Consistent behavioural traits and behavioural syndromes in pairs of the false clown anemonefish Amphiprion ocellaris

New cells added to the preoptic area during sex change in the common clownfish Amphiprion ocellaris

Sex differences in cell number in the preoptic area of the hypothalamus (POA) are documented across all major vertebrate lineages and contribute to differential regulation of the hypothalamic-pituitary-gonad axis and reproductive behavior between the sexes. Sex-changing fishes provide a unique opportunity to study mechanisms underlying sexual differentiation of the POA. In anemonefish (clownfish), which change sex from male to female, females have approximately twice the number of medium-sized cells in the anterior POA compared to males. This sex difference transitions from male-like to female-like during sex change. However, it is not known how this sex difference in POA cell number is established. This study tests the hypothesis that new cell addition plays a role. We initiated adult male-to-female sex change in 30 anemonefish (Amphiprion ocellaris) and administered BrdU to label new cells added to the POA at regular intervals throughout sex change. Sex-changing fish added more new cells to the anterior POA than non-changing fish, supporting the hypothesis. The observed effects could be accounted for by differences in POA volume, but they are also consistent with a steady trickle of new cells being gradually accumulated in the anterior POA before vitellogenic oocytes develop in the gonads. These results provide insight into the unique characteristics of protandrous sex change in anemonefish relative to other modes of sex change, and support the potential for future research in sex-changing fishes to provide a richer understanding of the mechanisms for sexual differentiation of the brain.

Colour patterns influence symbiosis and competition in the anemonefish-host anemone symbiosis system

Colour patterns in fish are often used as an important medium for communication. Anemonefish, characterized by specific patterns of white bars, inhabit host anemones and defend the area around an anemone as their territory. The host anemone is used not only by the anemonefish, but also by other fish species that use anemones as temporary shelters. Anemonefish may be able to identify potential competitors by their colour patterns. We first examined the colour patterns of fish using host anemones inhabited by Amphiprion ocellaris as shelter and compared them with the patterns of fish using surrounding scleractinian corals. There were no fish with bars sheltering in host anemones, although many fish with bars were found in surrounding corals. Next, two fish models, one with white bars and the other with white stripes on a black background, were presented to an A. ocellaris colony. The duration of aggressive behaviour towards the bar model was significantly longer than that towards the stripe model. We conclude that differences in aggressive behaviour by the anemonefish possibly select the colour patterns of cohabiting fish. This study indicates that colour patterns may influence not only intraspecific interactions but also interspecific interactions in coral reef ecosystems.

Stable and persistent male-like behavior during male-to-female sex change in the common clownfish Amphiprion ocellaris

Many fish species exhibit natural sex change as part of their life, providing unique opportunities to study sexually-differentiated social behaviors and their plasticity. Past research has shown that behavioral sex change in the female-to-male (protogynous) direction occurs rapidly and well before gonadal sex change. However, little is known about the timecourse of behavioral sex change in male-to-female (protandrous) sex-changing species, limiting our ability to compare patterns of behavioral sex change across species and identify conserved or divergent underlying mechanisms. Using the protandrous sex changing anemonefish Amphiprion ocellaris, we assessed behavior (aggression and parental care) and hormones (estradiol and 11-ketotestosterone) in fish over six months of sex change, and compared those fish against their non-changing partners as well as control males and females. Contrary to expectations, we found that sex-changing fish displayed behavior that was persistently male-like, and that their behavior did not become progressively female-like as sex change progressed. Hormones shifted to an intermediate profile between males and females and remained stable until gonads changed. These results support a new perspective that the timecourse for protandrous sex change in anemonefish is completely distinct from other well-established models, such that behavioral sex change does not occur until after gonadal sex change is complete, and that sex-changing fish have a stable and unique behavioral and hormonal phenotype that is distinct from a male-typical or female-typical phenotype. The results also identify aspects of sex change that may fundamentally differ between protandrous and protogynous modes, motivating further research into these remarkable examples of phenotypic plasticity.

Large host anemones can be shelters of a diverse assemblage of fish species, not just anemonefish

The mutualistic relationships between anemonefish (Amphiprion; Pomacentridae) and host sea anemones are well known, but host anemones are also used as shelter by damselfish (Pomacentridae), wrasses (Labridae) and cardinalfish (Apogonidae). The threespot dascyllus Dascyllus trimaculatus (Pomacentridae) is known to live on or near host anemones in their immature phase. Nonetheless, detailed studies on the use of host anemones by other fish species have not yet been conducted. To understand the factors that influence the use of host anemones by other fish species, this study investigated the fish biota around host anemones in the Ryukyu Archipelago. Other than Amphiprion spp. and D. trimaculatus, 10 additional species of fish (9 species of damselfish and 1 species of cardinalfish) were confirmed to temporarily use host anemones as shelter, and five species of wrasse including Labroides dimidiatus came to clean anemonefish. Logistic regression analyses (independent variable: presence or absence of other species of fish; dependent variables: anemonefish aggressiveness, size of host anemone, number of D. trimaculatus) indicated that the size of host anemones is important for the presence of fish, both in species using the anemone as temporary shelter and in cleaner species. Large host anemones can provide shelter and food resources for other species of coral reef fish as well as for anemonefish.

