The major measure of success of an organism’s fitness is that the extent to which its genes are propagated through reproduction. Why, then, would an organism forgo the chance to spread copies of their genes by never reproducing? Several answers to this question have been formulated through experiments and observations of eusocial species found in the insect orders Hymenoptera and Isoptera (Wilson et al. 2005, Robinson 1992). One explanation is that ecological constraints create situations where eusociality is advantageous for both the reproductive individual and the non-reproducers (Buckle 1980). For instance, nests are difficult and dangerous to come by for most eusocial species, reducing the chances that a solitary individual could find another unoccupied nest, which is necessary for juveniles’ survival (Michener et al. 1974). Even if a nest is found, the solitary individuals are unable to defend their nests while foraging for food, resulting in a low survivorship of young due to predation (Batra 1966). In simple social species such as Liostenogaster flavolineata, adult worker females have a chance of becoming queen when the current one dies; thus becoming the sole reproductive individual (Bridge et al 2007). By remaining in natal nests to assist the development of juvenile siblings, adults forgo their opportunity to reproduce but still increase their fitness (copies of genes in future generations) through indirect measures and kin selection (Box 1). Overall, in most social species including the sponge-dwelling shrimp Synalpheus regalis, eusociality arises due to ecological factors of limited food resources and shelter. These situations can be combated through the creation of castes that chiefly forage, defend, or reproduce—leading to the formation of eusociality (Crespi et al. 1995).

The organism used in this chapter, Synalpheus regalis (Picture 1), 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 (Box 4). 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). Table 3 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.

Picture 1: The interaction of two Synalpheus regalis facing off in an agonistic interaction
(permission obtained: http://web.vims.edu/bio/mobee/index.html?svr=www)

S. regalis occupies the small internal canals of tropical sponges, with colonies as large as several hundred individuals. The sponge-dwelling shrimp, Synalpheus regalis, has increased reproductive success though group living. Experiments show that larger groups out-reproduce smaller groups in this species. In Figure 2, it is documented that the larger the colony size, the more eggs produced by the colony, thus greater reproductive success (Duffy 1996b). Through the sampling of unmanipulated colonies, data shows the colonies’ reproductive yield increasing by a factor of 177 times in colony sizes ranging from 2-356 individuals (Duffy et al 2002). The colonies of S. regalis consist of large kin groups with adults of at least two generations, allowing for the non-reproducing individuals to still pass on their genes through rearing relatives—whether it be through foraging, defending, or any other job that increases juvenile survival—rather than having their own offspring.

Figure 2: Egg production of Synalpheus regalis based on total colony egg production and average per capita production, as a function of colony size (number of individuals). Average per capita represents the clutch size divided by the total number of individuals in the colony—the egg number is the size of the clutch. This figure shows that the larger the colony size, the increase in colony egg reproduction (Duffy et al. 2002).

Within the colony there are large males that defend the colony from intruders. Their sole duty is to protect the remaining two castes, the sole reproductive female and the juveniles (Picture 3). Experimental data by J. Emmett Duffy show the emergence of castes in the eusocial shrimp—with large males more likely to snap and attack heterospecific intruders than juveniles or female queens (Figure 4). Thus the first requirement of eusociality, reproductive division of labor, is satisfied in S. regalis (Duffy 1998). This is fulfilled by adults remaining in their natal nests due to the difficulty of colonizing another unoccupied nest. Attempting to join another colony is also difficult since conspecific organisms are also seen as intruders to the defenders, causing them to be attacked. Along with the reproductive division of labor, the overlapping of generations and the cooperative care of young are all traits commonly found in eusocial species (Duffy et al. 2002).

Picture 3: An adult male defender Synalpheus regalis guards an entrance into the host sponge Lissodendoryx colom biensis
(permission obtained: http://web.vims.edu/bio/mobee/index.html?svr=www)

Figure 4: The number of snaps per individual and colony in a Synalpheus regalis nest, which contains 15 juveniles, 15 adult males, and 1 queen. Both figures show that large males, rather than juveniles or females, are the most likely to attack any intruder, with heterospecific intruders being attack more than conspecificintruders. (Duffy et al. 2002).

