Seahorse Hippocampus Hypothesis

Seahorse Hippocampus Introduction

Seahorses are a prime example of species whose atypical biology and unusual global distribution leads to a series of evolutionary questions. Seahorses (genus Hippocampus) are a marine species that have extensively been studied because of their abnormal behaviors in the marine environment compared to other marine creatures. Many of the seahorse species have large ranges, both longitudinally (over a great horizontal distance across the ocean), and latitudinal (great vertical distance within the ocean), regardless of the fact that they are characterized as weak swimmers and lack any large structural fins for efficient swimming (Lourie et al., 1999a). Although they do have these large range environments, seahorses

typically reside within tight knit communities within an intimate range which they generally do not stray from (Perante et al., 1998).
Seahorse species are distributed globally, leading to an absence of reliable fossil records, which makes understanding of the origin of the Hippocampus genus difficult. Earlier studies have speculated that the Hippocampus genus originated before the formation of the Tethys Ocean during the Mesozoic era over 20 million years ago (McCoy and Heck, 1976). This time period would be during the Cretaceous period prior to the opening of the Indian and Atlantic oceans. Seahorses are a common within the marine taxa with the highest amount of species found in the Indian and Pacific Ocean. Southeast Asia continues to house the highest amount of seahorse diversity, upholding the title of the region being the origin of many marine taxa (McManus, 1985). Several studies have noted that regions with a highest concentration of species diversity, like seahorses in Southeast Asia, are correlated to a geographical location where the origin of speciation occurred (Baldwin et al., 1998).
The taxonomic knowledge within genus Hippocampus has thought to been incomplete, categorized by conflicting reports within the seahorse species distinction. Monophyly of the genus itself is supported by a series of synapomorhpic characters like a pre-hesile tail and the perpendicular nature of the head to the trunk, however the solo use of morphological traits can be misleading and the addition of molecular data and in particular molecular markers have been used to distinguish nuanced differences (Smith, 1963). This was previously successfully done within fish species and therefore in theory can be useful in distinguishing between seahorse species, allowing further study of the species (e.g., De Vargas et al., 1999).
Reference Research Goals & Summary
The reference study consisted of two goals.

The first goal was to accumulate the molecular phylogeny data for seahorses using the cytochrome b gene sequence information (S.P. Casey et al, 2004). In this particular study, the cytochrome b gene was used to investigate whether or not the Hippocampus was indeed pre-Tethyan in origin and to illuminate the relationship between Indo-Pacific and Atlantic seahorse species. Molecular markers like the cytochrome b gene were used to survey dispersal of seahorse species and to temporally define the evolutionary processes since much of the seahorse fossil record is deficient (Lourie et al.,

1999a).
The second goal of the study was to use the molecular phylogeny mentioned above in order to construct a well-supported seahorse species designation. This was based of the initial taxonomic speciation done by Laurie et al. in 1999, however with new genetic data and an extension of previous genome sequence, with discrepancies between the original morphological analysis and the new species distinctions identified and discussed. The study sought to determine the phylogenetic relationship between 93 specimens from 22 species of seahorses from the Altaic, Indian, and Pacific Ocean, all genus Hippocampus. Results from the study included a maximum divergence of 23.2% using the Kimura 2-parameter model, which affirmed the original statement that the Hippocampus genus was indeed pre-Tethyan in origin (S.P. Casey et al, 2004). Regardless of the wide variety of seashores species diversity in Southeast Asia, there was no evidence from the molecular markers that stated that the Indo-Pacific was the origin of the genus Hippocampus. Additional questions were also raised when analyzing the composite tree. Uncertainty in the composite phylogeny tree suggested that there were several species whose species designation was ambiguous, and indicated the need for additional phylogeographer studies to understand how seahorses were dispersed (S.P. Casey et al, 2004).
Original Research Goals
The published analysis included the production of a kimura-2 parameter corrected pairwise distance utilizing 29 seahorse haplotypes (representative of the 22 different species with multiples for those who are significantly morphologically different).

For the original analysis, the corrected pairwise distance will be calculated using the Jukes–Cantor and the Maximum Composite Likelihood Model. The Jukes–Cantor model assumes that the rate of nucleotide substitution or all nucleotides (C, A, T and G) are equal, that nucleotide frequencies are equal, that there is an equal rate of substitution among sites, and does not correct for the lower rate of transversion substitutes in comparison to transitional substitutions (Jukes and Cantor, 1969). The Maximum Composite Likelihood takes into account the phylogenic relationship between sequences, using the sum of the log likelihoods of the bases as the composite likelihood. Both pair wise distances and substitution parameters are estimated using the Maximum Composite Likelihood (Tamura et al. 2004). Both models should yield different maximum sequence divergence and average divergence that can then be compared to the original paper. With sequence divergence data, the temporal origin of the genus can be identified. The two alternate models to the Kimura-2 parameter will be analyzed to discuss which methods yield results closest to the expected time origin of the genus

Hippocampus.