diversity Review Population Structures and Levels of Connectivity for Scyphozoan and Cubozoan Jellyfish Michael J. Kingsford * , Jodie A. Schlaefer and Scott J. Morrissey Marine Biology and Aquaculture, College of Science and Engineering and ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, QLD 4811, Australia; [email protected] (J.A.S.); [email protected] (S.J.M.) * Correspondence: [email protected] Abstract: Understanding the hierarchy of populations from the scale of metapopulations to mesopop- ulations and member local populations is fundamental to understanding the population dynamics of any species. Jellyfish by definition are planktonic and it would be assumed that connectivity would be high among local populations, and that populations would minimally vary in both ecological and genetic clade-level differences over broad spatial scales (i.e., hundreds to thousands of km). Although data exists on the connectivity of scyphozoan jellyfish, there are few data on cubozoans. Cubozoans are capable swimmers and have more complex and sophisticated visual abilities than scyphozoans. We predict, therefore, that cubozoans have the potential to have finer spatial scale differences in population structure than their relatives, the scyphozoans. Here we review the data available on the population structures of scyphozoans and what is known about cubozoans. The evidence from realized connectivity and estimates of potential connectivity for scyphozoans indicates the following. Some jellyfish taxa have a large metapopulation and very large stocks (>1000 s of km), while others have clade-level differences on the scale of tens of km. Data on distributions, genetics of medusa and Citation: Kingsford, M.J.; Schlaefer, polyps, statolith shape, elemental chemistry of statoliths and biophysical modelling of connectivity J.A.; Morrissey, S.J. Population suggest that some of the ~50 species of cubozoans have populations of surprisingly small spatial Structures and Levels of Connectivity scales and low levels of connectivity. Despite their classification as plankton, therefore, some scypho- for Scyphozoan and Cubozoan zoans and cubozoans have stocks of small spatial scales. Causal factors that influence the population Jellyfish. Diversity 2021, 13, 174. structure in many taxa include the distribution of polyps, behavior of medusa, local geomorphology https://doi.org/10.3390/d13040174 and hydrodynamics. Finally, the resolution of patterns of connectivity and population structures will Academic Editors: Michael Wink, be greatest when multiple methods are used. Alenka Malej and Agustin Schiariti Keywords: medusa; scyphozoa; cubozoa; population; connectivity; population structure Received: 9 March 2021 Accepted: 12 April 2021 Published: 19 April 2021 1. Introduction Publisher’s Note: MDPI stays neutral Jellyfish are ecologically important as predators [1], prey [2–4] and structures in the with regard to jurisdictional claims in pelagic environment [5,6]. The abundance of jellyfish can vary greatly in space and time and published maps and institutional affil- increases in abundance can have detrimental effects on ecosystems and human livelihoods. iations. For example, predation on small planktonic food and specific groups (e.g., fish larvae) by jellyfish can result in important changes in marine food chains. These changes can in turn result in phase shifts that fundamentally alter trophic pathways, where in some cases baitfish may be largely replaced by jellyfish [7]. Jellyfish blooms can also affect human Copyright: © 2021 by the authors. infrastructure [8] such as aquaculture facilities, power plants that utilize seawater and Licensee MDPI, Basel, Switzerland. nets used to catch boney fishes [9]. Jellyfish have also been introduced in ballast water, This article is an open access article affecting native assemblages [10]. Further, jellyfish are venomous and when humans are distributed under the terms and stung the affects can range from nuisance-level irritations to severe responses and even conditions of the Creative Commons death; consequently, the presence of jellyfish affects tourist industries at multiple latitudes Attribution (CC BY) license (https:// globally [11]. In contrast, jellyfish are also an exploitable resource. They are targeted creativecommons.org/licenses/by/ by fisheries [12], and annual reported catches are about 0.9 Mt [13]. The magnitude of 4.0/). Diversity 2021, 13, 174. https://doi.org/10.3390/d13040174 https://www.mdpi.com/journal/diversity Diversity 2021, 13, x FOR PEER REVIEW 2 of 16 which have boom and bust abundance cycles. Accordingly, there are multiple reasons Diversity 2021, 13, 174 2 of 16 why knowledge is required on the sources, sinks, connectivity and related population structure