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Developmental plasticity during the early life histories of marine invertebrates is a fascinating opportunity to study the interplay between ecology and evolution. In particular, some embryos and larvae initiate asexual reproduction while completing their development. This chapter examines the mode, frequency, and taxonomic diversity of asexual reproduction that occurs between the zygotic and the juvenile stages. Special attention is given to the phylum Echinodermata, where asexual reproduction during embryonic and larval development has been best studied. An emphasis is also placed on the factors that have been identified as likely inducers of asexual reproduction and an assessment of the likelihood that asexual reproduction is an adaptive response to these factors. Lastly, several key open questions are identified as potential avenues for future research about the causes and consequences of asexual reproduction by the developmental stages of marine invertebrates.

The dominant mode of reproduction in eucaryotes is sexual. This has been described as a paradox given that sex is much more costly than reproducing asexually, such as by parthenogenesis. In the Crustacea, parthenogenesis is commonly found in the Ostracoda and Branchiopoda (Artemia and Cladocera), and studies of these species have made important contributions to understanding the ecological and evolutionary relationship between sexual and asexual reproduction. With respect to parthenogenesis, researchers have explored its taxonomic distribution and phylogeny, origin and mode, ecological genetics, and genomic signatures. Parthenogenetic Crustacea include both diploid and polyploid clones that have originated multiple times from related sexual species but appear to have a relatively limited evolutionary lifespan. Darwinulid ostracods may be one exception, with no known sexual forms and possibly an example of ancient asexuality, although this is controversial. Most parthenogenetic crustacean groups appear to have a wider geographic distribution than related sexual species and are often found in marginal habitats associated with higher latitudes and altitudes. Such patterns of geographic parthenogenesis have yet to be fully explained, but could possibly be due to colonization and adaptation advantages of asexuality; further studies are required to eliminate polyploidy alone as an explanation. There are many examples of parthenogenetic ostracods, cladocerans, and Artemia showing high levels of genetic diversity likely due to recent multiple origins from related sexual species. Phylogenetic analyses support this explanation and for Artemia and Daphnia, cases have been documented for rare functional males produced by parthenogenetic females that can mate with sexual females as a mechanism for generating new clonal lineages. The diversity of asexual species, combined with prior ecological and genetic information, suggests that crustaceans will continue as important models for understanding parthenogenesis, particularly with the application of new genomic tools.

This chapter examines the resilience of arctic biota to climatic fluctuations and disturbance. The ability to repair decimated populations by sexual reproduction or, as in the case with plants, also asexually is all the more astonishing given the climatic and resource limitations of high-latitude habitats. This ability of the arctic biota to recover from near extinction may be attributed to the reduced inter-specific competition that can be enjoyed in the sparseness of the arctic landscapes. The chapter discusses reproduction at high latitudes; human demography in the Arctic; longevity at high latitudes; predator-prey interactions; phenology and reproduction; and plant reproduction.

This chapter discusses the reproduction of conifers. For conifers, pollination involves the transfer of pollen grains from a pollen cone to a seed cone, where they will fertilize the ovules (seeds before they are fertilized by pollen). The process is called wind-pollination, and the lightweight grains are easily borne on the gentlest of breezes. After a pollen grain comes into contact with an ovule, it grows a pollen tube that passes through a small hole in the ovule's integument, or outer skin. The tube carries a sperm cell right into the ovule itself, and fertilization is achieved. The remainder of the chapter covers the anatomical difference between seed cones and pollen cones, and asexual plant reproduction via vegetative reproduction.

The diagnostic features that characterize Ostracoda, or seed shrimp, include distinctness of each pair of limbs from each other, body displaying little or no segmentation (although individual taxa can exhibit sclerite features that suggest possibly up to 11 trunk somites), an all-encompassing bivalved shell that lacks growth lines, and location of an adult male copulatory limb or organ just anterior to the caudal rami. The World Ostracoda Database indicates approximately 34,660 accepted species. The shell is typically calcareous, but in purely planktonic forms, calcium carbonate can be all but absent. The shell is often linked with a carapace, but this is not necessarily homolous with other pancrustacean carapaces. Once thought to be polyphyletic, the consensus, supported by molecule sequences, leans toward monophyly. The expression of the engrailed gene in Vargula hilgendorfii has allowed limb identifications. Initial hatching can happen at a variety of larval stages: three-segment nauplii, four-segment head larvae, or even longer segmental stages. Ostracods definitely occur in the Ordovician, but fossilized, ostracod-like limb fragments occur in the Cambrian. Sequencing of ostracods has helped reform the modern understanding of crustaceomorph evolution and taxonomy.