Heck, K. L. (1977). Comparative species richness, composition, and abundance of invertebrates in Caribbean seagrass (Thalassia testudinum) meadows (Panamá). Marine Biology, 41(4), 335–348. https://doi.org/10.1007/BF00389099

This previous work compares the species composition and abundance of invertebrates in the Caribbean coast of Panama and the Panama Canal Zone. While discussing some factors that could have affected this experiment, the author of this publication noted that the Panama Canal zone could have led to the displacement and death of many snapping shrimp. This previous work observed that species composition of invertebrates (which include snapping shrimp) were the same in tropical and subtropical seagrass meadows, however, different species of shrimp had a lower total abundance. This could mean that the formation of the Isthmus of Panama, followed by the construction of the Panama Canal, had an effect on speciation of several species of shrimp. (JP)

Knowlton, N. (1986). Cryptic and Sibling Species among the Decapod Crustacea. Journal of Crustacean Biology, 6(3), 356-363. doi:10.2307/1548175

This previous work defines the term sibling species. According to Knowlton, sibling species are species that have been shown to be very closely related using biochemical techniques such as electrophoretic analysis. Sibling species are not the same species. This publication also uses electrophoretic analyses, known as starch gel electrophoresis, to differentiate between species of snapping shrimp. (JP)

Schein, H. (1975). Aspects of the aggressive and sexual behaviour of alpheus heterochaelis say. Marine Behaviour and Physiology, 3(2), 83–96. https://doi.org/10.1080/10236247509378498

This previous work studies aggressive and sexual behavior in snapping shrimp. Although the snapping shrimps studied were found in North Carolina, their behavior reflects the behavior of the genus. Here they established that males seek females using chemical cues, and in general a male with a larger claw/larger body size favors larger females that can produce more offspring. Also, males with larger claws are stronger and can out compete smaller males. In relation to this publication, this previous work establishes a background of the behavior of the snapping shrimp. Understanding their natural behavior helps in understanding how the authors of this publication reached their conclusion. (JP)

Sexual selection and dimorphism in two demes of a symbiotic, pair-bonding snapping shrimp Knowlton, N. (1980), SEXUAL SELECTION AND DIMORPHISM IN TWO DEMES OF A SYMBIOTIC, PAIR-BONDING SNAPPING SHRIMP. Evolution, 34: 161–173. doi:10.1111/j.1558-5646.1980.tb04802.x

This previous work provides evidence on how differences between mates affects mate choice and, with environmental constraints, can lead to sexual dimorphism which is a distinct difference in size or appearance between the sexes of the same species. This study found that snapping shrimp are very aggressive and territorial to other shrimps of the same sex. By having these type of aggressive traits, compatibility would decrease as species are more separated. This supports one of the results found in this publication. (JP)

Phylogeny refers to the study of evolutionary history among groups of organisms or individuals. The phylogenetic tree is a diagram that illustrates the relationships among species; this diagram is based on similarities and differences in the physical or genetic characteristics of different organisms. This diagram shows that all organisms originate from a single ancestor but have diverged due to speciation over a long period of time. (JP)

Biological evidence supports an early and complex emergence of the Isthmus of Panama, Christine D. Bacon, PNAS, DOI: 10.1073/pnas.1423853112

https://phys.org/news/2015-04-evidence-isthmus-panama-earlier-thought.html

According to phys.org, a newly published paper suggests that the Isthmus of Panama was formed earlier than conventionally believed. The article states that the isthmus was formed in a stop and go manner over 20 million years ago; meaning that migration of organisms could have also followed the same trend. If migration of animals between North and South America started earlier, then the minimum time required for strong reproductive isolation may actually be longer than 3.5 million years. (JP)

Speciation is an evolutionary process where populations become distinct from each other over a long period of time and develop into a new species. (JP)

Knowlton, Nancy & Mills, DeEtta. (1992). The Systematic Importance of Color and Color Pattern: Evidence for Complexes of Sibling Species of Snapping Shrimp (Caridea: Alpheidae: AZ'heus) from the Caribbean and Pacific Coasts of Panama. Proceedings of the San Diego Society of Natural History. 18.

