I've previously blogged about the observation that hermit crabs, who live in the shells of other animals and have to get new shells as they grow, sometimes engage in chains of exchange, that resemble kidney exchange chains, or vacancy chains in labor markets.
In particular, they resemble kidney exchange chains initiated by a deceased donor, in this case initiated by an empty shell.
Here's a new article about hermit crabs which reports that they also engage in something that looks like organ trafficking, with a hermit crab being forcibly removed from its shell by two smaller crabs acting in concert, so that one of them may occupy the now vacant shell while the other moves into the shell of its partner in crime.
Laidre, Mark E. "The Architecture of Cooperation Among Non-kin: Coalitions to Move Up in Nature’s Housing Market." Frontiers in Ecology and Evolution (2021): 928.
"Coalitions typically involve two individuals (a pair), with a third individual being the target that the two-member coalition seeks to evict from its shell (Figure 1). Both members of the coalition have shells of their own, but these individuals and their shells are virtually always smaller than that of the target individual and its shell. Sometimes, based on the commotion and struggle generated during an attempted eviction, additional individuals—beyond the target and the core two-member coalition—are attracted to the area. These additional individuals—referred to as “third parties” or “bystanders”—are not part of the actual coalition, since they do not help at all to evict the target. Generally, third parties simply wait in the vicinity and sometimes position themselves in a social chain, which emanates from the back of the shell of one or both of the coalition members (Figure 2). This positioning in a social chain enables third parties to indirectly benefit, since in the event an eviction succeeds, it can catalyze a succession of back-to-back shell swaps (see Laidre, 2019a). Third parties are thus, in effect, “free riders” (Sigmund, 2010), since their positioning around the coalition offers no advantage whatsoever to the coalition itself as it works to evict the target. Indeed, whether third parties are positioned in a chain or not, they merely wait, performing no pulling actions and never adding any strength or providing any help to the two-member coalition. Interestingly, based on precisely where third parties position themselves, some may potentially even undermine the coalition (see below), effectively acting not merely as “free riders” but as “cheaters” (Sigmund, 2010). Finally, if too many bystanders accumulate, it can lead to chaotic jockeying and repositioning, with the original coalition separating.
"Whether with third parties present or not, the two members of the coalition attempt to physically evict the target. The target remains flipped on its back (i.e., with the dorsal side of its shell on the ground) and the opening of the target’s shell faces upward, allowing both coalition members to use their claws and legs to grab at and pull the anterior portion of the target’s body. As the coalition forcibly pulls, the target attempts to resist by clinging inside its shell. Typically, the two coalition members both pull simultaneously; though at times the two may alternate attempts at pulling, each doing so sequentially as one or the other member briefly rests. Both members of a coalition appear strongly involved, in terms of time and effort. Yet coalitions are not always successful. In some cases, one or both coalition members may give up; or the target individual may manage to flip itself over, escape from being pinned down, and run away. If a coalition is successful at evicting the target, the time till eviction occurs can vary widely, from just minutes up to hours (Laidre, personal observation). Once a coalition is successful and the target individual is evicted from its shell, then the evictee is pushed to the side and remains naked and shell-less as one of the coalition members moves into its now empty shell."
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