Anterior sensory organs in Sabellariidae (Annelida)

Abstract

Sensory organs in Annelida are very diverse and may be useful for assessments of morphological adaptation and character evolution. We used several methods to provide new insights into processes underlying the evolutionary radiation of anterior sensory organs in Sabellariidae. The presence and morphological diversity of the median organ (MO) found in the group was reviewed in order to test its phylogenetic significance and possible relationships to the distribution and ecological traits of the lineages. To test the intraspecific phenotypic plasticity of the MO, molecular analyses were conducted that focused on mitochondrial and nuclear genes from populations of Idanthyrsus australiensis exhibiting variation in the morphology of the MO. We used an integrative microscopical study of the ontogeny of Sabellaria alveolata to describe the anterior sensory structures present in the larvae and the morphological changes occurring before, during, and after settlement. In larval stages, the palps and the dorsal hump (DH) exhibit distinct innervation. The larval DH organ, which is likely to play a major role in chemoreception for settlement, is interpreted as being the incipient form of the adult MO. These results suggest that annelid sensory organs including the MO may be useful for phylogenetic and developmental investigations.Additional key words: chemoreception, gregarious settlement, marine invertebrates, polychaetes, radiation of lineages Invertebrate Biology 135(4): 423-447.

Phylogeography of the reef‐building polychaetes of the genus Phragmatopoma in the western Atlantic Region

“…While most sabellariids are solitary, the species that are reef‐building typically construct biogenic structures in intertidal or shallow subtidal environments (Faroni‐Perez et al . ). Reefs of Phragmatopoma caudata Krøyer in Mörch, 1863 are broadly distributed along the intertidal zone in the western Atlantic coastline, from Florida (USA) (34° N) to Santa Catarina (Brazil) (27° S), including many localities in the Caribbean (Kirtley, ).…”

“…These ecosystem engineers create complex habitats supporting high levels of biodiversity and provide ecosystem services such as coastal protection (Dubois et al, 2002;Noernberg et al, 2010;Ataide et al, 2014). While most sabellariids are solitary, the species that are reef-building typically construct biogenic structures in intertidal or shallow subtidal environments (Faroni-Perez et al 2016). Reefs of Phragmatopoma caudata Krøyer in M€ orch, 1863 are broadly distributed along the intertidal zone in the western Atlantic coastline, from Florida (USA) (34°N) to Santa Catarina (Brazil) (27°S), including many localities in the Caribbean (Kirtley, 1994).…”

“…While tolerance to salinity has not yet been quantified for larvae, adults can tolerate brackish waters of up to 30-40% seawater (Mauro, 1977). When metatrochophore larvae are competent for metamorphosis, they settle onto hard substrate, usually a conspecific pre-existing reef, induced by chemical cues (Pawlik, 1988) and mediated by the larval sensory organs, such as the dorsal hump and palps (Faroni-Perez et al, 2016). Finally, P. caudata has an estimated lifespan of 1-2 years (McCarthy et al, 2003).…”

Developmental studies provide new insights into the evolution of sense organs in Sabellariidae (Annelida)

“…For sabellariids, molecules present in the cement secreted by benthic juveniles and adults when building their tubes (free fatty acids), are known to be responsible for larval settlement [ 7 , 10 – 15 ]. In this respect, the larval palps and the dorsal hump (becoming the median organ in the adults) are presumed to participate in substrate selection during settlement [ 16 – 18 ].…”

“…Nevertheless, the median organ, a characteristic and apparently unique organ for Sabellariidae, of prostomial origin, digitiform in shape and located in the junction of the anterior opercular lobes, has also been attributed with a sensory function [ 10 , 18 , 30 – 32 ]. In fact, the larval dorsal hump, the incipient form of the adult median organ, has been suggested to play a major role in chemoreception for settlement [ 16 – 18 ]. Some sabellariid species also bear a band of presumed eyespots at both sides of the median organ [ 8 , 10 , 32 , 33 ] but their ultrastructure and composition have not yet been studied so far.…”