Sacoglossan sea slugs (Mollusca: Opisthobranchia) are one of the few groups of specialist herbivores in the marine environment. Sacoglossans feed suctorially on the cell sap of macroalgae, from which they "steal" chloroplasts (kleptoplasty) and deterrent substances (kleptochemistry), retaining intracellularly both host plastids and chemicals. The ingested chloroplasts continue to photosynthesize for periods ranging from a few hours or days up to 3 months in some species. Shelled, more primitive

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... sacoglossans feed only on the siphonalean green algal genus Caulerpa, and they do not have functional kleptoplasty. The diet of Sacoglossans has radiated out from this ancestral food. Among the shell-less Plakobranchidae (=Elysiidae), the more primitive species feed on other siphonales (families Derbesiaceae, Caulerpaceae, Bryopsidaceae and Codiaceae) and fix carbon, while the more "advanced" species within the Plakobranchidae and Limapontioidae have a more broad dietary range. Most of these "advanced" species are unable to fix carbon because the chloroplasts of their food algae are mechanically disrupted during ingestion. Mesoherbivores are likely to be eaten if they live on palatable seaweeds, their cryptic coloration and form not always keeping them safe from predators. Sacoglossans prefer to live on and eat chemically defended seaweeds, and they use ingested algal chemicals as deterrents of potential predators. The most ancestral shelled sacoglossans (Oxynoidae) and some Plakobranchidae such as Elysia translucens, Thuridilla hopei and Bosellia mimetica have developed a diet-derived chemical defense mechanism. Oxynoids and Thuridilla hopei are able to biomodify the algal metabolites. However, the Plakobranchidae Elysia timida and E. viridis, together with Limapontioidea species, are characterized by their ability to de novo synthesize polypropionate metabolites. A whole analysis of kleptoplasty and chemical defenses in Sacoglossans may offer a better understanding of the ecology and evolution of these specialized opisthobranchs. In this paper we summarize some of the latest findings, related mainly to Mediterranean species, and offer a plausible evolutionary scenario based on the biological and chemical trends we can distinguish in them. LUTIVAS. Los limacos marinos del orden Sacoglosos (Moluscos: Opistobranquios) son uno de los pocos grupos de herbívoros especializados en el ambiente marino. Los Sacoglosos se alimentan suctorialmente del jugo celular de microalgas, de las que "roban" cloroplastos (cleptoplastia) y sustancias disuasorias (cleptoquimia), y conservan intracelularmente tanto los plástidos como las sustancias químicas del patrón. Los cloroplastos ingeridos continúan fotosintetizando por períodos que van desde unas pocas horas o días hasta 3 meses en algunas especies. Los sacoglosos testáceos, más primitivos, se alimentan sólo de algas del género de sifonales Caulerpa, y no tienen cleptoplastia funcional. La dieta de los Sacoglosos ha radiado desde este alimento ancestral. Entre los Placobránquidos (=Elísidos), sin concha, las especies más primitivas se alimentan de otras algas sifonales (familias Derbesiáceas, Caulerpáceas, Briopsidáceas y Codiáceas) y fijan carbono, mientras que las especies más "avanzadas" pertenecientes a Placobránquidos y Limapontioideos poseen una panoplia alimentaria más amplia. La mayoría de estas especies "avanzadas" son incapaces de fijar carbono porque los cloroplastos de sus algas alimento se rompen mecánicamente durante la ingestión. Los mesoherbívoros están expuestos a ser comidos si viven sobre algas marinas comestibles, pues su coloración y forma crípticas no siempre los mantienen a salvo de los depredadores. Los Sacoglosos prefieren vivir sobre algas defendidas químicamente y comerlas, y utilizan sustancias químicas ingeridas como disuasores de depredadores potenciales. Los sacoglosos testáceos más primitivos (Oxinoidos) y algunos placobránquidos tales como Elysia translucens, Thuridilla hopei y Bosellia mimetica, han desarrollado un mecanismo defensivo derivado de la dieta. Los Oxinoidos y Thuridilla hopei pueden biomodificar los metabolitos algales. Sin embargo, los placobránquidos *If you are looking for alternatives to Darwinism, don't waste your time on opisthobranchs. G. Cimino and M.T. Ghiselin (2001) . Whatever the immediate future of the research on the consortia in which sacoglossans and other opisthobranchs are involved, it will presumably provide as many surprises and as much satisfaction as in the last few years. J.D. Ros and A. Marín (1991) . It has seemed to me convenient to introduce the term "kleptodefenses" to describe the defensive systems of any type which an opisthobranch obtains borrowed from another animal, from a plant or, even, from inert materials in the environment.