of jellyfish. There are about 200 morpho-species of “true jellyfish” (Class Scyphozoa [14]), with species influences on ecosystem functions and human activities is inherently linked to their truepopulation (genetic) dynamics, diversit andy likely this is being particularly double true [15] for. particular In contrast jellyfish only species ~50 species which have of box jellyfish (Classboom Cubozoa) and bust abundance have been cycles. described Accordingly, [16]. Our there knowledge are multiple reasons of biology why knowledgeand population ecol- ogyis by required class, on based the sources, on number sinks, connectivity of publications, and related is as population asymmetrical structure as species of jellyfish. richness [17]. Accordingly,There are many about general 200 morpho-species paradigms of on “true jellyfish jellyfish” are (Classbased Scyphozoa on scyphozoans. [14]), with For this rea- true (genetic) diversity likely being double [15]. In contrast only ~50 species of box jellyfish son(Class, we have Cubozoa) combined have been the described knowledge [16]. available Our knowledge on population of biology structures and population of both classes of jellyfishecology by to class, assess based their on commonality number of publications, and differences. is as asymmetrical as species rich- nessPopulation [17]. Accordingly, structures many generalhave generally paradigms been on jellyfish described are based according on scyphozoans. to three For levels. Meta- populations,this reason, wemesopopulations have combined the and knowledge local populations available on population [18]. Metapopulations structures of both are made up of classesa number of jellyfish of mesopopulations, to assess their commonality or in a and fisheries differences. context these are coined “stocks.” A Population structures have generally been described according to three levels. Metapop- metapopulationulations, mesopopulations often corresponds and local populations with the [biogeographic18]. Metapopulations range are of made a species. up of a For exam- ple,number the biogeographic of mesopopulations, range or in of a fisheriesthe three context described these are species coined “stocks.” of Chironex A metapop- are likely to be composedulation often of multiple corresponds stocks with (Figure the biogeographic 1). In some range cases of a, species.there may For example,be anti-tropical the distri- butionsbiogeographic of a species range where of the three metapopulations described species are of foundChironex inare both likely hemispheres, to be composed but are quite of multiple stocks (Figure1). In some cases, there may be anti-tropical distributions of a separate [19]. This scenario is likely for widespread taxa of jellyfish such as Carybdea species where metapopulations are found in both hemispheres, but are quite separate [19]. rastoniThis scenarioand Copula is likely sivickisi for widespread found in taxanorthern of jellyfish and such southern as Carybdea hemispheres rastoni and andCopula multiple seas [17]sivickisi. Separatefound metapopulations, in northern and southern or in some hemispheres cases more and multiple than one, seas likely [17]. Separate correspond to the Largemetapopulations, Marine Ecosystems or in some (LME) cases more coined than by one, Abboud likely correspond et al. [20] to. the Large Marine Ecosystems (LME) coined by Abboud et al. [20]. Figure 1. The geographic range of all know species of Chironex. Chironex fleckeri Southcott, 1956; FigureChironex 1. The yamaguchii geographic[21]; Chironex range indrasaksajiaeof all know[22 species], and unnamed of Chironex. species Chironex in northern fleckeri Australia Southcott, [23] 1956; Chironexand Thailand yamaguchii [24]; Taxonomy [21]; Chironex [25]). indrasaksajiae [22], and unnamed species in northern Australia [23] andPopulation Thailand connectivity[24]; Taxonomy is the [25 exchange]). of individuals among geographically sepa- rated population units [26]. Within metapopulations, a robust biological stock should have littlePopulation to no connectivity connectivity with other is stocks.the exchange Accordingly, of successfulindividuals immigration among andgeographically emigra- sepa- ratedtion, population involving any units life history [26]. stage,Within should metapopulations be very low or zero, a robust [27]. It isbiological possible that stock should have little to no connectivity with other stocks. Accordingly, successful immigration and emigration, involving any life history stage, should be very low or zero [27]. It is possible that an incipient stock is largely separate ecologically, but there is sufficient genetic ex- change that they do not constitute separate clades. Where separation has been sufficient enough to result