Kin Selection: Life Insurance and Fortress Defense

How does natural selection favor individuals cooperating to produce a greater, more successful whole, even if it means never reproducing? William D. Hamilton’s theory of kin selection helps to answer this question. Kin selection is rooted in genes producing copies in two manners: direct fitness by rearing offspring or indirect fitness by helping to care for relatives who also share these genes (Box 1). Therefore helping your relatives will always be advantageous. Natural selection favors this altruistic behavior when the cost of helping kin is less than the reproductive benefit achieved for the beneficiary of the altruism. This reproductive benefit can be evaluated by examining not only the benefit to the other individual, but also the degree of relatedness between the altruistic individual and the beneficiary (Box 2) (Hamilton 1964). Thus kin selection explains how eusocial behavior could arise in sponge-dwelling shrimp. The benefit in this ratio is very large, since juveniles (the sole means to pass your copy of genes) are incapable of fighting for themselves due to the lack of claws. The non-reproductive defenders ensure that these juveniles would survive, exalting a cost for not reproducing that is greatly overshadowed with the ascent of the juveniles into adulthood. Since juveniles never leave their natal nest, there is a high coefficient of relatedness in the sponges. By living with relatives, members of the colony can alter their behavior within the colony as a juvenile, breeding female, or a large male (Agrawal 2001). For example, even though some juveniles are not offspring of the large male sponge-dwelling shrimp, they share some of the same genes, resulting in an indirect fitness benefit for the large males when they ensure the survival of the juveniles. Allozyme data collected by Duffy et al. prove that the majority of colony members are full siblings—allowing for kin selection and indirect fitness to take place due to a high degree of relatedness.

Box 2: Calculating Hamilton’s Rule

Hamilton’s rule is a mathematical formula in determining if altruistic indirect fitness can be selected for by kin selection. The formula is C-B(R)<0, with C as the cost of the action for the actor, B as the benefits the recipient obtains from the action, and R is the relatedness between the recipient and the actor. For instance, suppose that a diploid female forgoes the opportunity to reproduce, instead remaining on her parents’ nest to assist in the development in her siblings. The juveniles of this species require tremendous parental care in terms of feeding, thus would greatly benefit from the assistance of a sibling. This species is monogamous resulting in siblings having the same parents, a genetic relatedness (R) of ½. The cost of not reproducing would be forgoing the chance to produce two offspring. However the benefits the parents receive through the helper daughter would be increasing the clutch size to five offspring. Therefore (2)-(1/2)(5)= - ½ which is less than 0. Since the value is less than zero, the presence of a helper daughter, that forgoes the opportunity to mate, will be selected for under kin selection.

Fortress defense is one of the two factors that influences organisms to help their relatives rather than reproduce on their own. Fortress defenders nest and feed in protected enclosures that can accommodate many individuals, especially a class specialized as soldiers for territorial defense. The main advantage of grouping for these social insects is to defend the valuable resource of the nest since it provides protection and a source of food for the juveniles—the idea that two or more snaps, a defense mechanism of S. regalis, is better than one against intruders. Since food is already present in the nests, the focus of grouping in this case is not for foraging, but rather for protection against predators. Examples of other fortress defenders include mole rats (Sherman et al. 1991; Jarvis et al. 1998), social shrimp (Duffy 1996a), thrips (Chapman et al. 2000), aphids (Benton et al. 1992), beetles (Schuster et al. 1985), and termites (Thorne 1997; Bartz 1979).

The second factor is life insurance which arises in social insects that forage for food outside of the nest, which exposes them to predators (Box 3). These insects must obtain nourishment outside of nest, unlike fortress defenders, since the young can not feed themselves and require food for development which the nest does not provide (Strassmann et al. 2007). The parent must undertake dangerous foraging for young in order for them to reach adulthood, but if the parent dies all of the offspring also die due to starvation—wasting the investment the parent had placed in the brood. However, an adult daughter can prevent this from occurring by staying in her natal nest to help protect and feed the dependent young. Therefore, if the parent dies while foraging the adult daughter will be able to take her place and raise the dependent brood, taking the role of reproduction as well. The different characteristics of Fortress Defenders and Life Insurers are portrayed in Table 2 (Queller and Strassmann 1998).

Box 3: Life insurers: Apis andreniformis

In the black dwarf honey bee, Apis andreniformis, the daughters of the queen care for the larvae, maintain and defend the hive, and forage for food outside of the nest (Picture 3). This species is a eusocial species of life insurers since they forage for food outside of the nest in order to feed the dependent juveniles (Arias et al. 2005; Keller et al. 1994). The queen bee smells each egg ensuring that all of the eggs are produced by the queen; if an egg smells foreign then it will be immediately removed from the nest by the queen (Pirk et al. 2004; Visscher et al. 1995). There can only be one queen and the chance of individual survival is very low making direct fitness unlikely for a solitary individual. The worker bee has a better chance of increasing its fitness through indirect fitness, rather than direct fitness, by helping the queen mother rear offspring.

Table 1: The three castes of Honey Bee
Apis andreniformis (permission obtained: NDSU)

A Queen Bee	A Drone Bee	A Worker Bee

Table 2: Table 2: Differences of two types of social insects, fortress defenders and life insurers (Queller and Strassmann 1998)

Characteristics	Fortress Defense	Life Insurers
Taxa	Thrips, aphids, beetle, termites	Ants, bees, wasps
Main advantage of grouping	Valuable, defensible resource	Overlap of adult gen
Food	Inside nest or protected site	Outside nest
Juveniles	Active, feed selves and may work	Helpless; need to feed
Nonsocial ancestors	Not necessarily parental	Highly parental
First specialized caste to evolve	Soldiers	Foragers
Colony Size	Usually small	Often large
Ecological Success	Usually limited	extensive

