Bunaken National Park has been surveyed for a fourth time in 14 years, in an attempt to establish the species composition of heterobranch sea slugs in a baseline study for monitoring programs and protection of this special park. These molluscs are potentially good indicators of the health of an ecosystem, as many are species-specific predators on a huge variety of marine benthic and sessile invertebrates from almost every taxonomic group. Additionally, they are known to contain bio-compounds of significance in the pharmaceutical industry. It is therefore of paramount importance not only to document the species composition from a zoogeographic point of view, but to assist in their protection for the future, both in terms of economics and aesthetics. These four surveys have documented more than 200 species, with an approximate 50% of each collection found only on that survey and not re-collected. Many species new to science have also been documented, highlighting the lack of knowledge in this field.

Indonesia is famous for its underwater biodiversity, which attracts many tourists, especially divers. This is also true for Sangihe Islands Regency, an area composed of several islands in the northern part of North Sulawesi. However, Sangihe Islands Regency is much less known than, e.g., Bunaken National Park (BNP, North Sulawesi). The main island, Sangihe, has recently experienced an increase in tourism and mining activities with potentially high impact on the environment. Recently, monitoring projects began around BNP using marine Heterobranchia as indicators for coral reef health. No information about this taxon exists from the remote islands in North Sulawesi. The present study represents the first monitoring study ever and focuses on marine Heterobranchia around Sangihe. In total, 250 specimens were collected, which could be assigned to Sacoglossa (3), Anthobranchia (19), and Cladobranchia (1). Despite the low number (23 versus 172 in BNP), at least eight species (35%) are not recorded from BNP, probably indicating differences in habitat, but also influence of a strong El Niño year in 2016. Here we also report for the first time a Chromodoris annae specimen mimicking C. elisabethina, and the discovery of a new Phyllidia species.

As ephemeral, benthic, secondary consumers usually associated with sessile coral reef organisms, marine heterobranchs are good indicators of the health of marine tropical habitats. Thus, marine Heterobranchia have recently become a major target for monitoring programs. For this work, an extensive survey was made in Bangka Archipelago, the first of its kind in this area. Bangka Archipelago is composed of small islands and the adjacent coastline of North Sulawesi. The substrate is dominated by biogenic reefs and volcanic rocks, thus forming highly diverse habitats. In total, 149 species were collected and/or photo-documented in September 2017 and September 2018, which represents the dry season. They can be assigned to the Cephalaspidea, Aplysiida, Pleurobranchida, Nudibranchia, and Eupulmonata. Thirty-three species are new to science, highlighting the lack of knowledge about the group and especially about this region. Our data provide a baseline for future monitoring surveys, as the anthropogenic pressures on Bangka Archipelago are increasing due to enhanced touristic activities, but also due to large scale environmental changes caused by previous mining operation activities.

Two new species of Phyllidia from North Sulawesi, Indonesia, Phyllidia fontjeisp. nov. and Phyllidia ovatasp. nov., are described based on morphology and molecular barcoding of CO1 and/or 16S. Both species are rare and distinctive and can be easily recognised by their colouration. Additionally, histological sections were made of the holotype of Phyllidia fontjeisp. nov. and a similarly sized Phyllidia ocellata and these morphologies are compared with the only other detailed histological examination of the Mediterranean Phyllidia flava.

From a collection of IsopodaAnthuroidea the new species Eisothistos minahasaensis sp. nov ., Mesanthura ratulangii sp. nov ., Mesanthura heliolatra sp. nov ., Haliophasma annaclericiae sp. nov ., Apanthuretta markuslasuti sp. nov ., Apanthuretta pinchans sp. nov ., Amakusanthura mamuajae sp. nov ., and Apanthurella cuspidobtusa gen. nov. et sp. nov . are described. The species were found in near-shore samples of coral rubble and sediment in North Sulawesi (Indonesia). The genus Haliophasma Haswell, 1881 is compared with Malacanthura Barnard, 1925 and the genus Exanthura Barnard, 1914 is reinstalled to accommodate species with a basal hook on the A1. Nemanthura Wägele, 1981 is synonymized with Exanthura Barnard, 1914. A new classification of the Apanthura group, including the diagnosis of the new genus Apanthurella gen. nov ., is provided together with lists of hitherto described species. Diagnoses for the genera Haliophasma , Malacanthura , Exanthura , Apanthura , Apanthuretta , Apanthuropsis , Amakusanthura , and Apanthurella are presented.

Banggai cardinalfish (Pterapogon kauderni) is an endangered, range-restricted apogonid endemic to the Banggai Archipelago (Banggai Islands), Indonesia, and a flagship species of the marine ornamental fish trade. Aquarium-mediated releases have established non-native populations, including in Lembeh Strait, North Sulawesi. This study compared population density, size structure, and microhabitat use of P. kauderni between introduced and native ranges using underwater visual census along 50-m x 5-m belt transects in Lembeh Strait (March 2021) and in the Banggai Islands (October 2021). Individuals were counted, assigned to three size classes (recruit, 1.8 cm, SL; juvenile, 1.8-3.5 cm, SL; adult, > 3.5 cm, SL), and recorded with associated protective microhabitats: branching coral, Diadema sea urchins, sea anemones, and seagrass. A total of 3,997 individuals from 45 colonies were recorded (42 with recorded microhabitat). Mean density was 2.00±0.62 ind m-2, with no significant difference between introduced and native ranges (Mann–Whitney U = 10.0, p = 0.69). Populations were adult-dominated overall (73.7%), while pooled descriptive proportions showed more juveniles in Lembeh and more adults in Banggai; however, station-level comparisons detected no significant regional differences. Coral and Diadema were the most frequent colony microhabitats, and 88.2% of individuals were associated with Diadema. Colonies with Diadema contained significantly more individuals (p = 0.005), and abundance increased with the number of protective microhabitats present (Spearman ρ = 0.34, p = 0.028). An exploratory colony-level comparison showed higher median abundance in colonies with Diadema rather than in colonies with none, but this result was interpreted cautiously because multiple colonies occured within the same stations. These findings provide evidence for habitat-based monitoring and management of native and introduced populations in Indonesian waters.