This thesis covers three important aspects of Antarctic heterobranchs: ecology, taxonomy, and systematics. The first section deals with ecological interactions of several nudibranchs. In Chapter 1, we chemically characterize a new natural product (a homosesterterpene) called granuloside, from Charcotia granulosa Vayssière, 1906; remarkably, this is the first record of this type of compound in marine organisms. In Chapter 2, we assess the origin, function, and distribution of granuloside in this nudibranch; we found glandular structures probably responsible for storing granuloside, as a defensive mechanism against predators, like the sympatric starfish, Odontaster validus Koehler, 1906. We also hypothesize that granuloside is de novo biosynthesized by C. granulosa. This chapter reflects how organisms from polar latitudes have similar defensive strategies to those of temperate and tropical zones. In Chapter 3, a new species of ectosymbiont copepod, Anthessius antarcticus n. sp., living on C. granulosa is described. This is the first record of such association in Antarctica and the first time that this copepod genus has been found living on a nudibranch. In Chapter 4, we study the development of two anthobranchs, Doris kerguelenensis (Bergh, 1884) and Bathydoris hodgsoni Eliot, 1907, both with intracapsular development; we provide new data on the egg masses characteristics, and embryos morphology and anatomy, throughout their development; we also studied at which ontogenetic stage their natural products appear. We concluded that both nudibranchs exhibit developmental periods of up to several years; their embryos are physically defended by a thick egg capsule, while juveniles already rely on de novo biosynthesized defensive compounds. In the second section of this thesis, our interdisciplinary taxonomic and systematic studies, including histology, tomography, electron microscopy, and molecular tools, allowed us to describe three new species of heterobranchs. In Chapter 5, we provide integrative taxonomic evidence for the establishment of a new family (Newnesiidae), and the description of a new species of Cephalaspidea (Newnesia joani n. sp.) with eurybathic and circumpolar distribution; this discovery traces the origin of the cephalaspideans (distributed worldwide) to Antarctica. In Chapter 6, we performed a three-dimensional (3D) anatomical reconstruction and compared the two nudibranchs Doto antarctica and the new species Doto carinova n. sp.; their phylogeny reveals intriguing questions concerning the development of the reproductive system in this genus; 3D reconstructions reveal also the presence of probable giant neurons associated with the nervous system, which were unknown in this genus so far. Finally, in Chapter 7 we provide new evidence of bipolar geographic distributions by describing a new species of nudibranch, Doridunculus punkus n. sp., using only non-destructive tomographic techniques. Our results highlight both the need and the relevance of multidisciplinary approaches to study biodiversity and ecological interactions in heterobranch molluscs from a poorly studied area of the planet, such as Antarctica.