Sibling species are common in all major marine groups and habitats. Their abundance reflects both inadequate study of morphological features of living organisms ("pseudo-sibling species") and divergence in habitat, life history, and chemical recognition systems without parallel divergence in morphology. Many marine sibling species are quite distinct genetically. Others, however, exhibit slight genetic differences whose significance is only clear in sympatry and in combination with other subtle but concordant patterns of differentiation. A large number of abundant, well-studied, or economically important taxa have recently been shown to be complexes of sibling species. The broad habitat and geographic distributions characteristic of many marine species require reevaluation in this context.

Sibling species in the sea

Within a population there are frequently several discrete morphs. While in some cases, particularly color polymorphisms, this variation can be explained by simple Mendelian modes of inheritance, in many cases the evidence suggests a polygenic pattern of inheritance. The threshold model of quantitative genetics, in which discrete morphs are determined by some underlying continuously distributed trait and a threshold(s) of expression, is applied appropriately in these cases. The discrete morphs exhibited in cyclomorphosis, pedomorphosis, pedogenesis, "protective" dimorphisms, trophic dimorphisms, wing dimorphism, and mating strategies can all be analyzed by using this model. Analyses of a wide range of different types of threshold traits show that there is typically a large additive genetic component, but that there is also strong environmental induction. A review of studies shows that no morph has a universally higher fitness, but that there is a tradeoff, with the relative fitnesses of two morphs being co...

The Evolution of Threshold Traits in Animals

This review summarises the current knowledge on cryptic species of polychaetes, one of the most dominant taxa in marine communities, and gives a brief overview of the different methods that has been used for their disclosure. Cryptic species constitute an important part of biodiversity and they are common among all kinds of polychaetes, with different life history traits, and may have sympatric as well as allopatric distributions. It is emphasised that cryptic species must not be neglected for a variety of reasons, and even though the magnitude of cryptic species is largely unknown the available data on polychaetes clearly demonstrate that morphology alone seriously underestimate the number of species. It is suggested that future research should focus on appropriately designed case-studies using combined approaches, as well as on large-scale whole sample analyses using next-generation sequencing in order to be able to answer the many questions that still remains.

Cryptic polychaete diversity: a review

Variability, flexibility and plasticity in life histories of marine invertebrates

The phylum Mollusca is highly speciose, and is the largest phylum in the marine realm. The great majority of molluscs are shelled, including nearly all bivalves, most gastropods and some cephalopods. The fabulous and diverse colours and patterns of molluscan shells are widely recognised and have been appreciated for hundreds of years by collectors and scientists alike. They serve taxonomists as characters that can be used to recognise and distinguish species, however their function for the animal is sometimes less clear and has been the focus of many ecological and evolutionary studies. Despite these studies, almost nothing is known about the evolution of colour in molluscan shells. This review summarises for the first time major findings of disparate studies relevant to the evolution of shell colour in Mollusca and discusses the importance of colour, including the effects of visual and non-visual selection, diet and abiotic factors. I also summarise the evidence for the heritability of shell colour in some taxa and recent efforts to understand the molecular mechanisms underpinning synthesis of shell colours. I describe some of the main shell pigments found in Mollusca (carotenoids, melanin and tetrapyrroles, including porphyrins and bile pigments), and their durability in the fossil record. Finally I suggest that pigments appear to be distributed in a phylogenetically relevant manner and that the synthesis of colour is likely to be energetically costly.

Molluscan shell colour

Poecilogony, defined as more than one mode of reproduction within a single species, has been reported in various invertebrates, including mollusks and polychaetes. Many cases that have been described involve planktotrophic and non-planktotrophic development in allopatric populations, or instances of planktonic larval stages and benthic juveniles being found together, but not associated with adults. There is always the possibility of mis-assignment of larvae to adults of the wrong species. Most cases that offer these kinds of evidence are now known to involve cryptic species, not poecilogony.There are a few species in which release of young occurs both at metamorphosis and a day or so before. There may be cases in which extracellular yolk or nurse-egg production is variable and allows the release of larvae at different stages, but no actual instance is known. A few instances are known of allopatric populations with different modes of development and other differences in reproductive characters that lack re...

An Assessment of Poecilogony in Marine Invertebrates: Phenomenon or Fantasy?

A case of shell color ploymorphism in the shallow water mesogastropod Crepidula convexa is described. Polymorphism of color in C. convexa appears to be based on disruptive selection, which occurs due to the availability of two contrasting shades of substrate, heterogeneous in time and space, combined with mortality from visual predators.Data on the relative survival of the two phentoypes of C. convexa in two patches of a heterogeneous environment are applied to the fitness set model developed by Levins (1968). The necessary and sufficient criteria for a stable polymorphism are met, but reasons are set forth for believing that this is, rather, a case of unstable polymorphism.Only one other species of the genus, C. adunca, shows a similar color polymorphism. Ecological factors uniquely shared by the two species are: visibility of inherited shell color, a major part of the population living in the intertidal zone, and an impact by visual predators. Color polymorphism is correlated with visual predation on a ...

K. Elaine Hoagland

Papers

An Assessment of Poecilogony in Marine Invertebrates: Phenomenon or Fantasy?

A gastropod color polymorphism: one adaptive strategy of phenotypic variation