This past publication explains how loosely related, or distantly related, species in the genus Afpheus Fabricius (snapping shrimp) can be distinguished based on color morphology such as differences in color patterns. This relates with this publication because both are related to the speciation of snapping shrimp in the Isthmus of Panama. Both publications find methods to differentiate between species; while this paper uses mtDNA and allozymes to compare genetic distance, the other paper uses careful analyses of color patterns to separate species. (JP)

Bermingham, E. & Lessios, H.A. (1993). “Rate Variation of Protein and Mitochondrial DNA Evolution as Revealed by Sea Urchins Separated by the Isthmus of Panama.” Proceedings of the National Academy of Sciences, 90(7): pp. 2734–2738.

Another research was made in regards to the closure of Isthmus of Panama 3 MA based on changes in oxygen/carbon ratios in sea water. The main subject studied was sea urchins. Bermingham and Lessios compared mtDNA and protein divergence among three differ species of sea urchins--Diadema, Echinometra, & Eucidaris. Results concluded that protein divergence between Atlantic and Pacific sea urchin species only differed by 1 order of magnitude, so no significant difference. The Diadema species had an overlap of allele frequencies whereas the other two species displayed fixed loci for differ alleles of both shores, which may hint towards gradual genetic drift or mutations.This proves that other marine life besides the Alpheus shrimp community were also affected by a change in the environment via closure of Panama Isthmus.

~(J.D.A.)

Read, A., McTeague, J., & Govind, C. (1991). "Morphology and behavior of an unusually flexible thoracic limb in the snapping shrimp, Alpheus heterochelis". The Biological Bulletin, 181(1): pp. 158-168.

This journal explains the morphology of the snapping shrimp Alpheus. Their second thoracic limb is thinner, flexible, bilaterally symmetrical, and possess larger ganglion. This type of limb is almost always active in regards to grooming and foraging for food. The major cheliped, or the larger claw, is used to produce loud popping noises as a defense mechanism against predators.

~(J.D.A.)

Knowlton, N. & Keller, B.D. (1982). "Symmetric Fights as a Measure of Escalation Potential in a Symbiotic, Territorial Snapping Shrimp." Behavioral Ecology And Sociobiology, 10(4): 289-292.

This journal goes into detail the behavioral aspect of the Alpheus shrimp. This species readily defends its territory using its large snapping claw. Results gathered focused on symmetric fights of shrimps in regards to sex, size, and resources. According to this study, larger females are more likely to defend their anemone territories and showed more aggressiveness than the male shrimps. The losers of these contests experienced severe injuries by these large females than any other type of shrimps.

~(J.D.A.)

Svitil, K.A. (1993). "Oceans Divided". Discover, pp.1-2.

A magazine article by Kathy Svitil talks about the closure of the Isthmus of Panama. She talked about how the rise of Panama 3.5 million years ago separated Atlantic and Pacific species before the land-bridge could even form. This was caused by the shift in tetonic plates, where the Pacific Ocean floor shifted underneath the Carribean plate before melting/solidifying by lava. She states how animals felt the influence of the rifts coming together, even quoting Nancy Knowlton on the speciation of snapping shrimps on either sides of Isthmus since these shrimps used to live in deeper depths of the ocean.

~(J.D.A.)

The formation of the Isthmus of Panama has been attributed to the ceasing of currents and changes to the depths of certain areas and levels of sand. It has been reinforced by a similar and more recent research that these changes are some of the major influences of divergence within the snapping shrimps species. (DV)

Read more in New dates and new rates for divergence across the Isthmus of Panama: http://rspb.royalsocietypublishing.org/content/265/1412/2257

The generation types (ex. F1, F2, etc.) and population size were taken into account for each shrimp pair. These factors were found to not support the phenomenon of divergence among shrimp pairs because no differences were found. ~J.D.A.

Taxonomy deals with naming and classifying organisms. A taxonomic group of any rank, such as a species, family, or class is a taxa. ~S.Z.

The mutation rate of biomolecules is calculated and used narrow down the time in prehistory when two or more organisms diverged. (SZ)

Taxa refers to taxonomical groups like species, family, or class. Taxa are used in classifying organisms based on relatedness of characteristics.

Sister taxa refers to two groups in which the organisms derived from a common ancestral organism, therefore these two groups are very closely related. (DV)

Divergence is the differentiation of something (like an organism's genetic information) from its common state.

