Citation: Vinn, O.; Hoeksema, B.W.; Kolodziej, B.; Kozlová, Z. When a Worm Loves a Coral: A Symbiotic Relationship from the Jurassic/ Cretaceous Boundary. Diversity 2023, 15, 147. https://doi.org/10.3390/ d15020147 Academic Editor: Michael Wink Received: 28 December 2022 Revised: 12 January 2023 Accepted: 18 January 2023 Published: 21 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diversity Article When a Worm Loves a Coral: A Symbiotic Relationship from the Jurassic/Cretaceous Boundary Olev Vinn 1 , Bert W. Hoeksema 2,3 , Boguslaw Kolodziej 4, * and Zuzana Kozlová 5 1 Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411 Tartu, Estonia 2 Taxonomy, Systematics and GeodiversityGroup, Naturalis Biodiversity Center, P.O. Box 9517, 2300 RA Leiden, The Netherlands 3 Groningen Institute for Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands 4 Institute of Geological Sciences, Jagiellonian University, ul. Gronostajowa 3a, 30-387 Kraków, Poland 5 Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Praha, Czech Republic * Correspondence: boguslaw.kolodziej@uj.edu.pl Abstract: Reefal limestones of the Štramberk Carbonate Platform are preserved as olistoliths and pebbles in deep-water flysch of the Outer Carpathians (Czech Republic, Poland). They contain the richest coral assemblages of the Jurassic/Cretaceous transition (Tithonian–Berriasian). Symbiotic as- sociations between corals and tube-dwelling macroorganisms were recognized only in the branching scleractinian corals Calamophylliopsis flabellum and Calamophylliopsis sp. One to seven calcareous tubes were recognized either in coral calyces, embedded in the wall, or attached to it. Two types of tubes were recognized: Type 1: tubes with a diameter of 0.3–0.6 mm and a very thin wall (ca. 0.05 mm), mostly occurring inside corallites, and nearly exclusively in the Štramberk-type limestone of Poland; Type 2: tubes with an outer diameter of 0.8–2.0 mm and a thick wall (ca. 0.1 mm, some even up to 0.2 mm), mostly found in the Štramberk Limestone of the Czech Republic. Growth lamellae were observed in some thick walls. Most tubes are almost straight, some are curved. The tubes were probably produced by serpulids or by embedment of organic-walled sabellid polychaete worms. It is likely the oldest record of a sabellid–coral association. Modification of the coral skeleton in contact with tubes indicates that the worms were associated with live corals. For many modern and fossil worm–coral associations, this symbiotic association is interpreted as mutualistic or commensal. Keywords: tube-dwelling worms; Polychaeta; Scleractinia; symbiosis; Carpathians 1. Introduction Coral reefs are among the world’s marine ecosystems with the highest biodiversity. Research on coral-associated organisms in terms of their diversity and functioning is urgent [1,2]. Corals themselves form highly diverse habitats for other organisms. Some coral-associated macro- and microorganisms have fossilization potential and are recorded in extinct corals. Even coral-associated microendoliths, as part of the coral microbiome, are reported from fossil material [3]. Worms of various taxa of the phylum Annelida are the most common macroorganisms reported as in vivo associated organisms in modern [4] and fossil corals, both in Paleozoic Rugosa and Tabulata, Mesozoic–Cenozoic scleractinian corals (Refs. [5–7] and references therein), as well as in stromatoporoid sponges, which were the most important reef builders in the Paleozoic (e.g., Ref. [8]). Palaeontological studies of coral-associated polychaetes and other organisms provide insight into the evolution of corals and reef ecosystems. The title of this paper has been inspired by a conference presentation by Martinell et al. [9] about associations between Pliocene solitary corals and worms. Long groove-shaped bioerosional structures running along the surface of the coral skeleton were left by eunicid polychaetes. This case of symbiosis was interpreted as commensalism [6]. In fact, many Diversity 2023, 15, 147. https://doi.org/10.3390/d15020147 https://www.mdpi.com/journal/diversity