Poecilogony in the caenogastropod Calyptraea lichen (Mollusca: Gastropoda)

In marine invertebrates, polymorphism and polyphenism in mode of development are known as “poecilogony.” Understanding the environmental correlates of poecilogony and the developmental mechanisms that produce it could contribute to a better understanding of evolutionary transitions in mode of development. However, poecilogony is rare in marine invertebrates, with only ten recognized, well-documented cases. Five examples occur in sacoglossan gastropods, and five occur in spionid polychaetes. Here, we document the eleventh case, and the first in a caenogastropod mollusc. Females of Calyptraea lichen collected in the field or reared in the laboratory often produce broods of planktotrophic larvae. They can also be collected with mixed broods, in which each capsule contains planktotrophic larvae, nurse embryos, and adelphophagic embryos. Adelphophages eat the nurse embryos and hatch as short-lived lecithotrophic larvae, or even as juveniles. Mitochondrial COI and 16S DNA sequences for females with different types of broods differ by less than 0.5%, supporting conspecific status. Some females collected in the field with mixed broods subsequently produced planktotrophic broods, demonstrating that females can produce two different kinds of broods. Calyptraea lichen is therefore polyphenic in two ways: mode of development can vary among embryos within a capsule, and females can change the types of broods they produce. Additional key words: adelphophagy, Calyptraeidae, polyphenism, mode of development, plasticity Alternative phenotypes, the expression of more than one discrete phenotype either during the lifetime of an individual or among individuals of the same species, are thought to play a major role in generating biological diversity (West-Eberhard 2003; Schwander & Leimar 2011). Evolutionary transitions between different genetically fixed discrete monomorphic states may occur by passage through transitional stages that could include polyphenic intermediates (Schwander & Leimar 2011). The alternative phenotypes expressed by a polyphenic species can result from genetic polymorphism, a plastic response to different environmental cues, or a combination of the two. These underlying modes of control may be evolutionarily labile, changing repeatedly between environmental and genetic control through evolutionary time and across phylogeny (Schwander & Leimar 2011). Most species of marine invertebrates are monomorphic with respect to mode of development (Thorson 1950; Hoagland & Robertson 1988; Allen & Pernet 2007). Some species produce offspring that develop as small swimming larvae that must feed to reach metamorphosis (planktotrophic development). Other species produce larger non-feeding lecithotrophic larvae, and yet others produce large embryos that bypass a free-living larval stage and hatch as crawling juveniles (direct development). Although many groups of marine invertebrates express interspecific variation in mode of development, very few intermediates or evolutionary transitional forms between these two common modes of development have been documented (Thorson 1950; Allen & Pernet 2007; Collin 2012). Understanding transitions among the different modes of development is a major goal of invertebrate biologists, and studies of potentially transitional forms are considered key in this endeavor (Strathmann 1978a,b, 1985; Allen & Pernet 2007; Collin 2012; Knott & McHugh 2012). Author for correspondence. E-mail: [email protected] Invertebrate Biology x(x): 1–8. © 2014, The American Microscopical Society, Inc. DOI: 10.1111/ivb.12057 The production of alternative phenotypes in mode of development is termed poecilogony (Hoagland & Robertson 1988; Bouchet 1989; Chia et al. 1996; Knott & McHugh 2012). As populations or females of poecilogonous species are capable of producing different offspring types, such species could represent a transitional stage between indirect and direct modes of development (Knott & McHugh 2012). An alternate view is that poecilogony is not a transitional but a stable strategy, adaptive in variable environments (Chia et al. 1996; Collin 2012). Both of these views could apply to poecilogonous species, as the term poecilogony has been used to describe polyphenism at different scales, from variation among populations to plasticity within a female (Collin 2012). Poecilogony is extremely