The Galápagos Islands are recognized for their unique biota and are one of the world’s largest marine protected areas. While invasions by non-indigenous species are common and recognized as a significant conservation threat in terrestrial habitats of the Archipelago, little is known about the magnitude of invasions in its coastal marine waters. Based upon recent field surveys, available literature, and analysis of the biogeographic status of previously reported taxa, we report 53 non-indigenous species of marine invertebrates in the Galápagos Islands. Forty-eight (90.6%) of these species are newly reported or newly recognized as introduced, a nearly ten-fold increase from the five species previously recognized as non-indigenous. Of these 48 species, 30 (62.5%) were newly discovered in surveys commenced in 2015. Ascidians (11 species), bryozoans (10), polychaetes (9), and hydroids (8) account for 38 (71.7%) of the introduced species. Our analyses further detected 33 cryptogenic invertebrate and algal species and one littoral vascular plant. Most taxonomic groups remain to be assessed for the presence of non-indigenous species. Importantly, the recent field surveys were restricted predominantly to one habitat (harbor biofouling) on two islands, further suggesting that introduced species richness for the Galápagos Islands may be considerably higher. Most of the introduced species treated here were likely brought to the Galápagos by ships. While we presume that most if not all of the many thousands of vessels arriving in the Galápagos Islands since the 1500s had marine animals and plants attached to their hulls, we hypothesize that the general absence in the Islands of extensive shoreline structures (in the form of wharves, docks, pilings, and buoys) until the last half of the 20th century may have constrained extensive colonization by fouling species. The proliferation of shoreline structures may have both provided expanded habitat for non-indigenous species that had earlier colonized natural substrates, as well as having facilitated a 20^th and 21^st century wave of new invasions in the Galápagos Islands. Our results represent the greatest reported increase in the recognition of the number of invasions for any tropical marine environment in the world. This work suggests that the number and potential ecological impacts of non-indigenous species in tropical marine and maritime habitats may be substantially underestimated in other regions of the world. Our study demonstrates that tropical marine invasions deserve significant attention, not only in a biogeographical, historical, and ecological context, but also from a management perspective, especially in the Galápagos and other high-value conservation areas.

An account is given of hydroids collected in 2015 and 2016 from port and harbor fouling communities in the Galápagos Islands. Also included is the hydroid of Ectopleura media, discovered on the wreck of the tanker Jessica near Isla San Cristóbal in 2001. Among 20 species reported herein were six anthoathecates and 14 leptothecates. Most common in the samples were the kirchenpaueriid Ventromma halecioides and the halopteridid Halopteris alternata. Eight species (Bougainvillia muscus, Bimeria vestita, Clytia elongata, C. obliqua, C. thornelyi, Obelia oxydentata, Eucheilota sp., and Halecium labiatum) are reported for the first time from the Galápagos archipelago. Three of them (Clytia elongata, C. thornelyi, and Halecium labiatum) are also new to the eastern Pacific. Seven species treated here are considered introduced by shipping to the islands, bringing to eight the number of introduced hydroids. In addition, we treat four species as cryptogenic, bringing to five the total number of the latter in the Galápagos. The binomena Obelia thornelyi Nutting, 1927, Clytia stolonifera Blackburn, 1938, and C. latitheca Millard and Bouillon, 1973 are regarded as synonyms, with the first of these having nomenclatural priority. Meanwhile, the senior synonym O. thornelyi is reassigned to the genus Clytia Lamouroux, 1812 as C. thornelyi. Another species in the samples, Dynamena distans Lamouroux, 1816 (also widely known as Sertularia distans and Tridentata distans), is combined for the first time with the genus Amphisbetia L. Agassiz, 1862, as A. distans.

The biofouling community on Santa Cruz and Baltra Islands, Galapagos, was surveyed in 2016 based on samples from settlement plates deployed in 2015 and 2016 at three different sites. We report numerous new records for the Galapagos fauna: one novel family (Opheliidae), nine novel genera, and 15 novel species records were documented in a total of seven families. Six introduced species were found, the syllid Myrianida pachycera, the sabellids Branchiomma bairdi, Branchiomma sp. and Pseudobranchiomma schizogenica, and the serpulids Hydroides elegans, Hydroides sanctaecrucis, along with one cryptogenic species, the spionid Dipolydora armata. Non-native species come from either the warm waters of the Western Atlantic or Indo-West Pacific Oceans.

Bryozoans have been infrequently studied in the Galapagos Islands, and even less so in nearshore biofouling assemblages. Based on surveys of Galapagos fouling communities in 2015 and 2016, we report 18 bryozoan species. At least 10 species are new records for the Galapagos, including Amathia sp. 1 (Bowerbankia group), Beania klugei Cook, 1968, Bugulina stolonifera (Ryland, 1960), Caulibugula cf. dendrograpta (Waters, 1913), Celleporaria inaudita Tilbrook, Hayward and Gordon, 2001, Hippopodina tahitiensis (Leca and d’Hondt, 1993), Nolella stipata Gosse, 1855, Schizoporella pungens (Canu and Bassler, 1928), Sundanella sp. and Watersipora subtorquata (d’Orbigny, 1852) sensu Vieira et al. 2014a. More than half of these are considered likely ship-borne introductions. Schizoporella pungens was the most abundant bryozoan. Continuing explorations of Galapagos biofouling communities will likely reveal additional introduced and native bryozoan species.

The Galápagos Marine Bioinvasions Expedition (April 24–May 4, 2016) carried out a survey of the biofouling communities on a number of floating docks and pilings, as well as numerous previously deployed settlement panels both uncaged and caged, on two of the Galápagos Islands: Santa Cruz and neighboring Baltra. The suspended panels had been submerged for either 3 months or 14 months. Studies were concentrated at two sites on Santa Cruz Island, which is the location of the main town Puerto Ayora and has the most vessel traffic. The small adjacent Baltra Island contains the airport and there is frequent boat traffic there, as well as a Navy base with a floating dock. Ascidians were the dominant foulers on most panels, especially the caged ones, with a dramatic difference in coverage between caged and uncaged which is presumed to result from differential fish predation pressure since the panels were all suspended with none touching the benthos. There were schools of small fish around all the docks observed to be constantly nipping at the dock surfaces. Eighteen species of ascidians were collected, comprising 12 new records. Of the 14 identified to species level, 11 are considered introduced and 3 cryptogenic. The most common species included Didemnum perlucidum Monniot, 1983, Diplosoma listerianum (Milne Edwards, 1841), Polyclinum constellatum Savigny, 1816, Ascidia sydneiensis Stimpson, 1855, Ascidia ceratodes (Huntsman, 1912), an Ascidia sp. with long tunic spines, Polyandrocarpa zorritensis (Van Name, 1931), Styela canopus (Savigny, 1816), and Pyura haustor (Stimpson, 1864). Other foulers included small colonies of Botrylloides niger Herdman, 1886, a few small colonies of Symplegma brakenhielmi (Michaelsen, 1904), a dark Didemnum sp. and another unidentified didemnid, a pink Symplegma rubra Monniot, 1972, three very tiny Molgula sp. and a single specimen of Microcosmus exasperatus Heller, 1878. Several colonies of Botrylloides giganteus (Pérès, 1949) were retrieved from plates on the Baltra Navy dock. Two large specimens (~ 6 cm) of Halocynthia dumosa (Stimpson, 1855) (usually incorrectly referred to as H. hispida (Herdman, 1881)) were collected from the Baltra dock and one by SCUBA at 12.2 m off of Bartolome Island. A complete species list is presented, with a comparison with previous published and unpublished records.