Considering the
vast diversity of chordates, it may seem odd that few of them are parasitic.
Particularly among fishes there are some parasitic groups, cookie cutter sharks
and lampreys being the most common, but there is another group of fishes that
is not as well understood: the pearlfishes. Although not all members of this
family are parasitic. Commensalism is represented in this family as well. Commensalism refers to a type of symbiotic relationships
in which the individual interacts with and relies on its host for survival while
the host is negligibly affected by said interaction. Pearlfish are a group of
ophidiiforms in the family Carapidae with eight currently recognized genera and
thirty-six species who represent each of these strategies. A pearlfish’s host
often depends on the species of the fish. Some can be found parasitizing
bivalves or living commensally inside tunicates others live in echinoderms like
sea stars and sea cucumbers. The phylogeny of the Carapidae is still under
debate as its genetic relatedness with other members of ifs genus and family
are still being contested. We will be exploring the Carapidae family through
the lens of one particular species: Encheliophis
boraborensis formerly called Carapus
boraborensis whose adult form is spent commensally inside sea cucumbers.
Illustration of the Pinhead Pearlfish Encheliophis boraborensis. Source. |
It may be
necessary to provide a brief note on sea cucumber anatomy: sea cucumbers have a
mouth on the anterior end and an anus on the posterior near the posterior is a
pair of organs called a respiratory tree, these sticky organs are ejected by
the cucumber when threatened and are able to produce toxins, however many
pearlfish show increased resistance to these toxins compared to other reef
dwelling fish. This allows the pearlfish to be protected by the cucumber
without being poisoned by it. Additionally, the anal cavity is near the gonads,
in the case of parasitic pearlfish these are the tissues most commonly found in
pearlfish’s stomachs suggesting that parasitic pearlfish tend to eat the gonads
of their host while still being able to exploit the cucumber for protection as
its respiratory tree is left intact as the cucumber is able to remain healthy
but the individuals fecundity could be severely reduced depending on the time
needed until the gonads regrow (Parmentier and Vandewalle 2004).
Pearlfish are
mainly found in the Pacific Ocean, specifically the tropics on reef habitats
where their hosts can be most commonly found. The pinhead pearlfish, Encheliophis boraborensis, can be found
near French Polynesia living in the cloacal vent of any number of local sea
cucumber species and in some cases many pearlfish can be found in a single host
(Parmentier and Vanderwalle 2004). Pearlfish such as the pinhead pearlfish rely
on their sleek elongated body shapes to enter their hosts tail first to enter
their host cucumbers. However, with species who inhabit hosts, such as
tunicates or oysters, the orientation is less significant.
Click here to watch video of a pearlfish.
Pinhead Pearlfish with leopard sea cucumber Bohadschia argus. Source: Project Noah. |
When building a
phylogeny, it is useful to use morphological traits as well as molecular
analysis to form plausible hypotheses. Much can be said about a pearlfish’s
life history based on morphology alone as, upon analysis, certain patterns
arise that help distinguish the separate life strategies the fish employs. For
example, jaw structure and size in the parasitic fishes tends to be weaker in
comparison with commensal species. This weakening of their feeding apparatus
can be attributed to the commensal species need to leave its host in order to
hunt the small crustaceans on which it feeds as opposed to the pearlfish species
who do not need to leave their host and are able to feed exclusively on their
hosts soft tissues. Another useful morphological trait is the presence of sound
generating structures connected to the pearlfish’s swim bladder (mainly used
for communication with conspecifics) vary greatly (Lagardère et. al. 2005). The
diversity of these mechanisms is emblematic of the speciation within the
Carapidae family but offered very little in the way of clues to establish any
evolutionary relationships within the taxa (Parmentier et.al. 2016). However,
as previously mentioned a host may be inhabited by multiple individuals, but
these may not necessarily all be members of the same species in a laboratory
study by Garilao, interspecies competition was observed. Thus, it can be
extrapolated that the variation in calls and sound generating structures unique
to each species help distinguish one species from another and play a role in
competition for a host.
To explain the
relationship between parasitic and commensal pearlfish one must also consider
their life history. Pearlfish larvae have two stages before reaching their
final adult forms, these stages are the vexilifer larvae which are free
swimming meroplankton until they reach the tenuis stage in which their bodies
shorten in length and they begin to search for a host. It is believed that the
parasitic species are derived from the commensal pearlfish as the result of
pedomorphosis within the parasites. Pedomorphosis occurs when the adult form of
a species retains traits from its juvenile forms in the case of the pearlfish
this is reminiscent of the relationship between free living and parasitic
lampreys where, once again, only the adult form becomes parasitic.
References
Froese, R. and D.
Pauly. Editors. 2019. FishBase. Encheliophis
boraborensis summary page. World Wide Web electronic publication. [accessed 2019 May 1].
http://www.fishbase.org/summary/Encheliophis-boraborensis.html.
Lagardère JP, Millot S, Parmentier E. 2005.
Aspects of sound communication in the pearlfish Carapus boraborensis and Carapus
homei (Carapidae). Journal of
Experimental Zoology Part A: Comparative Experimental Biology. 303A(12):1066–1074.
Parmentier E, Castillo G, Chardon M,
Vandewalle P. 2001. Phylogenetic analysis of the pearlfish tribe Carapini
(Pisces: Carapidae). Acta Zoologica 81(4):293–306.
doi:10.1046/j.1463-6395.2000.00059.x
Parmentier E, Vandewalle P. 2004. Further insight on carapid —holothuroid
relationships. Marine Biology 146(3):455–465.
doi:10.1007/s00227-004-1467-7
Parmentier E, Das K. 2004. Commensal vs. parasitic relationship between
Carapini fish and their hosts: some further insight through δ13C and δ15N
measurements. Journal of Experimental
Marine Biology and Ecology. 310(1):47–58. doi:10.1016/j.jembe.2004.03.019
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