Review Article

Currents in the Pacific Ocean as Pathways for Dispersal of Bivalve Mollusks

VV Sukhanov*

AV Zhirmunsky National Research Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences, Russia

Submission:September 14, 2024; Published: October 09, 2024

*Correspondence author: VV Sukhanov, AV Zhirmunsky National Research Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences, Vladivostok 690041, Russia

How to cite this article: VV Sukhanov. Currents in the Pacific Ocean as Pathways for Dispersal of Bivalve Mollusks. Oceanogr Fish Open Access J. 2024; 17(5): 555973. DOI: 10.19080/OFOAJ.2024.17.555973

Abstract

Common boundaries of distribution ranges of Bivalvia can be identified at sites of their aggregations. According to the Bernard’s Catalogue of the Living Bivalvia, these aggregations are distributed from the Bering Strait to Cape Horn along the eastern Pacific coast. They are confined to the major currents caused by trade winds along Equator. The issues to be addressed are formulated.

Keywords: Ocean currents; Trade winds; Synperates; Frequency distribution

Introduction

When designing a new method for biogeographical zoning Sukhanov et al. [1], we selected the zonal/geographical status of a species for its biogeographical characterization, or, in other words, the type of species range in the latitudinal zoning. All these types were replaced by ordinal numbers on the special scale of correspondence. It was done as follows.

Three planetary latitudinal zones are distinguished in the Northern Hemisphere: arctic, boreal, and tropical. Each of these zones is divided into two sub-zones: the arctic zone is divided into high- and low-arctic sub-zones; the boreal zone, into high- and low-boreal sub-zones; the tropical zone, into subtropical and equatorial sub-zones. We consider each of these sub-zones as an elementary, indivisible minimal unit. By assuming the equatorial line to be the zero point of reference, we assigned ranks from 1 to 6 to the boundaries of each of these sub-zones. The ranks were assigned to the boundaries of these sub-zones sequentially as they increased from the tropics to the northern latitudes. Thus, the Equator was ranked 0; the North Pole as 6. We referred to the chain of these ranks as the Latitudinal Zone Index Scale.

This index scale was extended to the Southern Hemisphere, with ranks for the boundaries between the sub-zones having negative values. These ranks formed the following chain of values: 0, –1, –2, …, –6 from the Equator to the South pole through the notal to the high-Antarctic region. The Equator had a rank equal to 0 again; the South Pole was ranked –6.

All these innovations were designed to calculate the value of the latitudinal zoning index averaged for all species that got in one or another sample with specific geographical coordinates. Then, based on a set of such samples, one can compose a map of spatial distribution of values of this index over a water body. In other words, biogeographical zoning can be performed. Using the map of the isolines of this index, it is easy to identify areas with different ranks. Afterwards, one may switch back from the numerical ranks to their verbal names. When searching for information about certain nekton species, we found that a large portion of the species, in addition to the biogeographical characteristics of the latitudinal zoning, was accompanied by descriptions of the types of “longitudinal zoning”. However, many of nekton species were attributed to the eastern coast of the Pacific Ocean. It was unclear to us, how this information could be used. Therefore, we postponed this problem “to the future”.

That “future” came when the author read the Bernard’s Catalogue of the Living Bivalvia [2] that contains information about 83 families, including 302 genera and 1,306 species of bivalve mollusks. These bivalves inhabit the eastern Pacific coast from the Bering Strait (66° N) to Cape Horn (56° S). Each species has its own range (minimum and maximum boundaries and a center of the range in degrees of geographical coordinates), minimum and maximum depths of finding, minimum and maximum temperatures of its habitat, and geological age set on the ordered scale.

Not all of these parameters are available in the Catalogue for each species: 12% of 10,448 values are missing. Nevertheless, this dataset is unique in its coverage, which can help in formulating a number of issues. The author obtained this Catalogue already in a digitized form, as a text file, not as images of its pages in the Internet. This circumstance saved time to prepare the material for computer processing. Nevertheless, the source file was not strictly and entirely formatted as declared and, therefore, was not suitable for computer processing due to numerous errors. The latter took a long time to fix. For this reason, there were few publications based on the Catalogue Bernard et al. [3].

Synperates

The term synperate consists of the Greek roots συν (meaning “together”) and πέρας (“margin”, “limit”, or “boundary”). It was proposed by Kuznetsov [4] to refer to sites where outlines of distribution ranges of different animal species coincide. He considered synperates as the most important boundaries of species’ ranges. When plotting the distribution ranges of different species together in a map, one can find that the boundaries run very close to each other in some areas. The line of aggregation of such boundaries is a synperate. There must be some kind of natural barrier near that line which makes it difficult or even impossible for the species to disperse in space. See also comments by Kafanov [5]. Identification of synperates in a 2D space (on a plane) poses a challenge. A popular method to address it is typification of ranges, where “areas of aggregation of range boundaries” are searched by a certain technology Zhirkov [6].

