greater thermotolerance in this vital symbiotic association (Buerger et al. 2020). I
would be remiss if I did not mention the types of photosymbiotic relationships that
similarly exist between dinoflagellates and anemones such as Exaiptasia (Foo et al.
2020), and the photosymbiotic relationships that exist between dinoflagellates and
Cassiopea jellyfish. Hydra, the anemones, corals, and jellyfish are, of course, all
members of the phylum Cnidaria which suggests that members of this animal
phylum may have been cheating with their photosymbionts for a long while.
Sponges that live in shallow water often contain viable photosynthetic intracellular endosymbionts that variously may be either eukaryotic algae, or cyanobacteria
(Erwin and Thacker 2007), or dinoflagellates. These endosymbionts can be inherited
and they provide the host animal with products of photosynthesis. The fact that these
types of photosynthetic symbionts found in sponges vary so greatly suggests that the
sponges may have been trying to level the competitive playing field when they
partnered with their photosynthetic symbionts. Figure 26.18 shows examples of
shallow-water sponges. Deep water sponges may contain bacterial symbionts that
provide nutritional benefits to their host.
Molluscs have cheated by using photosymbiosis in several ways! There are
groups of bivalve molluscs that house photosynthetically active dinoflagellates
(Armstrong et al. 2018; Li et al. 2018). Their molluscan relatives the sea slugs use
kleptoplasts, which is an interesting story. The molluscan clade Sacoglossa contains
many species of sea slugs that feed upon algae and retain only the algal chloroplasts,
which are kept as organelles, within the animals intestinal cells. These captured
chloroplasts can continue to function for a few months, providing a photosynthetic
benefit to their host. This method of acquiring endosymbiont chloroplasts is called
kleptoplasty (Laetz and Wägele 2017) meaning the chloroplasts have been stolen.
The green pigment additionally may provide camouflage to the slugs. These animals
sometimes are described as being ‘solar powered’ slugs. Figure 26.19 shows four
species representing three genera of kleptoplastic sea slugs. Elysia chlorotica, shown
upper left in Fig. 26.19, has been described as containing a constellation of stars in
reference to its chloroplasts. Costasiella kuroshimae, shown upper right in
Fig. 26.19, has a strange cuteness that resembles a child who has discovered green
hair dye. There are several additional genera of seas slugs including Bosellia that
form kleptoplasts but are not represented in Fig. 26.19. Evolutionarily, the first slug
which learned to form kleptoplasts would have been cheating. The other
kleptoplastic sea slugs might be perceived as either descendants of that first kleptic
sea slug, or competitors that kleptically have leveled the playing field.
Flat worms also can form kleptoplasts! There are marine flatworms, including
Baicalellia solaris and Pogaina paranygulgus, that accumulate kleptoplasts while
feeding upon their diet of diatoms. Which cheated first? Was it the flat worms or the
slugs?
26 The Game of Evolution Is Won by Competitive Cheating
565
would be remiss if I did not mention the types of photosymbiotic relationships that
similarly exist between dinoflagellates and anemones such as Exaiptasia (Foo et al.
2020), and the photosymbiotic relationships that exist between dinoflagellates and
Cassiopea jellyfish. Hydra, the anemones, corals, and jellyfish are, of course, all
members of the phylum Cnidaria which suggests that members of this animal
phylum may have been cheating with their photosymbionts for a long while.
Sponges that live in shallow water often contain viable photosynthetic intracellular endosymbionts that variously may be either eukaryotic algae, or cyanobacteria
(Erwin and Thacker 2007), or dinoflagellates. These endosymbionts can be inherited
and they provide the host animal with products of photosynthesis. The fact that these
types of photosynthetic symbionts found in sponges vary so greatly suggests that the
sponges may have been trying to level the competitive playing field when they
partnered with their photosynthetic symbionts. Figure 26.18 shows examples of
shallow-water sponges. Deep water sponges may contain bacterial symbionts that
provide nutritional benefits to their host.
Molluscs have cheated by using photosymbiosis in several ways! There are
groups of bivalve molluscs that house photosynthetically active dinoflagellates
(Armstrong et al. 2018; Li et al. 2018). Their molluscan relatives the sea slugs use
kleptoplasts, which is an interesting story. The molluscan clade Sacoglossa contains
many species of sea slugs that feed upon algae and retain only the algal chloroplasts,
which are kept as organelles, within the animals intestinal cells. These captured
chloroplasts can continue to function for a few months, providing a photosynthetic
benefit to their host. This method of acquiring endosymbiont chloroplasts is called
kleptoplasty (Laetz and Wägele 2017) meaning the chloroplasts have been stolen.
The green pigment additionally may provide camouflage to the slugs. These animals
sometimes are described as being ‘solar powered’ slugs. Figure 26.19 shows four
species representing three genera of kleptoplastic sea slugs. Elysia chlorotica, shown
upper left in Fig. 26.19, has been described as containing a constellation of stars in
reference to its chloroplasts. Costasiella kuroshimae, shown upper right in
Fig. 26.19, has a strange cuteness that resembles a child who has discovered green
hair dye. There are several additional genera of seas slugs including Bosellia that
form kleptoplasts but are not represented in Fig. 26.19. Evolutionarily, the first slug
which learned to form kleptoplasts would have been cheating. The other
kleptoplastic sea slugs might be perceived as either descendants of that first kleptic
sea slug, or competitors that kleptically have leveled the playing field.
Flat worms also can form kleptoplasts! There are marine flatworms, including
Baicalellia solaris and Pogaina paranygulgus, that accumulate kleptoplasts while
feeding upon their diet of diatoms. Which cheated first? Was it the flat worms or the
slugs?
26 The Game of Evolution Is Won by Competitive Cheating
565