Molecular divergence is the differentiation between the nucleotide sequence in DNA or amino acid sequences in proteins of organisms. (DV)

Transisthmian refers to the opposite sides of an isthmus. The authors utilize this term to portray that the shrimps that were measured were from opposite sides (Pacific side and Caribbean side) of the Isthmus of Panama. ~J.D.A. (DV)

How the formation of the Isthmus of Panama has been an impacting event in the divergence of species like that of snapping shrimp, that of which can be seen through comparison of protein, mitochondrial DNA, and reproductive compatibility. (DV)

Glynn, P.W. (1985). “El Nino-Associated disturbance to coral reefs and post disturbance mortality by Acanthaster planci.” Marine Ecology Progress Series, 26(17): pp. 295–300.

Another study shows how in 1985 the coral reefs in eastern Pacific (Panama included) got ruined by El Nino due to very high water temperatures (average of 31 Celsius) and depth penetration (over 100 m). Since most habitats in coral reefs were destroyed, many species were easily exposed to predators. This may affect the results obtained by Mills and co. because barely a decade has passed for the marine habitats in Panama to recuperate.

~(J.D.A.)

Supports that a physical barrier will increase the chances of divergence between species creating two or more sub-species decedents . ~S.Z.

This paper contains discussion of species concepts and their application, morphological species characters and sibling species, biological properties of species, isolating mechanisms, hybridization, the variation and genetics of populations, storage and protection of genetic variation, the unity of the genotype, geographic variation, the polytypic species of the taxonomist, the population structure of species, kinds of species, multiplication of species, geographic speciation, the genetics of speciation, the ecology of speciation, and species and transpecific evolution. All of which can contribute a great deal to the topic of this paper. ~S.Z.

Sporadic production is referring to production that occurs in an irregular manner. the fertile clutches will occur in at an inconsistent interval. ~S.Z.

The less compatible they are the more they have diverged. ~S.Z.

The shrimp located closer to the sea floor had a slimmer chance of being pushed over the isthmus; therefore had a higher chance of isolation and divergence. ~S.Z.

The physical location of the shrimp could effect the divergence associated with the gradual rise of isthmus. ~S.Z

P5-P6 pair were isolated from each other just before the final closing of the Panama seaway. This was due to shoaling of water and the environmental change that came with it. ~S.Z.

Rate of divergence for the first four pairs of shrimp was calculated by dividing the allozyme and mtDNA divergence values by the time elapsed since final closure. ~S.Z.

Basically, the null is rejected because it contradicts the observed data. The null states that isolation occurred around at the same time, but the observed data shows divergence indicators such as metabolic enzymes, mtDNA, and mate recognition, all of which are not associated at a single mechanistic basis required for concurrent divergence. ~S.Z.

Gel Electrophoresis is a method of separating macromolecules such as DNA, RNA, or proteins. The separation is based on the molecules size and charge. The shorter molecules will travel further in the gel agarose than longer molecules. ~ S.Z.

straightforward, clear. (SZ)

The physical barrier created by the Isthmus of Panama prevented the exchange of genes; therefore, allowing the organism to develop differently on wither side of the barrier or diverge. (SZ)

The divergence occurred at different points in time rather than at one specific time. (S.Z)

A process by which a species evolves into two or more descendant or different forms (SZ)

INTRODUCTION The divergence in allozymes, mitochondrial DNA (mtDNA), and reproductive compatibility for seven shallow water transisthmian snapping shrimp (Alpheus) was studied to test the accuracy of molecular clocks. In order to interpret inconsistencies and estimate differences the relationship among species in divergence rates there must be an increase in the number of taxa and characters studied. This research will provide information on transisthmian pairs of sister taxa in snapping shrimp genus Alpheus. These shrimp are close enough to be studied at the same time, but are completely separated by a physical barrier. How will genetic and geological factors play a role in the divergence of these organisms? What is the minimum time required for speciations or divergence to occur?

(S.Z)

Species so closely related showed two differ clutches. Isolation could've been geographical, though nothing that could completely cut them off from each other since they are similar. If anything, this could infer that they could've been selective in mating, or aggressive in behavior to even mate, thus preventing creation of offspring between them. ~J.D.A

Because of the long separation from one another, when some connection was established between either sides of the Isthmus, the shrimp pairs no longer had preferences to each other. Their extended sexual isolation probably had them adapted into altering their breeding behavior. ~J.D.A.

Foraminifera are single-celled planktonic animals. They have an external shell and a granular ectoplasm for catching their food. When they die their shell becomes part of seafloor matter. ~J.D.A.