In one-dimensional space (along a line), such a problem is much easier to solve. Materials from the Bernard’s Catalogue make it possible to draw synperates for bivalve mollusks all along the eastern coastline of the Pacific Ocean. (Figure 1) shows such synperates, i.e., aggregations at the southern and northern boundaries of distribution ranges of marine Bivalvia species. In the present study, we provide geographical coordinates on the modern scale, from –60° to 80° through zero at the Equator, rather than according to the classical British grid, from 60° S till 80° N (as in the Bernard’s Catalogue).

f1

This histogram of synperates shows the taxonomic poorness of the Southern Hemisphere. Only 13% of species have the centers of their ranges located south of the Equator. There may be several possible explanations for this observation. First, the extent of knowledge of the Southern Hemisphere is low. There the countries situated on the coasts have low economic and, therefore, weak scientific potential. It is obvious that this knowledge should increase as many new studies are undertaken. Nevertheless, the small number of synperates compared to those in the North may also indicate a low biological diversity of environments in this region that cannot support high species richness. Another responsible factor is the low inflow of migrants from the West to the East due to the small area of lands in the southern West and, therefore, the small extent of coasts and, respectively, shallow waters as sources of dispersal.

The local peaks in the histogram indicate the sites of aggregation of boundaries. At least six such peaks can be seen. An aggregation of boundaries from –55° to –50° is noticeable in the left part of the graph. A series of three small maxima spaced to a distance of 10° from each other is located right. Most likely these are artifacts caused by rounding of raw data. Since the Southern Hemisphere remains relatively poorly studied to date, the sites of rare findings of mollusks might be determined very approximately.

The well-visible tall peak near the Equator is located further right in the histogram (from –5 to 0°, about 300 cases), followed by a maximum at latitudes between 5° and 10°, then by another large peak near 30°, a lower in height maximum near 45°, and then by a peak at a latitude of approximately 60°. This sequence ends with a low maximum at 70°. The southern and northern boundaries of the species ranges are combined into a single histogram (Figure 2). The centers of all ranges are assigned the zero value here, and the boundaries are represented as positive and negative deflections from the centers.

The northern and southern boundaries of any species range are located almost symmetrically relative to its center, and their spreads (standard deviations) are almost similar. The average southern boundary ± its standard error is equal to –8.59 ± 0.28°, and the standard deviation of this boundary is 8.97°. The average northern boundary with its standard error is 8.26 ± 0.28°, and the standard deviation is 9.17°. Hence, the average extent of the species range of Bivalvia is 16.85 ± 0.40° or 1,875 ± 44 km in metric units.

(Figure 3) shows the frequency distribution of these extents (lengths) of species ranges. The frequencies vary slightly along the Y-axis up to the X-axis at 35° inclusive and then they sharply drop and gradually approach zero in the right part of the graph. Prior to this study, the author expected the histograms of lengths of species ranges to have either a Gaussian “bell curve” shape or an L-shaped distribution with a maximum for the smallest ranges and a descending tail for larger ones. But it has turned out to be even more interesting. The sum of all frequencies for the species ranges from 36° to 80° makes up only 9% of all species. This group comprises abnormally large ranges. In contrast, 91% of frequencies from zero to 35° inclusive have a characteristic, rarely found distribution: frequencies slightly fluctuate in the vicinity of the same level for about 120 species.

This is the example where the frequency distribution proved to be close to even. A statistical interpretation of even distribution means constancy of the density function. Thus, the probability of a species range to get in one or another length class is the same and does not depend on the extent of the range. It is a kind of dice game that Nature plays here, where six or one are equally possible to happen. Any biogeographical interpretation of this phenomenon and its mathematical model are not yet available. Nevertheless, they would help to understand how species ranges are formed. This pattern is likely to be even more complicated. The histogram in (Figure 3) can be interpreted as a mixture of normal distribution truncated at the modal value and abnormal frequency outliers with a length of ranges near 30°-35°.

A bipolar range (of a species that lives in the North and South but is absent from the middle latitudes where it died out in the past) in the Bernard’s Catalogue may look like a very long, continuous area. Such ranges “hide” among abnormally large ranges. They can be found among species with well-known northern and southern boundaries but unknown centers of ranges. We found 32 species like these in the Catalogue (2.4% of all). Thus, bipolar species probably exist here but there are too few of them to explain this phenomenon.

