hermaphrodite.• Protandrous: male first sequential hermaphrodite (slipper shells (Mollucsa: Gastropoda).• Most sponges are hermaphrodites, with each individual producing both sperm andeggs.– Gametes arise from choanocytes or amoebocytes.– The eggs are retained, but sperm are carried out the osculum by the water current.– Sperm are drawn into neighboring individuals and fertilize eggs in the mesohyl.– The zygotes develop into flagellated, swimming larvae that disperse from the parent.– When a larva finds a suitable substratum, it develops into a sessile adult.• Sponges are capable of extensive regeneration, the replacement of lost parts.– They use regeneration not only for repair but also to reproduce asexually from fragments broken off a parent sponge.• Planarians can reproduce asexually through budding and regeneration.– The parent constricts in the middle, and each half regenerates the missing end.• Planarians can also reproduce sexually.– These hermaphrodites cross-fertilize.• Internal fertilization requires cooperative behavior that leads to copulation.Internal and external fertilization both depend on mechanisms ensuring that mature sperm encounter fertile eggs of the same species• External fertilization requires a moist habitat that will protect a developing egg from desiccation and heat stress.– Specific mating behaviors assure that sperm and egg will be in the same place at the same time.• Internal fertilization usually results in the production of fewer zygotes than does external fertilization.• However, the survival rate is lower for external fertilization than it is for internal fertilization.Species with internal fertilization usually produce fewer zygotes but provide more parental protection than species with external fertilization– Parental care ofoffspring may occurregardless of whetherfertilization is externalor internal.• The least complex reproductive systems lackgonads.• Polychaete worms lack gonads.• Eggs and sperm develop from undifferentiated cells lining thecoelom.• Some reproductive systems, such as that seen in parasitic flatworms, can be very complex.Complex reproductive systems have evolved in many animal phyla• Most insects have separate sexes with complex reproductive systems.• In many species the female reproductive system includes a spermatheca, a sac in which sperm may be stored for a year or more.• Reproductive cycles and life cycles are controlled by both environmental and hormonal cues.• Animals may be solely asexual or sexual.• Or they may alternate between the two modes depending on environmental conditions.• Daphnia reproduce by parthenogenesis under favorable conditions and sexually during times of environmental stress (Crustacea).Reproductive cycles and Life Cycles vary extensively among animalsComplex Life Cycles and Alternation of Generations• Most invertebrates have complex life cycles– They pass through morphologically distinct life stages such as planktonic larval stages before transforming into an adult that has a different form and might live in a different habitat.• Some invertebrate also have alternating generations that reproduce sexually or asexually.Complex Life Cycles• Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia.– The polyp stage, often a colony of interconnected polyps, is more conspicuous than the medusas.• Hydras, among the few freshwater cnidarians, are unusual members of the class Hydrozoa in that they exist only in the polyp form.– When environmental conditions are favorable, a hydra reproduces asexually by budding, the formation of outgrowths that pinch off from the parent to live independently.– When environmental conditions deteriorate, hydras form resistant zygotes that remain dormant until conditions improve.• Class Trematoda