A gene coding for the cytochrome oxidase enzyme. This enzyme is found in the mitochondrial membrane. It is responsible for catalyzing transfer of electrons to the oxygen atom. This is for the electron transport chain to make ATP as a product. ~J.D.A.

Their collection may have been affected. It was mentioned by the author that she and her colleagues collected from muddy environments since the sites were sewage dump locations. Most of the shrimps were covered in mud, therefore hard to distinguish. Also, the author mentioned transit ships traveling between the Atlantic and Pacific sides of Isthmus. These ships would migrate the shrimps away from their homes; may affect how the team paired shrimps based on their mtDNA. ~J.D.A.

Allozyme is an enzyme. They are coded by differ alleles found at the same locus of the gene. They were studied in this experiment via taxonomic branching to map out closely related pairs. ~J.D.A.

This explains the process of gene transfer. Usually this is observed from shifts in genes between different populations. So lets say a blue bird from one population migrates out of its territory and mates with a brown bird from the new population, the allele frequency shifts. ~J.D.A.

mtDNA traced pairs back to million years ago. ~J.D.A.

Main subjects studied. Planktonic larva is significant in this study because larva shrimp would migrate to different regions of the Isthmus of Panama which may be a significant contributor to speciation events ~J.D.A. (+ JP)

In general, this graph illustrates that as genetic distance between species increases (this is measured by Nei's D) then mtDNA divergence increases as well. Overall, compatibility between two different organisms will be less if they have high mtDNA divergence and a high Nei's D value. (JP)

mtDNA CO1 is a gene found within the mitochondria, it is used to measure the genetic difference between organisms. A higher mean indicates a larger difference between two organisms, which is indicative of being a different species. (JP)

The authors performed a similarly designed experiment to the one cited here which was done on drosophila (flies). This is to show that there are other species that have undergone staggered isolation through similar or even different events. (DV)

The authors used this source as a bases to analyze the snapping shrimps mitochondrial DNA data and arrange the organisms in a phylogenetic tree as seen in figure 1. (DV)

The authors site this book as they are referring to the importance of having and maintaining genetic variation within a population. Also they might have utilized the mathematical theory of selection in a fluctuating environments, since the paper focuses on environmental and geographical changes affects on isolation. (DV)

The classic allopatric model is also known as geographic speciation or the dumbbell model. Classic allopatric model is a kind of speciation that occurs when biological populations of the same species become isolated from each other. This isolation is what will prevent genetic exchange between the separated populations. ~S.Z.

"Intra" means within. Divergence is when something separates. Thus, this word could mean how species diverge from each other from within the same oceanic community. ~J.D.A.

Strombinid refers to the taxa Strombidae. It's a family of medium-sized conch sea snails. Gastropods is also another term used to describe the Mollusca phylum. ~J.D.A.

The word shoal describes a shallow body of water. This is synonymous to sandbars or elevated sand in beaches. Therefore, something that "shoals" means that it doesn't have much depth. ~J.D.A.

Genetic divergence was observed to have occured at different moments when oceanic changes like the haulting of certain currents across the Panama seaway and the shallowing of certain areas. (DV)

Concordant means to agree. A variation of this word would be "concor", which has the same meaning. For this paper, the word may be used to describe how data collected seem to show the same variation or consistency. ~J.D.A.

Tenable is defined as something that can be "holdable". It could mean that something must have enough support for it to be upheld. In other words, if someone has an argument against an idea, the explanation should be enough to defend the concept. ~J.D.A.

The intolerant behavior that has been observed in snapping shrimp known as snapping is the production of a water jet created by the larger claw that creates a loud "pop" sound. Although, it has been discovered that the actual threat in this behavior is the subsequent shock wave that comes after the water jet. (DV)

Read more in Shrimp shootouts end with a shock wave bang: http://www.sciencemag.org/news/2017/10/shrimp-shootouts-end-shock-wave-bang?utm_campaign=news_daily_2017-10-30&et_rid=17774509&et_cid=1632982

Staggered suggests that the occurrence or arrangement of events did not happen at the same time. Staggered isolation is the thought of organisms become genetically isolated at different events or moments in time, therefore leading to speciation of these organisms. (DV)

Parsimony analysis deals with simplicity. In regards to science, there will be various explanations to certain phenomenon. According to parsimony analysis, the simplest explanation is the best one. ~J.D.A.