Currents

Dispersal of adult bivalve mollusks is impossible: they either move very slowly or live an attached life on the substrate. They disperse mainly via larvae passively carried by sea currents. Once the metamorphosis is complete, these larvae settle onto the seabed. The importance of currents in the distribution of marine organisms is widely discussed in the literature Mileykovsky [7]. Under the influence of the theory of island biogeography MacArthur, Wilson [8], the author built a simple mathematical model that described dispersal of littoral animals along the Greater Kuril Chain Sukhanov [8]. This chain of islands is washed by two currents. The cold Oyashio Current flows from north to south along the ocean side. The warm Soya Current flows from south to north along the Sea of Okhotsk side. These currents carry passively migrating larvae with them. The northern source of these migrants is Kamchatka, and the southern source is Hokkaido. During the migrations, most of larvae die. Few of them survive before reaching their destination. The model described well the actual distribution of littoral animals along the islands: residual variance of the model is only 17.5% of total. Thus, the mechanism that provides dispersal of most marine bivalves is based on the transport of larvae by currents.

A diagram of the major currents in the Pacific Ocean Ocean Currents Map [9] (see also [10]) is shown in (Figure 4). A large cyclonic (clockwise) gyre is located above the Equator. It is referred to as the North Pacific Gyre and consists of a chain of four currents: Kuroshio, North Pacific, California and North Equatorial. The California Current is cold, while the other three currents are warm. Two smaller cyclonic eddies act within this large gyre. Each of them contains a large garbage patch consisting of man-made solid waste. There is a smaller anticyclonic (counter-clockwise) gyre above the North Pacific Gyre, referred to as the Subpolar Gyre. It is formed by the warm Alaska Current and the cold Kamchatka Current. The cold Oyashio Current, discussed above in relation to the mathematical model, is shown in the upper left corner of the map.

The South Pacific Gyre is shown below the Equator. It is formed by the warm South Equatorial Current and the cold Peru (Humboldt) Current. The warm Equatorial Counter Current flows eastward along the Equator in the area between the North and South Pacific Gyres. Running along the Equator in the westerly direction, the North and South Pacific Gyres accumulate heat. Then they deflect to the respective poles because of the Coriolis force. They cool down there and bring cold water back to the American coast.

The Antarctic Circumpolar Current flows from west to east along latitude –55° near Cape Horn. It forms the southernmost synperate (Figure 1) and acts as a barrier to dispersal of larvae to Antarctica. Another current passes along the coasts of Chukotka and Alaska from west to east above the Arctic Circle at a latitude of 70° Bondarenko [11] (Figure 1). It blocks the transport of larvae to the Arctic Ocean and forms the northernmost synperate (Figure 1). Trade winds are the driving force of two large gyres in the Pacific Ocean. In the past, the strong and constant westerlies were actively used by sailors on the trade routes in the Atlantic Ocean to travel from the Old World to the New World.

Air flows heated near the Equator are directed westward due to the rotation of the Earth. Constant trade winds drive surface waters also from east to west along the Equator. The Coriolis force gradually deflects these flows: those above the Equator move to the north off the west coast, and those below the Equator move to the south. This way the main North and South Pacific Gyres are formed. Due to the action of trade winds, the wind-driven excess of water accumulates off the western coast near the Equator. It causes a compensatory Equatorial Counter Current flowing in the opposite direction along the Equator, from west to east.

Possible explanations for the existence of aggregations of boundaries can be found by comparing the map of currents in (Figure 4) and the histogram of synperates in (Figure 1). The synperate at latitudes from –5° to 0° and a peak at a level from 5° to 10° are explained as follows: shoreward currents are oriented east to the coast, bringing larvae to shore and preventing them from migrating west. The synperate at latitudes 25°–30° is explained by two counterflowing, colliding currents (the California Current flows south, and the Equatorial Counter Current turns north near the coast). The aggregation of the boundaries at latitudes level of 55° to 60° (the northern part of the Alaska Current) does not have any simple explanation. The migration of larvae in this area is likely prevented by the chain of the Aleutian Islands. The synperates at latitudes –55° to –50° and at a level of 70° to 75° have already been explained by the blocking currents. Recall that the cartographic diagram in (Figure 4) may have an inaccuracy of approximately 5°.

A strong ocean current provides transport of larvae only along the central part of its “channel”. In its transverse (cross) directions, larvae move at low speeds. Therefore, the peripheral areas of this current are a kind of restraining limits that cause formation of synperates, like river banks that prevent spreading of aquatic life to the land. To illustrate this statement, the distribution of findings of bivalve mollusks in the space of synperates and habitat depths is shown in (Figure 5). To avoid overloading of the figure, only southern synperates (“left” boundaries of ranges in (Figure 2)) and minimum habitat depths. The depth scale is made substantially nonlinear because most of the findings were at shallow depths.

