Rediscovering the unusual, solitary bryozoan Monobryozoon ambulans Remane, 1936: first molecular and new morphological data clarify its phylogenetic position

Abstract

Background One of the most peculiar groups of the mostly colonial phylum Bryozoa is the taxon Monobryozoon, whose name already implies non-colonial members of the phylum. Its peculiarity and highly unusual lifestyle as a meiobenthic clade living on sand grains has fascinated many biologists. In particular its systematic relationship to other bryozoans remains a mystery. Despite numerous searches for M. ambulans in its type locality Helgoland, a locality with a long-lasting marine station and tradition of numerous courses and workshops, it has never been reencountered until today. Here we report the first observations of this almost mythical species, Monobryozoon ambulans. Results For the first time since 1938, we present new modern, morphological analyses of this species as well as the first ever molecular data. Our detailed morphological analysis confirms most previous descriptions, but also ascertains the presence of special ambulatory polymorphic zooids. We consider these as bud anlagen that ultimately consecutively separate from the animal rendering it pseudo-colonial. The remaining morphological data show strong ties to alcyonidioidean ctenostome bryozoans. Our morphological data is in accordance with the phylogenomic analysis, which clusters it with species of Alcyonidium as a sister group to multiporate ctenostomes. Divergence time estimation and ancestral state reconstruction recover the solitary state of M. ambulans as a derived character that probably evolved in the Late Cretaceous. In this study, we also provide the entire mitogenome of M. ambulans, which—despite the momentary lack of comparable data—provides important data of a unique and rare species for comparative aspects in the future. Conclusions We were able to provide first sequence data and modern morphological data for the unique bryozoan, M. ambulans, which are both supporting an alcyonidioidean relationship within ctenostome bryozoans.

Boring systematics: A genome skimmed phylogeny of ctenostome bryozoans and their endolithic family Penetrantiidae with the description of one new species

“…Monobryozoidae was traditionally placed either among arachnidioids, primarily due to the presence of non‐kenozooidal cystid appendages (Jebram, 1986 ), or as incertae sedis (D'Hondt, 1983 ). More recent investigations suggest an affinity of monobryozoids with alcyonidioids (Schwaha, 2020c ; Schwaha et al., 2024 ), which is also confirmed in our study. This affinity is reflected by alcyonidioid‐like characters such as a circular orifice, the presence of a prominent orifical sphincter and a vestibular anus (Schwaha, 2020c ; Schwaha et al., 2024 ).…”

“…More recent investigations suggest an affinity of monobryozoids with alcyonidioids (Schwaha, 2020c;Schwaha et al, 2024), which is also confirmed in our study. This affinity is reflected by alcyonidioidlike characters such as a circular orifice, the presence of a prominent orifical sphincter and a vestibular anus (Schwaha, 2020c;Schwaha et al, 2024).…”

“…Our study also suggests a sister‐group relationship between the genus Alcyonidium and M. ambulans . The latter is a solitary bryozoan species living in sandy marine sediments as part of the meiofauna and was just recently rediscovered (Remane, 1936 ; Schwaha et al., 2024 ). Monobryozoidae was traditionally placed either among arachnidioids, primarily due to the presence of non‐kenozooidal cystid appendages (Jebram, 1986 ), or as incertae sedis (D'Hondt, 1983 ).…”

“…As a result, the atp8 gene was excluded from our final data matrix, together with 18S and 28S of Terebripora sp. The remaining sequences of these 27 samples were combined with published sequences of the ctenostome Monobryozoon ambulans (Schwaha et al., 2024 ), nine cheilostome species (Orr et al., 2021 ) and the phylactolaemate Pectinatella magnifica as outgroup (Fuchs et al., 2009 ; Gim et al., 2018 ; Waeschenbach et al., 2009 ; Table A2 ).…”