Mirror image stimulation could reverse social-isolation-induced aggressiveness in the high-level subsocial lactating spider.

Conspecific aggressiveness often increases after social isolation for species that are not entirely solitary, and this increased aggression could also be reversed after resocialization. However, literature on this aggression plasticity refers to either permanently social or low-level subsocial species in invertebrates. Examinations of conspecific aggressiveness reversibility in high-level subsocial invertebrates, in which offspring cohabitate with parents for a certain period of time after sexual maturation, would enhance the understanding of the role of conspecific-aggression plasticity in social evolution. Here, using the lactating spider Toxeus magnus, which exhibits extremely high-level subsociality, we assessed three questions. (1) Is its conspecific aggression affected by social living and/or kinship? The results indicated that conspecific aggression increased after social isolation, while kinship did not affect aggressiveness. (2) Could the social-isolation-induced higher aggression be reversed after resocialization? The results showed that the increased aggression of the spiders could be reversed 3 days after resocialization. (3) What is the proximate mechanism that caused the aggression reversibility by resocialization? A simulated resocialization experiment in which single spider was provided with mirrors demonstrated that the visual cues of conspecifics alone could reverse the aggression after 6 days. These results indicate that the high-level subsocial invertebrate showed aggressiveness reversibility without chemical cues. This is more similar to permanently social species rather than to low-level subsocial species, and visual cues could be vital to induce aggression change. These results suggest that conspecific-aggression reversibility might play a key role in social evolution and may functionally enhance species' adaptiveness under variable conditions.

Spatial distributions, feeding ecologies, and behavioral interactions of four rabbitfish species (Siganus unimaculatus, S. virgatus, S. corallinus, and S. puellus).

Clarifying the underlying mechanisms that enable closely related species to coexist in a particular environment is a fundamental aspect of ecology. Coral reefs support a high diversity of marine organisms, among which rabbitfishes (family Siganidae) are a major component The present study aimed to reveal the mechanism that allows rabbitfishes to coexist on coral reefs in Okinawa, Japan, by investigating the spatial distributions, feeding ecologies, and behavioral interactions of four species: Siganus unimaculatus, S. virgatus, S. corallinus, and S. puellus. All four species had a size-specific spatial distribution, whereby small individuals were found in sheltered areas that were covered by branching and bottlebrush Acropora spp. and large individuals were found in both sheltered and exposed rocky areas. However, no clear species-specific spatial distribution was observed. There was some variation in the food items taken, with S. unimaculatus primarily feeding on brown foliose algae, red foliose algae, and red styloid algae, and S. virgatus and S. puellus preferring brown foliose algae and sponges, respectively. However, S. corallinus did not show any clear differences in food preferences from S. virgatus or S. unimaculatus, mainly feeding on brown foliose algae and red styloid algae. The four species exhibited differences in foraging substrate use, which was probably related to differences in their body shape characteristics: S. unimaculatus has a slender body with a remarkably protruding snout and mainly used concave substrates for feeding, whereas S. virgatus has a deeper body with a low degree of snout protrusion and mainly used convex substrates. The other two species have a low degree of snout protrusion combined with a deeper body in the case of S. corallinus and a slender body in the case of S. puellus and used concave, flat, and convex substrates to an equal degree for feeding. Behavioral interactions were categorized into "agonistic behaviors" (attack and agonistic displays) and "no interactions." For all four species, a greater frequency of agonistic behaviors was observed when two conspecific pairs approached each other than when two heterospecific individuals encountered each other. Together, these results suggest that food item partitioning is one of the main factors enabling the coexistence of these four syntopic rabbitfish species, which is enhanced by species-specific differences in feeding substrates as a result of their different body shape and behavioral characteristics.