Frog crabs (Family Raninidae) are cryptic, burying marine brachyuran crabs adapted for inhabiting soft and sandy bottoms across a wide bathymetric range of tropical to low-latitude temperate regions. The present account encompasses re-description of Lysirude channeri from a depth range of 614–655 m in Bay of Bengal, India. Morphological examination of 76 specimens agreed with earlier type descriptions in having two antero-lateral spines, but this contradicts with the specimens from the South China Sea and off the Philippines. In addition, some specimens from the present study revealed the presence of two carpal spines instead of one described before. However, the genetic congruency of the collected specimens were inferred by developing molecular marker viz. mitochondrial cytochrome oxidase subunit I (mtCOI) gene sequences, representing the first molecular data for Lysirude channeri. Phylogram and genetic distance data (up to 0.60%) justified the genetic congruency of Lysirude channeri as well as the interspecific divergence (up to 15.2%) between Lysirude channeri and Lyreidus brevifrons. Hence, the present study provides complete morphological and molecular data for re-describing the frog crab Lysirude channeri and also delineates its speciation from other related brachyuran crabs.

Several poorly known species of the family Syllidae (Annelida) have been revised and re-described. New diagnosis and taxonomic remarks are provided for Erinaceusyllis erinaceus, Opisthosyllis neglecta comb. nov., Parapionosyllis parapionosylliformis comb. nov., Prosphaerosyllis semiverrucosa comb. nov. and Amblyosyllis granosa.

The occurrence of Nemanthus californicus on the Pacific coast of Costa Rica, the first record outside the type locality in Baja California, is reported. A re-description of its anatomy and notes on its biology are provided. Nemanthus californicus settled on the antipatharian Myriopathes panamensis with the pedal disc surrounding the branch in part or totally, and showed traces of asexual reproduction, an expected adaptation to colonize the black coral. Damage to colonies was significant, causing the death of the black coral. This type of relationship could be parasitism.

Several type series of unusual and poorly known genera of Syllidae have been examined. New diagnoses for the following genera and re-descriptions of their type species are provided: Anguillosyllis, Clavisyllis, Lamellisyllis and Nuchalosyllis; Brachysyllis, previously synonymized with Dioplosyllis, is herein considered to be a valid taxon. The species Brachysyllis infuscata is also re-described. Three genera are considered to be non-valid taxa: Braniella, synonymous with Anguillosyllis; Alluaudella, synonymous with Odontosyllis; and Exogonella, synonymous with Parexogone. Their type species herein are re-described and assigned to their corresponding valid genus. Finally, Exogonoides is considered nomina dubia since its relationships with other syllids could not be established. Keys to species of Anguillosyllis and Brachysyllis are also included.