REVIEW Behavioural and neuronal basis of olfactory imprinting and kin recognition in larval fish Gabriele Gerlach 1,2,3, *, Kristin Tietje 1 , Daniela Biechl 4 , Iori Namekawa 5 , Gregor Schalm 1 and Astrid Sulmann 1 ABSTRACT Imprinting is a specific form of long-term memory of a cue acquired during a sensitive phase of development. To ensure that organisms memorize the right cue, the learning process must happen during a specific short time period, mostly soon after hatching, which should end before irrelevant or misleading signals are encountered. A well- known case of olfactory imprinting in the aquatic environment is that of the anadromous Atlantic and Pacific salmon, which prefer the olfactory cues of natal rivers to which they return after migrating several years in the open ocean. Recent research has shown that olfactory imprinting and olfactory guided navigation in the marine realm are far more common than previously assumed. Here, we present evidence for the involvement of olfactory imprinting in the navigation behaviour of coral reef fish, which prefer their home reef odour over that of other reefs. Two main olfactory imprinting processes can be differentiated: (1) imprinting on environmental cues and (2) imprinting on chemical compounds released by kin, which is based on genetic relatedness among conspecifics. While the first process allows for plasticity, so that organisms can imprint on a variety of chemical signals, the latter seems to be restricted to specific genetically determined kin signals. We focus on the second, elucidating the behavioural and neuronal basis of the imprinting process on kin cues using larval zebrafish (Danio rerio) as a model. Our data suggest that the process of imprinting is not confined to the central nervous system but also triggers some changes in the olfactory epithelium. KEY WORDS: Coral reef fish, Larval dispersal, Orientation, MHC peptides, Olfaction, Crypt cell Introduction Olfactory guided navigation in coral reef fish Like many aquatic organisms, coral reef fish show a dual life stage, where settled adults produce dispersing larvae. At hatching, planktonic larvae drift away from the reef to spend a species- dependent time (larval dispersal duration) in the open ocean – probably to avoid high predation in the reef. These millimetre-sized larvae quickly (within a few days of dispersal) turn into relatively powerful juvenile swimmers, reaching swimming speeds of several centimetres per second (Fisher et al., 2000; Fisher and Wilson, 2004; Stobutzki and Bellwood, 1994, 1997). Until recently, the distribution and settlement of coral reef fish were assumed to be purely driven by currents and stochastic storm events. However, the persistence of marine populations at small isolated oceanic islands requires that a significant number of juveniles return to the natal habitat after their pelagic dispersal phase (Robertson, 2001). Here, and even in less isolated habitats, self-recruitment and natal homing has been assumed to be (far) greater than purely planktonic dispersal would predict from modelling approaches (Armsworth, 2000; Cowen et al., 2000; Staaterman and Paris, 2014; Wolanski et al., 1997). We used population genetic analysis to demonstrate that up to 60% of juvenile cardinalfish, Ostorhinchus doederleini, could be assigned to the adult reef population where they were about to settle (Gerlach et al., 2007a, 2016). Natal homing of other species, e.g. clownfish juveniles (Amphiprion spp.) was also confirmed by mark–recapture studies, otolith tagging and microchemistry studies (Jones et al., 1999; Swearer et al., 1999; Thorrold et al., 2006). As it is not possible to track larvae in the ocean, dispersal distances are based on catching larvae in plankton tows and by modelling approaches using the pelagic larval duration as a proxy (but see Weersing and Toonen, 2009). Dispersal distances of coral reef fish larvae are assumed to be shorter than 150 km depending on the species (Burgess et al., 2007; Paris and Cowen, 2004). Despite a potentially wide distribution, numbers of homing coral reef fish juveniles are far higher than expected by random movement. Orientation capabilities could help them to find their way back to their natal reefs. Finding the way back to a natal reef, river or beach after week- or year-long dispersal might require a learning and memory process involving time-dependent specific parameters of this natal place. For orientation-guided homing to natal reefs, coral reef fish larvae must remember sensory parameters that they experienced directly after hatching, which is the only time in which they can obtain reliable ‘home cues’, as larvae – with fully developed olfactory and visual sensory systems – start dispersing into the open ocean on the night of hatching. To explain navigation over long distances, we provided evidence that the juvenile cardinalfish (O. doederleini) can use time- compensated sun compass orientation during the day (Mouritsen et al., 2013) and a magnetic compass at night (Bottesch et al., 2016). For orientation at closer distances, there is evidence that reef fish larvae can also respond to acoustic cues for orientation (Radford et al., 2010, 2011). Additionally, we have documented a pronounced importance of olfaction in the homing of coral reef fish larvae (Atema et al., 2002; Gerlach et al., 2007a; Paris et al., 2013). Coral reef fishes such as O. doederleini, Pomacentrus coelestis and several other species of apogonid and pomacentrid juveniles were shown to prefer the smell of water collected from a reef to that of open ocean water (Atema et al., 2002). Both species were capable of distinguishing between chemical cues from reefs 1 Institute of Biology and Environmental Sciences, Carl von Ossietzky University Oldenburg, 26129 Oldenburg, Germany. 2 Helmholtz Institute for Functional Marine Biodiversity Oldenburg (HIFMB), 26129 Oldenburg, Germany. 3 Centre of Excellence for Coral Reef Studies and School of Marine and Tropical Biology, James Cook University, QLD 4811, Australia. 4 Graduate School of Systemic Neurosciences & Department Biology II, Ludwig-Maximilians-Universita ̈ t Munich, 82152 Planegg-Martinsried, Germany. 5 Friedrich Miescher Institute for Biomedical Research, 4058 Basel, Switzerland. *Author for correspondence (gabriele.gerlach@uni-oldenburg.de) G.G., 0000-0001-5246-944X; I.N., 0000-0002-1012-9823 1 © 2019. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2019) 222, jeb189746. doi:10.1242/jeb.189746 Journal of Experimental Biology