Interstitial Annelida

Members of the following marine annelid families are found almost exclusively in theinterstitial environment and are highly adapted to move between sand grains, relying mostly onciliary locomotion: Apharyngtidae n. fam., Dinophilidae, Diurodrilidae, Nerillidae,Lobatocerebridae, Parergodrilidae, Polygordiidae, Protodrilidae, Protodriloididae, Psammodrilidaeand Saccocirridae. This article provides a review of the evolution, systematics, and diversity of thesefamilies, with the exception of Parergodrilidae, which was detailed in the review of Orbiniida byMeca, Zhadan, and Struck within this Special Issue. While several of the discussed families havepreviously only been known by a few described species, recent surveys inclusive of molecularapproaches have increased the number of species, showing that all of the aforementioned familiesexhibit a high degree of cryptic diversity shadowed by a limited number of recognizablemorphological traits. This is a challenge for studies of the evolution, taxonomy, and diversity ofinterstitial families as well as for their identification and incorporation into ecological surveys. Bycompiling a comprehensive and updated review on these interstitial families, we hope to promotenew studies on their intriguing evolutionary histories, adapted life forms and high and hiddendiversity.