The boundaries of range aggregation in (Figure 5) are represented by vertical chains of points. Larvae are caught by the currents, mainly by the trade winds, flowing from east to west. Then, during the migration, the larvae gradually settle to the seabed in the form of plumes, thus, creating bivalve beds. Where the velocities of the currents are slow, metamorphosis and settlement of larvae are completed not only in shallow waters but also over great depths. Larvae involved by the fastest part of a current are carried far away from the coast and can reach the western coast of the Pacific Ocean. Thus, the habitat depth itself does not always determine the ecological niche of a species. Metamorphosis and settlement of larvae caught in sea currents occur regardless of depth over which they move.

Conclusion

Our study clarifies the mechanism of settlement of bivalve mollusks in the Pacific Ocean. We have shown the key effect that the major ocean currents exert on this process. An analysis of the aggregations at the boundaries of the ranges helped to relate them to these currents. Longitudinal studies are not suitable for describing biogeographical zoning maps with two-dimensional ranges. Nevertheless, vector maps, not landscape ones, that describe movements by arrows are necessary. These are suitable for demonstrating migration routes of larvae and characterizing relationships between taxa from different regions.

An extensive biogeographic classification of bivalve taxa of various ranks based on their synperates and centers of habitats needs to be carried out in the future. I would also like to work with a catalogue like the Bernard’s one for the western coast of the Pacific Ocean. It would clarify the ranges and routes of exchange of pelagic larvae between both coasts. It is also necessary to design an objective method for detecting synperates in a 2D-space, based on a computer algorithm. There are still no clear answers to the following two questions. Why is the southern half of the Pacific Ocean so poor in bivalve taxa? What is the factor responsible for the unusual pattern of size–frequency distribution in extents of species ranges?

Acknowledgment

The Bernard’s Catalogue was given to the author by prof. A.I. Kafanov (A.V. Zhirmunsky Institute of Marine Biology, Far East Branch, Russian Academy of Sciences), and this study is dedicated to his memory. Many thanks are due to Richard Arterbury, Founder/CEO, Richard@oceanblueproject.(org) for his kind permission to reproduce the map of currents of the Pacific Ocean (Figure 4).

Funding

This work was supported by ongoing institutional funding. No additional grants to carry out or direct this particular research were obtained. Ethics approval and consent to participate. This work does not contain any studies involving human and animal subjects.

Conflict of interest

The author of this work declares that he has no conflicts of interest.

References

1. Sukhanov VV, Ivanov OA (2009) A new approach to biogeographical subdivision. Russ J of Marine Biology 35(1): 79-86.
2. Bernard FR, McKinnell SM, Jamieson GS (1991) Distribution and zoogeography of the Bivalvia of the eastern Pacific Ocean. Can Spec Publ Fish Aquat Sci 112: 60.
3. Bernard FR (2023) Catalogue of the living Bivalvia of the eastern Pacific Ocean: Bering Strait to Cape Horn. Can Spec Publ Fish Aquat Sci 61: 102.
4. Kuznetsov BA (1936) O nekotoryh zakonomernostjah rasprostranenija mlekopitajushhih po evropejskoj chasti SSSR. Ch. 1 About some regularities of distribution of mammals on the European part of the USSR. Part 1. Zool J 15(1): 96-127.
5. Kafanov AI (2005) Istoriko-metodicheskie aspekty obshhej i morskoj biogeografii. [Historical and methodological aspects of the general and marine biogeography] Vladivostok: Far-Eastern Univ Press, p. 208.
6. Zhirkov IA (2016) Bio-geografija. Obshhaja i chastnaja: sushi, morja i kontinental'nyh vodojomov. [Biogeography. Private and General: Land, Seas and Continental Waters] Moscow: Partnership of scientific publications KMK, p. 566.
7. Milejkovskij SA (1997) Lichinki donnyh bespozvonochnyh [Larvae of bottom invertebrates]. Biologija okeana. [Biology of Ocean]. Moscow: Nauka Press, pp. 96-106.
8. Sukhanov VV (1982) Model' raspredelenija vidovogo obilija na litorali ostrovnoj grjady [Model of distribution of species abundance on the littoral of the island range]. Morskaja biogeografija (predmet, metody, principy rajonirovanija). [Marine biogeography (Subject, Methods, Principles of zoning]. Moscow: Nauka Press, pp. 52-75.
9. (2023) Why are Ocean Currents Important? Ocean Currents Map.
10. MacArthur RH, Wilson EO (1967) The theory of island biogeography. Princeton: Princeton Univ. Press, p. 293.
11. Bondarenko AL (2003) Obshhie predstavlenija o techenijah Mirovogo okeana. Shemy techenij. General understanding of the currents of the oceans. Flow diagrams.