Negotiations over parental care: a test of alternative hypotheses in the clown anemonefish

In species with biparental care, conflict arises over how much each parent provides to their offspring because both parents benefit from shifting the burden of care to the other. Here, we tested alternative hypotheses for how parents will negotiate offspring care using a wild population of clownfish (Amphiprion percula). We experimentally handicapped parents by fin-clipping the female in 23 groups, the male in 23 groups, and neither parent in 23 groups and measured changes in indicators of female, male, and pair effort in response to handicapping. First, we found that handicapping resulted in a decrease in the number of eggs laid by fin-clipped females and a decrease in the amount of parental care by fin-clipped males. Second, contrary to predictions, female effort did not change in response to the male being handicapped, or vice versa. Finally, the number of embryos that matured to hatching, an indicator of pair effort, was not influenced by the manipulation, suggesting that although the handicap was effective, clownfish do not face the predicted “cost to conflict” when one parent is handicapped. Together, these results test the generality of theoretical predictions and uncover novel questions about whether and how negotiations operate in systems where interests are aligned.

Ecological and social constraints combine to promote evolution of non-breeding strategies in clownfish

Individuals that forgo their own reproduction in animal societies represent an evolutionary paradox because it is not immediately apparent how natural selection can preserve the genes that underlie non-breeding strategies. Cooperative breeding theory provides a solution to the paradox: non-breeders benefit by helping relatives and/or inheriting breeding positions; non-breeders do not disperse to breed elsewhere because of ecological constraints. However, the question of why non-breeders do not contest to breed within their group has rarely been addressed. Here, we use a wild population of clownfish (Amphiprion percula), where non-breeders wait peacefully for years to inherit breeding positions, to show non-breeders will disperse when ecological constraints (risk of mortality during dispersal) are experimentally weakened. In addition, we show non-breeders will contest when social constraints (risk of eviction during contest) are experimentally relaxed. Our results show it is the combination of ecological and social constraints that promote the evolution of non-breeding strategies. The findings highlight parallels between, and potential for fruitful exchange between, cooperative breeding theory and economic bargaining theory: individuals will forgo their own reproduction and wait peacefully to inherit breeding positions (engage in cooperative options) when there are harsh ecological constraints (poor outside options) and harsh social constraints (poor inside options).

Scaring Nemo: Contrasting effects of observer presence on two anemonefish species

Behaviours of Clark's anemonefish Amphiprion clarkii and the dusky anemonefish Amphiprion melanopus were studied in Vanuatu. Six anemones and their resident fish were observed for typical behaviours (hiding, watching, roaming, inter-, and intraspecific behaviour) with and without the presence of a snorkelling observer. Observer presence had significant but contrasting effects on hiding behaviour in A. clarkii and A. melanopus. Bolder anemonefish species may be able to outcompete other species in areas with high human presence.

Response to underwater laser pointer in the Orange-finned Anemonefish Amphiprion chrysopterus and three-spot damselfish Dascyllus trimaculatus

Response of orange-finned anemonefish Amphiprion chrysopterus and three-spot damselfish Dascyllus trimaculatus to red laser-pointer light was studied in Mo'orea, French Polynesia. Four magnificent anemones Heteractis magnifica and their resident fish were observed for typical behaviours (biting, chasing, hiding, posing, lunging and retreating) with and without exposure to laser-pointer light. Lunging behaviour increased significantly for both fish species upon exposure to laser-pointer light; none of the other behaviours changed significantly. We advance the hypothesis that orange-finned anemonefish and three-spot damselfish interpret laser pointer stimulation as a territorial threat.

Host choice and fitness of anemonefish Amphiprion ocellaris (Perciformes: Pomacentridae) living with host anemones (Anthozoa: Actiniaria) in captive conditions

In this study, we investigated the host choice of naïve Amphiprion ocellaris, a specialist, at two different stages of development (newly settling juveniles and post-settlement juveniles). The fish were exposed to their natural and unnatural host species in the laboratory and their fitness was assessed in terms of activity and growth rate. Newly settling juveniles exhibited little host preference, while post-settlement juveniles immediately associated with their most common host in the wild. The analysis of fish activity confirmed that A. ocellaris is diurnal; they are most active in the morning, less at midday and barely move at night. The average travelling distance of juveniles was shorter in the groups living with their natural host, increasing in the groups living with an unnatural host and was highest in groups that did not become associated with any other unnatural host species. Post-settlement juveniles living with the natural host species grew better than those living with unnatural hosts or without anemone contact. These results suggest that the welfare of A. ocellaris in captivity will be optimized by keeping them with their natural anemone host species, although more generalist Amphiprion species may survive in association with other hosts.

Behavioural consistency and group conformity in humbug damselfish

Humbug damselfish, Dascyllus aruanus, are a common coral reef fish that form stable social groups with size-based social hierarchies. Here we caught whole wild groups of damselfish and tested whether social groups tended to be comprised of animals that are more similar to one another in terms of their behavioural type, than expected by chance. First we found that individuals were repeatable in their level of activity and exploration, and that this was independent of both absolute size and within-group dominance rank, indicating that animals were behaviourally consistent. Secondly, despite the fact that individuals were tested independently, the behaviour of members of the same groups was significantly more similar than expected under a null model, suggesting that individual behaviour develops and is shaped by conformity to the behaviour of other group members. This is one of the first studies to demonstrate this group-level behavioural conformity in wild-caught groups.