On the other hand some individual clam species may associate with multiple host
species. Among them there are species associating with closely related hosts e.g.
Pseudopythina macrophtalmensis is an associate of six species of Macrophtalmus
crab (Sato et al. 2011), or Pseudopythina subsinuata live in a close commensal
association with the stomatopod crustaceans—members of the genera Oratosquilla,
Harpiosquilla or Squilla (Appukuttan 1972; Morton 1972). Others associate with
even strikingly different hosts. For example, Naeromya rugifera associates with
Upogebia pugettensis shrimp or the polychaete sea mouse Aphrodita spp. (Li and
O’Foighil 2012). The latter example provides proof for the ecophenotypic plasticity
of N. rugifera, whose shell differs morphologically depending on the host (but no
host-specific genetic structuring was recorded). This finding is confirmed by the
results of Sato et al. (2011), who found that Koreamya arcuata associated with two
species of Lingula having different shell morphologies (differing mostly by the size
of the adult shell), but DNA sequences of COI and ITS1 confirmed genetic similarity of both morphotypes. Another galaeommatid fits in this category—Kurtiella
pedroana (formerly Mysella), which associates with different genera of mole crabs
(Blepharipoda occidentalis, Emerita analoga) and hermit crab (Isocheles pilosus)
settles on their exoskeletons, or—more commonly—inside their gill chambers,
attached by their byssal threads, however it also is able to live free (Lafferty 1993;
Boyko and Mikkelsen 2002; Carpenter 2005; Bhaduri et al. 2017; Table 5.1).
Recently Bhaduri et al. (2017) described an unusual location for this species—
inside the body of a mole crab. These authors discovered several adult clams (and
some veligers) that were living directly in the hemocoel of E. analoga. These clams
lacked byssal threads and were smaller than external bivalves. It is not clear, if the
crabs’ hemocoel is an accidental locality for the settlement of K. pedroana, neither
how the bivalves infected this locality (likely as veligers), nor how they obtain oxygen or food (possibly from hemolymph of crab). It is ambiguous if they can reproduce. This bivalve is a simultaneous hermaphrodite with a female-only phase
(Boyko and Mikkelsen 2002), thus enclosed veligers could be an offspring of
enclosed bivalves, but it is not clear if enclosed veligers can leave the crab. The
answer is presumably not for both reproduction and larvae evacuation, which is
interpreted as a likely dead-end for the bivalve in the evolution of a host-parasite
relationship. Furthermore no wound reactions or pathologies were associated with
presence of internal bivalves, suggesting no immune reaction of the crabs to bivalves
in their hemocoel. This example may be considered as a possible transition from a
commensal to a possible endoparasitic lifestyle.
Similarly, although all other members of Entovalva genus are endocommensals
of holothurians, Entovalva nhatrangensis, an associate in the esophagus of two
holothurian species from Vietnam may be considered not as a commensal, but as a
possible parasite (Bristow et al. 2010a). It is regarded as a most modified member
of the genus by having its outer surface heavily folded, ensuring by this a large surface of contact with its host’s tissues. This folded surface is presumed to serve as
absorption/excretion organ. Its foot is curved and not movable, which ensures firmer
contact with the host. The byssus is secreted through the gaps between folds and
glue the bivalve to the host tissues. E. nhatrangensis is confirmed to reproduce in
species. Among them there are species associating with closely related hosts e.g.
Pseudopythina macrophtalmensis is an associate of six species of Macrophtalmus
crab (Sato et al. 2011), or Pseudopythina subsinuata live in a close commensal
association with the stomatopod crustaceans—members of the genera Oratosquilla,
Harpiosquilla or Squilla (Appukuttan 1972; Morton 1972). Others associate with
even strikingly different hosts. For example, Naeromya rugifera associates with
Upogebia pugettensis shrimp or the polychaete sea mouse Aphrodita spp. (Li and
O’Foighil 2012). The latter example provides proof for the ecophenotypic plasticity
of N. rugifera, whose shell differs morphologically depending on the host (but no
host-specific genetic structuring was recorded). This finding is confirmed by the
results of Sato et al. (2011), who found that Koreamya arcuata associated with two
species of Lingula having different shell morphologies (differing mostly by the size
of the adult shell), but DNA sequences of COI and ITS1 confirmed genetic similarity of both morphotypes. Another galaeommatid fits in this category—Kurtiella
pedroana (formerly Mysella), which associates with different genera of mole crabs
(Blepharipoda occidentalis, Emerita analoga) and hermit crab (Isocheles pilosus)
settles on their exoskeletons, or—more commonly—inside their gill chambers,
attached by their byssal threads, however it also is able to live free (Lafferty 1993;
Boyko and Mikkelsen 2002; Carpenter 2005; Bhaduri et al. 2017; Table 5.1).
Recently Bhaduri et al. (2017) described an unusual location for this species—
inside the body of a mole crab. These authors discovered several adult clams (and
some veligers) that were living directly in the hemocoel of E. analoga. These clams
lacked byssal threads and were smaller than external bivalves. It is not clear, if the
crabs’ hemocoel is an accidental locality for the settlement of K. pedroana, neither
how the bivalves infected this locality (likely as veligers), nor how they obtain oxygen or food (possibly from hemolymph of crab). It is ambiguous if they can reproduce. This bivalve is a simultaneous hermaphrodite with a female-only phase
(Boyko and Mikkelsen 2002), thus enclosed veligers could be an offspring of
enclosed bivalves, but it is not clear if enclosed veligers can leave the crab. The
answer is presumably not for both reproduction and larvae evacuation, which is
interpreted as a likely dead-end for the bivalve in the evolution of a host-parasite
relationship. Furthermore no wound reactions or pathologies were associated with
presence of internal bivalves, suggesting no immune reaction of the crabs to bivalves
in their hemocoel. This example may be considered as a possible transition from a
commensal to a possible endoparasitic lifestyle.
Similarly, although all other members of Entovalva genus are endocommensals
of holothurians, Entovalva nhatrangensis, an associate in the esophagus of two
holothurian species from Vietnam may be considered not as a commensal, but as a
possible parasite (Bristow et al. 2010a). It is regarded as a most modified member
of the genus by having its outer surface heavily folded, ensuring by this a large surface of contact with its host’s tissues. This folded surface is presumed to serve as
absorption/excretion organ. Its foot is curved and not movable, which ensures firmer
contact with the host. The byssus is secreted through the gaps between folds and
glue the bivalve to the host tissues. E. nhatrangensis is confirmed to reproduce in