Sponge-dwelling shrimp are fortress defenders (Table 2). The risk of predation is not most severe outside of the nest, but rather inside of it from intruders. Thus larger colony members, most of who do not breed, defend the sponge against any intruder impinging on the sponge. The defenders of the colony are large and overtly aggressive, possessing snapping claws to use against intruders, and tend to be older than the rest of the colony. This age related polytheism is a common trait in labor specializing social insects that also express eusociality (Wilson 1971; Oster and Wilson 1978). These large shrimp allocate their energy to protecting the nest, rather than parenting (Table 2). The attacks of predators against shrimp can lead to wounds and even death. By allocating the fighting to just the male defenders, it is ensured that the reproductive individual will be protected and will survive to reproduce (Robinson 1992). These fortress defenders are also protecting the most valuable resource of the nest, the sponge itself that provides food and nourishment for the juveniles. Since most of the defenders do not breed, the only way to secure their genes in future generations is to protect their juvenile siblings, allowing them to grow to adulthood free from predation.

Examples of Eusocial Fortress Defenders

Synalpheus regalis has been accepted as a eusocial species due to the vast importance division of labor has provided for populations. For all populations, the juvenile is a precious commodity that must be brought to adulthood to ensure the spread of gene copies. With a caste system, S. regalis is capable of doing just this, having a defender caste that ensures the safety of the nest’s juvenile as well as the queen. Other eusocial species have also classified as either fortress defenders or life insurers; with the primary distinction being the importance of the nest as a food source seen in fortress defenders. The eusocial termites also classify as fortress defenders (Thorne 1997). Similar to sponge-dwelling shrimp, nests of termites provide nourishment for juveniles and adults, reducing the need for foraging. Termites are similar to S. regalis due to the presence of a soldier caste to defend the colony against intruders—in both organisms the nest is very important since it provides nourishment, houses juveniles and reproductive individuals, and serves as protection. However unlike S. regalis, in the termite species Zootermopsis angusticollis female soldiers are also present. Normal female pre-soldiers (callow soldiers that will molt into soldiers) have oviducts, a seminal receptacle, and eggs, thus is capable of reproduction. Once the pre-soldier molts into a mature female soldier, the reproductive organs ceases development, making the female infertile (Thorne 1997). Using molecular analysis, all species of termites have been found to differentiate into castes not genetically, but via developmental instructions that allow them to become any one of the castes depending on hormonal stimuli (Figure 5). The first developmental pathway is the sexual line recognized by the presence of wing buds; the second pathway, apterous, leads individuals to become workers (Watson et al. 1985). Further research is necessary in Synalpheus regalis to determine what developmental pathways, or any other mechanism of differentiation, is used in the formation of soldiers versus the reproductive individual.

Figure 5: Developmental pathways present in the all species of termites that contain flexible developmental options—transitions can occur at any of the instars. For instance, the larva, nymphs, and pseudergate are all workers of the colony but can differentiate into fertile alates or neotenic reproductives (soldiers). (Thorne 1997).

Aphids are another fortress defender eusocial species, sharing several similarities with sponge-dwelling shrimp and termites. The gall-forming Pemphigus spyrothecae, along with all other aphid species, contain a caste of soldiers common to Synalpheus regalis and termite species (Benton et al. 1992). These soldiers cluster around the entrance to the gall, actively attacking predators with their sylets and hind legs. This act is very altruistic since it is common to die while defending their relatives. During summertime of peak reproduction, three hundred aphids could be found in a gall of which 50% are soldiers (Foster 1990). However, the soldiers also perform another function of housekeeping by removing any honeydew, exuviae (shed skins), and any other waste including dead aphids from the gall. This cleaning of the nest allows for growth of the juvenile aphids —yet is still a dangerous act due to the chance of falling from the gall entrance (Benton et al. 1992). Further research is necessary to determine alternative functions of the soldiers of Synalpheus regalis. The acts of soldiers, whether defense as in S. regalis or non-defensive as in gall-forming aphids, are seen as forms of altruism. By sacrificing their safety to house-keep or attack intruders, these fortress-defenders protect the important commodities of the nest, the reproductive individual, and of course the related juveniles that will ensure copies of their genes will pass to future generations.

The species of Synalpheus has been discovered to be eusocial species due to its adherence to the three contingencies of eusociality: cooperative care of young, reproductive division of labor, and overlapping of generations. As fortress defenders, a caste is present in all five Synalpheus species that constantly defend the precious commodity of the sponge. The sponge is important since it provides nourishment for the growing juveniles, while providing shelter for the colony. Ecological constraints limit individuals from dispersing due to the scarcity of unoccupied sponges (Table 3). Also, conspecific species are still regarded as intruders, resulting in an attack from the sponge’s defenders—making the joining of a new sponge colony very difficult. Further research is necessary to determine what developmental pathways produce reproductive females and the soldiers. Perhaps even the soldiers of Synalpheus posses another function, similar to the eusocial aphids.