Polka-dotted treasures: Revising a clade of ascidian- and bivalve-associated shrimps (Caridea: Palaemonidae)

Abstract

Most marine shrimp species of the family Palaemonidae are characterized by symbiotic associations with hosts that belong to a wide range of invertebrate phyla. One clade of related endosymbiotic species has evolved to live inside the branchial chambers of ascidians and the mantle cavities of bivalve molluscs. The phylogeny of this clade (comprising 15 bivalve-associated species in the genera Anchistus, Neoanchistus, and Paranchistus, and three species of ascidian-associated species in the genus Dasella) is the topic of the present study, which is based on both morphological and molecular data. A concatenated phylogeny reconstruction was built by using the markers coi and 16S. With the help of a total evidence approach (with a scored morphological datamatrix), species could be added for which no molecular data were available. An ancestral character state analysis was performed to detect host switches. In contrast to another endosymbiotic clade, the ancestral host state was found to be slightly in favour of bivalves. The phylogenetic relevance of morphological features is discussed with a focus on a tympanal organ located in the major chelipeds of some bivalve-associated shrimp species. This little-known anatomical structure is illustrated by sem and µCT scans. Its possible function is discussed. In the phylogeny reconstructions, both Anchistus and Paranchistus were found to be polyphyletic. They were reclassified to obtain a more natural classification: Anchistus custoides and Anchistus custos were transferred to the resurrected genus Ensiger. Paranchistus liui, P. nobilii, P. pycnodontae, and P. spondylis were transferred to Polkamenes gen. nov. Anchistus pectinis and P. ornatus were transferred to Tympanicheles gen. nov. Per genus, a key to the species is provided.

Ecomorphological convergence in the walking leg dactyli of two clades of ascidian‐ and mollusc‐associated shrimps (Decapoda: Caridea: Palaemonidae)

“…(2016; see below), these events are hypothesized to have happened much more frequent (de Gier et al., 2022). The endosymbiotic mollusc‐ and ascidian‐associated palaemonid species are generally grouped in two clades (Figure 1; de Gier et al., 2022; de Gier & Fransen, 2023), from hereon called the ‘ Conchodytes clade’ and the ‘ Anchistus clade’. The first clade includes the larger genera Ascidonia Fransen, 2002, Conchodytes Peters, 1852, Dactylonia Fransen, 2002, Odontonia Fransen, 2002 and Pontonia Latreille, 1829, in addition to some minor, often monotypic genera (de Gier et al., 2022).…”

“…The first clade includes the larger genera Ascidonia Fransen, 2002, Conchodytes Peters, 1852, Dactylonia Fransen, 2002, Odontonia Fransen, 2002 and Pontonia Latreille, 1829, in addition to some minor, often monotypic genera (de Gier et al., 2022). The second clade includes the genera Anchistus Borradaile, 1898; Dasella Labour, 1945; Ensiger Borradaile, 1915; Neoanchistus Bruce, 1975; Paranchistus Holthuis 1952; Polkamenes de Gier & Fransen, 2023; and Tympanicheles de Gier & Fransen, 2023 (de Gier & Fransen, 2023). The two clades are not directly related, and the placement of the larger clades within the Palaemonidae is still a point of discussion: in the study by Horká et al.…”

“…Ultrametric phylogeny reconstructions based on TE‐analyses from de Gier et al. (2022) (a) and de Gier and Fransen (2023) (b). (c–m) Lateral views of a number of representatives from the two clades.…”

“…(c–m) Lateral views of a number of representatives from the two clades. Representatives are underlined in the phylogeny trees: (c) Conchodytes meleagrinae Peters, 1852—after Fransen & Reijen (2013); (d) Odontonia katoi (Kubo, 1940)—after Hayashi (2003); (e) Anchiopontonia hurii (Holthuis, 1981)—after Bruce (1992); (f) Pontonia mexicana Guérin‐Méneville, 1855—after an illustration by Alberto Guerra, for BioDiversidad Marina de Yucatán (BDMY); (g) Ascidonia quasipusilla (Chace, 1972)—new illustration of RMNH.CRUS.D.51678; (h) Dactylonia monnioti (Bruce, 1990)—after Bruce (1990); (i) Anchistus miersi (De Man, 1888)—after de Gier and Fransen (2023); (j) Neoanchistus cardiodytes Bruce, 1975—after Bruce (1975); (k) Polkamenes nobilii (Holthuis, 1952)—after de Gier and Fransen (2023); (l) Ensiger custoides (Bruce, 1977)—after de Gier and Fransen (2023); (m) Dasella ansoni Bruce, 1983—after Bruce (1983). Shrimp dimensions not to scale.…”

“…Compared to other morphological structures which are often used in taxonomic contexts (e.g. the rostrum and telson spines; see for example de Gier & Fransen, 2023), the walking leg dactyli showcase almost no intraspecific variance (Chow et al., 2021). The walking legs are thought to be one of the first features to adapt to new surroundings, and are therefore prime candidates for ecomorphological studies on the taxa (Fransen, 2002).…”