Kimura is a Japanese biologist whose evolutionary theory was based at the molecular level and believed that most genetic differences between species were neutral and not driven by natural selection.

Kimura's corrected percent sequence divergence is an analysis that compares the genomes of species and allows to determine the divergence between their genetic code which can help conclude moment of speciation. (DV)

Conspecific is also like heterospecific as it pertains to classifying species. In the sentence it seems to be an opposing word to heterospecific. If heterospecific means organisms of differ species, then conspecfics must mean the opposite--must mean organisms of same species. ~J.D.A.

Heterospecific is used to classify organisms. "Hetero" means differ, and "specific" pertains to a particular subject/concept. Heterospecific means organisms that aren't the same species. ~J.D.A.

Isthmus is a stretch of narrow land that connects two large landmasses and separates two bodies of water.

The Isthmus of Panama formed 3 million years ago connecting North America and South America, while separating the Pacific Ocean and the Caribbean. (DV)

The snapping shrimp (Alpheus) is a species of shrimp well known for their asymmetrical claws which when closed produce a loud snapping noise. They borrow in the sand and attack their prey by quickly reaching out with their larger claw to capture it. (DV)

Nei found the average heterozygosity and genetic distance from a small number of individuals.This paper explains how biases arise in calculations when small samples are used. However, this paper establishes an average that reduces bias. (JP)

This paper, through studying 3 different species of Bathygobius, found that morphological divergence is not correlated with reproductive isolation. Their experiment was testing the extent in which these 3 species had evolved reproductive isolation in the Isthmus of Panama. (JP)

This source demonstrates that mitochondrial DNA is able to provide fairly accurate estimates of times since separation of a species in a 3 million year range. Here they also used organisms from the Isthmus of Panama and gel electrophoresis to deduce the time of speciation. (JP)

The authors of this paper found that, through parsimony analysis of the sequence divergence estimates and of sequence polymorphisms of the Holarctic fish's mtDNA, different Holarctic fish species arose from a geographical event that occurred during the beginning of the mid-Pliocene period.

The authors of this paper cited this source because this source conducts a similar study in deducing a time frame in which speciation of the Holarctic fish occurred. (JP)

This source published in 1979 to Nature tests the reliability of the molecular clock hypothesis by using Panamanian sea urchins. The author argues that the molecular clock hypothesis is not tenable or supportable. (JP)

Spearman's rank correlation is a nonparametric measure of rank correlation. This measure whether data sets are linear or not linear by assuming that the data set does not follow a normal distribution. A higher r value signifies more linearity. This measure demonstrates a relationship between two variables. (JP)

By quantifying behavioral tolerance and intolerance of male-female transisthmian pairs, the researchers are able to compare interactions between closely related and distantly related species of snapping shrimp.

For example, while keeping figure 1 and table 2 in mind, more closely related species have higher compatibility compared to more distantly related species. Although this is not an indicator of having viable offspring, this shows that closely related species may share similar behaviors or may have similar niches. (JP)

PAUP stands for Phylogenetic Analysis Using Parsimony. This is a computational phylogenetics program that is used to develop phylogenetic trees. PAUP implements parsimony into its calculations, meaning that the simplest explanation is the most preferred. (JP)

Figure 1 is a visual representation of the relationship between differing species of snapping shrimps. This diagram was made using the mitochondria DNA sequences of the organisms and the PAUP program which calculates the level of relatedness between the sibling species of snapping shrimp.

Comparing Table 1 with Figure 1, it is found that species more closely related, such as P2 and C2, had a lower mtDNA mean value (6.6). Meanwhile, species that were more distantly related, such as P7' and C7, had a higher mtDNA mean value (19.7). (JP)

Nei's D is a formula that measures genetic distance. This measure assumes that genetic differences are caused by mutation and genetic drift. Genetic distance is a measure of the genetic divergence between species.

The changes in structure or protein sequences of allozymes was used in conjunction with this formula to find the genetic distance between different species of snapping shrimp. (JP)

The molecular clock is a hypothesis that states that DNA and protein sequences evolve at the same rate over time across different organisms. The molecular clock uses the mutation rate of biomolecules to estimate the time in prehistory when two or more life forms diverge. By using the molecular clock model, researchers are able to determine when an organism diverged from its ancestor. (JP)