Posts
Of all the reproductive strategies that have evolved over time, the
mating habits of the Ceratoid Anglerfish are by far the most bizarre to have
developed. If the Twilight series were
to be crossed with Fifty Shades of Grey, we would expect to see something like
what many of these fish do everyday: sexual parasitism. As to the question of
why many of these fish evolved into such a strange way of copulating, it seems
as though to be driven by the pressure of finding a mate in the deep dark sea.
If you rarely can a partner in the world, you better make sure that when you do
find one, they can never get away.
Anglerfish live so
far down in the ocean that it is a challenge for researchers to study them.
There have been few occasions for the fish to be filmed. The techniques used to
view most species in-situ is so costly that many researchers cannot afford to
do them, so they stick with traditional methods such as trawling (Luck and
Pietsch 2008). Often, most captured specimens cannot survive the ascent to the
surface or they will die shortly after arrival at the surface (Luck and Pietsch
2008). Although it very difficult to study the behavioral characteristics,
researchers have discovered much about the fish with the evidence they can
gather.
So how do they ‘do
it?’ In the case of the obligate sexual parasitism, males will find a female
and then bite into her. He then holds on to her with his jaws long enough until
their two bodies fuse together. The female’s body basically consumes the male,
with only things of him remaining are his circulatory system and his
reproductive organs; all of his other organs degenerate (Pietsch 1975). The
exact mechanism as to how the two different epidermal tissues fuse together is
still unknown.
There are two other
types of sexual parasitism in Ceratioids; the males have a temporary
non-parasitic attachment or they are facultative parasites. These types do not
share all of the same characteristics of obligate parasitism such as
undeveloped mechanisms for digestion.
Facultative parasitism has only been observed in two families of the
Ceratioidei: Caulophryne and Bertella. It was assumed that all Caratioidei were
obligate parasites until a pregnant female was discovered that was
un-parasitized (Pietsch 2005). This discovery led to two possibilities: either
the female could reproduce asexually or there was chance that males could
detach themselves after handling their business; facultative attachment.
 |
| Male attached to female host anglerfish, Cryptopsurus. Photo by T. Pietsch (2005) |
In species with
facultative relationships, fertilization can either occur during a temporary or
permanent attachment, both options are viable. “Males of facultative forms
probably attach to females whenever the two meet regardless of sexual
readiness. If both partners are in a state of readiness at the time of
attachment, spawning and fertilization take place, after which the male
releases his hold on the female and is then presumably capable of beginning a
new search for another mate” (Pietsch 2005). If partners aren’t ready to begin
spawning, the males will remain attached until the time is right, and the
longer that a male remains attached, the greater his chances of a permanent
attachment (Pietsch 2005). This is
believed because to be the case because females have been sighted fully
developed sexually without males attached, leading researchers to believe that
that sexual development is not a factor of attachment as with obligate
parasitism.
The last method of
sexual reproduction is non-parasitic temporary attachment. In this method,
males do not need to fuse with the females at all, they only chomp down for a
short ride. The collections around the world are devoid of examples of
temporary sexual couplings because it is believed that males will detach
themselves under the stress of fishing gears. The only known examples are two
pairs of Melanocetus, which are
firmly attached without a fusion of the epidermis (Pietsch 2005). These males
are free to float around to other ladies they may find, chomping down whenever
they smell something good in the water. I would propose that temporary attachment
to females would increase the chances of survival for the species because it
would allow for mating to be more selective.
It was originally
believed that males locate the females through olfactory cues and visual
stimulus. This idea is supported by the presence of well-developed eyes and
nostrils in the free-living males. “According to this theory, the males use
their large and sensitive nostrils and eyes to pick up upon conspecific female
olfactory and visual cues such as pheromones and bioluminescent lures” (Carley
et al. 2010). The eyes of the males were driven to develop to due the dark
conditions and the olfactory system was refined to search the open waters. Not
every species of has the same capabilities of sight and sense; some species
with underdeveloped nostrils have well-developed eyes, or vice versa (Pietsch
2005). So it is believed that males
either use sight or smell depending on species, but not both.
In the species with
obligate sexual parasitism, both the males and females do not develop sexually
until attachment has occurred; well-developed sexual organs have never been
seen in un-parasitized females and free-living males. It seems that males never
mature sexually unless they have become attached to a female, and, likewise,
females will not produce eggs until stimulated by the male parasitic attachment
(Parr 1930). Unlike most species of fish where sexual development is a factor
of size or age, sexual development in obligate types is brought on by the
parasitic attachment (Pietsch 2005).
Another
strange characteristic of free-living males of the obligate type is the lack of
jaws and digestive tracts capable of digesting food. Males that do not find a
female to attach to will not only fail to reproduce, but will also die of
starvation within the first few months of their lives (Pietsch 2005). The only
purpose of the male’s life is to find a female and reproduce. Males have
evolved to simply serve as sperm donors to the females and are good for nothing
else, not a completely bad life.
How did such a
strange love-making strategy start? It was originally believed that Ceratioid
were monophyletic based off of morphological-based phylogenies. This was based
on characteristics that have been highly conserved such as extreme sexual
dimorphism, loss of pelvic fins, repositioning of pectoral fins, reduced
density due to loss of bony parts, and infusion of lipids throughout the body
(Carley et al. 2010). However, when more molecular data became available, it
showed that the phylogeny is most likely paraphyletic, indicating that obligate
parasitism has evolved more than once and then lost by other species. “The
maximum likelihood molecular reconstruction of the phylogenetic relationships
also suggests that temporary attachment to females was a precursor to
facultative and obligate parasitism, rather than a reduction of an ancestral
parasitic behavior” (Carley et al. 2010).
In a 1926 paper
about sexual parasitism in anglerfish, the discoverer of sexual parasitism,
C.T. Regan, almost assumed correctly about the suborder when he stated, “The
reason why the Ceratioid, alone amongst Vertebrates, have males of this kind is
evident. They are necessarily few in numbers in comparison with the more active
fishes on which they prey, and they lead a solitary life, floating about in the
darkness of the middle depths of the ocean. Under such circumstances it would
be very difficult for a mature fish to find a mate, but this difficulty appears
to some extent to have been got over by the males, soon after they are hatched,
when they are relatively numerous, attaching themselves to the females, if they
are fortunate enough to meet them, and remaining attached throughout life.” The
pressure of living in a place where life is challenging has caused the amazing
relationship developed by Ceratioid anglerfish. The anglerfish provide another
example of the majesty in the survival of the fittest, that life will stretch
the realm of possibility in order to continue living into the future.
References
Luck, G. T., and T. W. Pietsch. 2008. In-situ observation of a deep-sea
ceratioid anglerfish of the genus Oneirodes
(Lophiiformers: Oneirodidae). Copeia, 2008 (2): 446-451.
Parr, A. 1930. On the probable identity, life-history and anatomy of the
free-living and attached males of the ceratioid Fishes. Copeia, 1930 (4):129-135
Pietsch, T. W. 1975. Precocious sexual parasitism in the deep sea
ceratioid anglerfish, Cryptosaras couesi
Gill. Nature 256:38-40.
Pietsch, T. W. 2005. Dimorphism, parasitism, and sex revisited: modes
of reproduction among deep-sea ceratioid anglerfishes (Teleostei:
Lophiiformes). Ichthyological Research 52: 207-236.
Regan, C.T. 1926. The pediculate fishes of the suborder Ceratioidea.
Dana Oceanographic Report 2:1–45
No comments:
Post a Comment