rare. There are five documented cases in sacoglossan gastropods (Vendetti et al. 2012) and five cases in spionid polychaetes (Collin 2012). In six of these species, development varies either among populations (Costasiella ocellifera (SIMROTH 1895); Elysia chlorotica GOULD 1870; Pygospio elegans CLAPAR EDE 1863) or among females (Elysia zuleicae ORTEA & ESPINOSA 2002; Elysia pusilla (BERGH 1871); Streblospio benedicti WEBSTER 1879). Also, only four of these ten poecilogonous species produce both planktotrophic larvae and non-feeding offspring (reviewed in Collin 2012). In the spionid polychaete Polydora cornuta BOSC 1802 and the sacoglossan gastropod Alderia willowi KRUG, ELLINGSON, BURTON & VALD ES 2007, females can change the kind of offspring produced between consecutive broods. Sometimes during these transitions they produce both kinds of offspring in the same clutch, but this is not typical (Krug 1998; MacKay & Gibson 1999; Krug et al. 2012). The spionids Boccardia polybranchia (HASWELL 1885) and Boccardia proboscidea HARTMAN 1940 normally produce both kinds of offspring from the same brood (Duchêne 1984; Gibson 1997); some of the embryos in each capsule of these mixed broods develop into planktotrophic larvae, while others within the same capsule consume nurse eggs, grow larger, and hatch as lecithotrophic larvae (Gibson 1997; Gibson et al. 1999; Oyarzun & Strathmann 2011; Oyarzun et al. 2011). Here, we report the discovery of an eleventh case of poecilogony, in the calyptraeid gastropod Calyptraea lichen BRODERIP 1834. This species shares features with P. cornuta and A. willowi (changes in brood composition between broods of the same female), and also with B. proboscidea and B. polybranchia (mixed broods). This report aims to describe the phenomenon and provide a basis for future research. Methods Individuals of Calyptraea lichen live on large cobbles and rock rubble in the low intertidal of the Bay of Panama. They are patchily distributed; of the calyptraeid species collected in the same location, Crucibulum spinosum (G.B. SOWERBY 1824), Crucibulum radiata (BRODERIP 1834), and Crepidula navicella (LESSON 1831) are frequently found together with C. lichen, but C. lichen seems to be less abundant than members of the other species. Like all calyptraeids, they are protandrous suspension-feeders that brood thin-walled transparent egg capsules between the substrate and neck lappets (Fig. 1A). Calyptraeids can produce broods of small eggs that hatch as planktotrophic larvae (~50% of species), broods of large eggs that hatch as crawling juveniles or very short-lived lechithotrophic larvae (~30% of species), or broods of adelphophagic embryos that consume nurse eggs/ embryos and hatch as crawling juveniles (Collin 2003a). Calyptraea lichen has previously been reported to produce planktotrophic larvae (Collin 2003a). Adults of C. lichen were collected from Playa Venado near Veracruz on the Pacific coast of Panama (8°52.95670N, 79°35.87780W) in 2010 and 2011. Any broods present were collected with their mother. Adults were kept in the laboratory individually or in pairs, if they were collected as pairs, in 350-mL plastic cups, and fed Isochrysis galbana PARKE 1949 daily (following Collin & Salazar 2010). Broods removed from their mothers were raised in UV-sterilized seawater passed through a 0.22-lm membrane filter, with added antibiotics (6.3910 5 mol L 1 streptomycin sulfate salt, 1.4910 4 mol L 1 penicillin G potassium salt). Broods deposited in the laboratory were kept with the mother and allowed to hatch naturally to determine the hatching stage. Three females with planktotrophic development and two females with mixed broods were preserved in 95% ethanol, and DNA was extracted using a phenol-chloroform extraction protocol. Fragments of two mitochondrial genes were sequenced; 650 base pairs of mitochondrial Cytochrome Oxidase subunit 1 (COI), and 500 base pairs of 16S rDNA. These were amplified and sequenced following the protocol and using the universal primers reported in Collin (2001). A fragment of the nuclear gene histone H3 was also amplified and sequenced following methods in Colgan et al. (2000). The COI and 16S sequences were aligned with previously published sequences of species in the Panamanian Calyptraea clade (planktotrophic C. lichen: AF546067, AF546007; Calyptraea conica BRODERIP 1834: Invertebrate Biology vol. x, no. x, xxx 2014 2 McDonald, Collin, & Lesoway