Gain of palps within a lineage of ancestrally burrowing annelids (Scalibregmatidae)

Abstract

Scalibregmatidae is a small annelid family of subsurface deposit feeders in sand or mud, which are generally well adapted to infaunal burrowing. The overall morphology of Scalibregmatidae is very similar, with thick bodies, small parapodia, and no prostomial appendages or short horns. The only exception is members of the genera Axiokebuita and Speleobregma that most frequently inhabit crevices or gravel and possess extensive ventral ciliated palps and globular adhesive pygidium. Character reconstruction using maximum likelihood and Bayesian methods show that ciliated palps and adhesive pygidium are synapomorphies of the Axiokebuita–Speleobregma clade. The most likely transformation series is from horns to ciliated palps, the origin of which correlates with the occurrence of Axiokebuita and Speleobregma in crevices or gravel. The wide spaces among rocks or granules yield high permeability and inertial water flow, preventing deposition of organic matter. Under these flow conditions that differ significantly from those of sand or mud bottom, ciliated palps aid to the collection of suspended particles and an adhesive pygidium provides attachment. With palps being a highly debated character in annelid evolution, it is remarkable that prominent ciliated palps are gained within a lineage of ancestrally nonpalpate annelids, most likely increasing their fitness when colonizing a new environment.

Implied weighting and its utility in palaeontological datasets: a study using modelled phylogenetic matrices

“…; Martínez et al . )). However, we modified this method, following Wright & Hillis () to set the parameter that defines character change by randomly drawing from a gamma distribution (with a shape parameter of 0.5 and a rate parameter of 5) for each character within the matrix.…”

Anophthalmia and elongation of body appendages in cave scale worms (Annelida: Aphroditiformia)

“…). In comparison, other non‐scale worm annelids inhabiting caves are non‐predatory and exhibit elongated palps, serving both in sensing and in capturing suspended organic particles from the water column (Martínez et al., ; Worsaae et al., ).…”

“…Four independent analyses, with four chains (three heated and one cold), were run for 30 million generations. For each Bayesian Markov chain, three hundred trees were obtained by sampling at intervals of 20k generations, ensuring statistical independence of the samples (Martínez, Di Domenico, & Worsaae, ; Pagel, Meade, & Barker, ). Burn‐in was set to 10 million generations.…”

New species of Scalibregmatidae (Annelida, Polychaeta) from the East Antarctic Peninsula including a description of the ecology and post-larval development of species of Scalibregma and Oligobregma

“…However, a small group of scalibregmatids of the genera Axiokebuita and Speleobregma have very elongate horns that are ciliated and with shallow grooves that suggest they function to support feeding rather than burrowing (Parapar et al 2011;Martínez et al (2013Martínez et al ( , 2014 and have been termed 'palps." Use of these "palps" in feeding was confirmed for two marine cave-dwelling species of these genera that inhabit coarse sediments rather than the soft muddy sediment habitat typical of most scalibregmatids (Martínez et al 2013).…”

“…This character is an additional aspect of the overall morphology of scalibregmatids that deserves further study. The recent papers by Parapar et al (2011b) and Martínez et al (2013Martínez et al ( , 2014 on two closely related scalibregmatid genera, Axiokebuita and Speleobregma, suggest that a ventral groove or ridge line is completely absent in all described species of these two genera. If so, then, in conjunction with the complete absence of lyrate setae, acicular spines, branchiae, and parapodial cirri, together with the presence of an unusual prostomium with long ciliated lateral horns (termed palps by these authors) and an unusual peristomial sensory structure, this clade as reported by these authors differs considerably from other scalibregmatids.…”