Editorial Preface for Special Issue “Reproduction in Sponges: Diversity, Strategies, and Regulation”

Abstract

Sponges have long fascinated biologists, and they remain central to Evo-Devo research. This status rests on their phylogenetic position as one of the early-diverging metazoan groups and on their unique anatomical and histological organization. Despite numerous debates regarding who diverged first, Porifera or Ctenophora, sponges remain a key group for understanding such fundamental questions as the origin of animal multicellularity and mechanisms of tissue integration, evolution of epithelium and nervous system, etc. From the Evo-Devo point of view, studies of sponges are crucial to shed light upon the nature of the somatic/germ line segregation in animals, the origin and early evolution of gametogenesis, embryonic and postembryonic development, asexual reproduction, regeneration, and morphogenesis itself (Lanna 2015; Colgren and Nichols 2019).

Despite more than 170 years of sponge developmental research (Ereskovsky 2010; Cajado and Lanna, this issue), many obscure and unexplored topics remain in the developmental biology of these ancient animals. While sharing the basic features of reproduction (gametogenesis, fertilization, development from zygote, active cell movements during embryonic development, etc.) with eumetazoans, sponges demonstrate a great diversity of reproduction strategies and regulation, morphogenesis, and cell behavior, many of which are unique for the phylum. This special issue, “Reproduction in Sponges: Diversity, Strategies, and Regulation,” aims to present some of these poorly explored aspects in the developmental biology of Porifera.

Sponges demonstrate remarkably diverse sexual systems, ranging from stable gonochorism to sequential hermaphroditism and sex reversal (Sarà. 1993). Surprisingly, the sexual system can be labile even with a single species being changed from year to year (Fell and Jacob 1979). Furthermore, mechanisms of sex determination in sponges are poorly documented because studies are at least partially hindered by the absence of sexual dimorphism and structurally separated gonads in these animals. Lorente-Sorolla and Riesgo contribute a review that synthesizes the current knowledge on the sexual systems, environmental and genetic cues driving sexual fate, and the evolutionary origins of sexual conditions in sponges. They scrupulously analyze the scarce evidence of environmental and genotypic sex determination in sponges, giving proper attention to physiological and organizational features of these animals. Overall, Lorente-Sorolla and Riesgo argue that sponges exhibit an evolutionarily labile system in which sexual fate may emerge from the interaction of conserved genetic modules, environmental cues, and physiological constraints.

In turn, Cajado and Lanna present a wide and unique historical panorama of studies of reproduction and development in calcareous sponges for the last 200 years. Scrutinizing over 200 articles ranging from 1825 to 2026, the authors not only present a vast historical perspective on our visions of calcareous sponge developmental biology, but they also provide the current state of knowledge in this area, identify critical unanswered questions, and outline priorities for future research, making this review an exceptional framework for any researcher diving into the developmental biology of sponges.

The studies of the molecular mechanisms guiding the animal body plan formation through embryonic development are a cutting-edge research area in Evo-Devo for the last few decades (Gilbert and Barresi 2016). However, studies focusing on non-bilaterian species are still few (Lanna 2015; Degnan et al. 2015). Borisenko et al. present the bulk transcriptomic analysis of embryogenesis in demosponge Halisarca dujardinii supplemented with the visualization of the spatial pattern of gene expression using whole-mount in situ hybridization. Authors characterize gene expression across three key stages: cleavage, blastula, and prelarva. They identified significant transcriptional shifts and analyzed the expression of numerous transcriptional factors as promising proxies controlling gene regulatory networks and major morphological rearrangements. Overall, the study by Borisenko et al. gives new insights into how morphologically simple animals utilize complex molecular mechanisms for their development.

Despite the long research history, the embryonic development in many sponge orders remains completely unknown. Nonetheless, the collected data provide a rich material for comparative embryological studies: it shows that the embryonic development in sponges is surprisingly diverse, and eight main developmental types are identified across various phylogenetic lines and different taxa (Ereskovsky 2004, 2010). Among the developmental types, the direct development (without larvae) in viviparous demosponges has been the least studied so far. Cárdenas et al. present a detailed description of the direct embryonic development in four Craniella species (Tetractinellida) with extensive histological studies. Previously, the direct embryonic development was studied histologically only once in another demosponge, Stylocordyla chupachups (Sarà et al. 2002). The results obtained by Cárdenas et al. corroborate the distinction of the direct, non-larval development as a separate type of sponge embryonic development.

Metamorphosis is one of the key stages in sponge development. It entails a fundamental restructuring from the larval to the adult body plan. This developmental period is marked by the reestablishment of the main body axis and profound transdifferentiation of larval cells. The sponge metamorphosis has attracted considerable attention from researchers since the 19th century. And many of these studies are dealing with the metamorphosis of calcareous sponges (Ereskovsky 2010; Cajado and Lanna, this issue) as they are the only sponge class giving opportunity to explore the emergence of various types of the aquiferous system. Lanna and Klautau characterized the morphogenetic processes during metamorphosis and aquiferous system development in two calcareous sponges: syconoid Sycettusa hastifera and, for the first time, in leuconoid Paraleucilla magna. In both species, the development of the adult aquiferous system starts with the asconoid system of the young olynthus and proceeds through the epithelial morphogenesis, but of different types. The description of the post-embryonic development of S. hastifera and P. magna will allow these species to be used as models in future experimental studies investigating morphogenesis during embryonic development and post-embryogenesis in sponges that were fundamental not only to the evolution of Porifera but also to the entire Metazoa.

Finally, the review by Ereskovsky and Lavrov provides a current critical overview of the modes of asexual reproduction in sponges. While asexual reproduction is widespread in sponges and has even become an integral part of an animal's life cycle in many sponge species, it remained on the margins of sponge development investigation for a long time. However, in recent decades, some forms of asexual reproduction (gemmulation in freshwater demosponges and budding in marine homoscleromorphs) have emerged as promising models for Evo-Devo studies (Funayama 2010; Okamoto et al. 2012; Rocher et al. 2024; Leys et al. 2025). In their review, Ereskovsky and Lavrov have summarized available data on the morphogenesis accompanying different types of asexual reproduction in four sponge classes and indicate aspects of sponge asexual reproduction requiring research in the future.

With this issue, we hope to shed light on several current questions in sponge developmental and reproductive biology. We believe this collection of research and review articles will not only interest developmental and evolutionary biologists but also inspire new research on this fascinating group of animals.

Alexander V. Ereskovsky: conceptualization, writing – original draft. Andrey I. Lavrov: conceptualization, writing – original draft.

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.