A new species of porcelain crab, Megalobrachium lemaitrei n. sp., is described and illustrated from the Pacific coasts of Panama and Colombia. The new species is similar to its northern counterpart M. erosum (Glassell, 1936) in the general eroded appearance of the carapace and chelipeds, but is distinguished by the details of the carapace and cheliped ornamentation and pereopod setation. For comparison, Megalobrachium erosum is re-described on the basis of the type and other material.

Thirty-eight specimens collected along the intertidal zone from the southern Pacific of Mexico were examined, resulting in the extension of the distribution ranges for Eucinetops lucasii Stimpson, 1860 from Mazatlán, Sinaloa to San Agustín Bay, Oaxaca (approximately 1488 km southward), E. rubellulus Rathbun, 1923 from Maruata, Michoacán to La Tijera Beach, Oaxaca (802 km southward), and Uca musica Rathbun, 1914 from San Blas, Nayarit to La Encrucijada Biosphere Reserve, Chiapas (1763 km southward).

A new species of Paraseison (Rotifera: Seisonacea: Seisonidae) is described from the eastern Pacific Ocean off the coast of California. Female and male specimens of the new species were attached inside the carapace of the leptostracan crustacean Nebalia hessleri, collected at 19 m depth in La Jolla Canyon. This recording represents the first properly identified and described species of Seisonacea for the American coasts. It is furthermore the second known species of the genus Paraseison, which until now included the species P. annulatus only, reported for the Mediterranean Sea and European west Atlantic coast. The new species was diagnosed mostly by its species-specific trophi morphology and in particular by the shape of its fulcrum. Paraseison kisfaludyi sp. nov. is the fourth described species of the order Seisonacea, which accommodates also Seison nebaliae and the recently described S. africanus, all of which live attached to species in the leptostracan genus Nebalia. Even though the Seisonacea are placed in a crucial position within the phylum and may constitute the link between rotifers and other platyzoans, this taxon has in many respects been neglected in morphological studies. Herein we provide new information on the group's taxonomy, morphology and distribution, which will be useful for future phylogenetic and zoogeographical studies.

Three new species of coral reef boring sponges were found in remote coral reefs from Revillagigedo Island, an archipelago that is 386 km from the continent. Cliona medinae sp. nov. is a sponge with orange-yellow papillae characterized by short almost straight spirasters. Cliona tropicalis sp. nov., is a yellow papillate sponge with a spicule complement similar to the species included in the Cliona viridis complex. However, the new species differs from the rest of the species mainly in its external morphology and by differences in the size and shape of spicules. Thoosa purpurea sp. nov. is characterized by its purple colour, and the spicular complement formed by tylostyles, two amphiaster categories, bi- tri- and tetra-radiate oxyasters and smooth or microspined centrotylote oxeas. In addition, Cliothosa tylostrongylata sp. nov. is also described from coral reefs from the southern Mexican Pacific Ocean. This is a light red species, with tylostyles and tylostrongyles as megascleres and ramose and nodulose amphiasters as microscleres. The four species were found exclusively excavating skeletons of live or dead corals of the genus Pocillopora. This study increases the number of boring sponges known from the Mexican Pacific Ocean to 22 species and it is the first study on marine sponge fauna from the Revillagigedo archipelago.