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Several times in the paper, the idea of indirect fitness has been mentioned to replace notions of direct fitness. Fitness is measured in terms of genes contributed to the next generation by an individual; this can be done through indirect or direct measures (Burt 1995). Direct fitness occurs when individuals themselves reproduce, ensuring that their genes are passed onto their own offspring. The sole reproductive female in S. regalis propagates its genes through direct fitness, since the offspring owe their existence to her and no one else. On the other hand, indirect fitness entails an individual helping to raise related kin rather than reproducing on their own. By doing so the individual makes certain that the offspring will survive into adulthood, passing on some of their own genes due to relatedness (Burt 1995). The concept of kin selection asserts that indirect fitness can be selected for as long as Hamilton’s Rule of C-B(R)<0 is upheld (Box 3). The male defenders spread their genes to future generations with indirect fitness through caring for juvenile siblings. Therefore, eusocial individuals can only have their genes passed to the next generation through indirect fitness by facilitating the growth of siblings into adulthood—only the sole reproductive individual take part in direct fitness within eusocial species.
The organism used in this chapter, Synalpheus regalis ( [link] ) , is commonly found in sponges within the tropical West Atlantic along with four other eusocial species of shrimp: Synalpheus filidigitus, Synalpheus chacei, Synalpheus brooksi , and Synalpheus “rathbunae A” (Duffy 1992). These sympatric species of social Synalpheus rarely overlap in host sponges used and never co-occur in individual sponges so it looks like the shrimp exclude heterospecific competitors (Duffy et al. 2002). The populations in sponges contain a few hundred individuals, each with two generations of kin. For their entire lives, these shrimp live in the internal canals of the host sponge, using it as a food resource and shelter ( [link] ). The shrimp feed on the host tissues as well as on the detritus, which includes bodies of dead organisms or fecal material. Since there is no planktonic stage in this species and no other known mechanism of dispersal, juveniles stay in their natal sponge for the duration of their lives (Duffy et al. 2002). [link] shows the sampling of 64 sponges from two species that are commonly occupied by sponge dwelling shrimp. Fewer than 5% of the individual sponges used by Synalpheus regalis in Carrie Bow Cay, Belize were left unoccupied, implying a strong competition for suitable nest sites that could discourage solitary living. This ecological constraint results colonies with closely related individuals and outside non-related individuals are seen as intruders who are attacked (Duffy et al. 2002). Mature females are easily identified by their ovaries which are visible through their transparent bodies in S. regalis . The other shrimp without ovaries are either mature males (large bodied) or juveniles (small bodied). The only morphological caste difference is found in the queen, the sole reproductive individual, who lacks the large snapping claw and instead has a minor claw used for feeding (Duffy et al. 2002). This is feasible since the queen does not need a large claw since she has a caste of defenders, whose primary duty is to ensure her survival.
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