Various interactions involving macroalgae and other organisms are known in the marine environment, among these, some represent true examples of mutualism. Biotic pollination can be considered a mutualistic interaction. In the case of terrestrial plants, it is generally defined as the process of transferring pollen from male to female reproductive structures of the same species. However, the use of this term has been extended to marine macroalgae thanks to the study of Lavaut and colleagues (2022), who had used the term “pollination” to refer to the transport of male gametes (spermatia), by isopod crustaceans of the species Idotea balthica, from male to female specimens of the red alga Gracilaria gracilis. These authors suggested the existence of a mutualistic relationship with mutual benefits for the two species. The aim of the present study was to investigate to what extent this interaction can be considered mutualistic and what are the potential benefits obtained by both partners. The association of isopods of the genus Idotea with various algal species (G. gracilis, Fucus spp., Ulva spp.) collected in the field has been evaluated. The results showed that at three locations in northern France, frequency of isopods of the genus Idotea was much higher on G. gracilis than on other algae. Furthermore, isopods associated with G. gracilis were smaller than those found on Fucus spp. Assuming that, from exposure to I. balthica specimens, G. gracilis may gain an advantage in terms of growth increase, a laboratory experiment was planned to test the null hypothesis that there was no difference in the growth of G. gracilis in presence and absence of I. balthica. The experiment was conducted for one month using aerated glass balloons, placing 3 fragments of G. gracilis, belonging to different life stages and sexes, in each balloon under the following three conditions C1: algal fragments not exposed to isopods; C2: exposed to 5 isopods free to move; C3: exposed to 5 isopods caged in a small plastic tube. The results showed that the isopods were able to graze macroepiphytes grown on the algal thallus during the experiment. Furthermore, no significant difference was found in the growth rate of G. gracilis either between the different experimental conditions or between the different life stages/sexes of the alga, demonstrating that G. gracilis did not achieve a higher growth in presence of I. balthica. Considering the possibility that the release of spermatia from male gametophytes of G. gracilis may be favoured by the presence of I. balthica, a controlled laboratory experiment was designed to test the null hypothesis that there was no difference in the amount of spermatia released by G. gracilis in presence and absence of the isopod. Aquaria containing a composition of algal fragments from different male gametophytes have been used. The number of spermatia released per gram of alga was obtained for the four different experimental conditions tested: C1: algal fragments not exposed to isopods; C2: exposed to 5 isopods; C3: exposed to 10 isopods; C4: exposed to 20 isopods. The results suggested that the presence of I. balthica favours the release of spermatia from the alga. In particular, isopods favoured a higher spermatia release compared to the control (C1), but only when algae were exposed to the highest density of I. balthica specimens (C4) among those tested. In conclusion, results of this thesis have brought further evidences to support the already proposed hypothesis of the existence of a mutualistic interaction with mutual benefits between G. gracilis and I. balthica. Furthermore, the obtained results provide new insights into the nature of the relationships existing between macroalgae and small invertebrates, suggesting the importance of studying these associations between other species as well.
In ambiente marino sono note diverse interazioni che coinvolgono macroalghe ed altri organismi, tra queste, alcune rappresentano veri esempi di mutualismo. L'impollinazione biotica è considerata un’interazione mutualistica. L’uso del termine “impollinazione” è stato recentemente esteso alle macroalghe marine grazie al lavoro di Lavaut et al. (2022), che lo hanno utilizzato per indicare il trasporto di gameti maschili (spermazi), da parte di crostacei isopodi della specie Idotea balthica, dagli esemplari maschili a quelli femminili dell’alga rossa Gracilaria gracilis. Questi autori hanno suggerito l’esistenza di una relazione mutualistica con benefici reciproci per le due specie. Lo scopo del presente studio è stato quello di indagare la natura mutualistica di questa interazione e i potenziali benefici ottenuti da entrambi i partner. È stata perciò valutata la frequenza di isopodi del genere Idotea su diverse specie algali (G. gracilis, Fucus spp., Ulva spp.) raccolte su campo. I risultati mostrano che in tre località della Francia settentrionale la frequenza di isopodi del genere Idotea è risultata molto più alta su esemplari di G. gracilis rispetto ad altre alghe. Inoltre, gli isopodi associati a G. gracilis sono risultati più piccoli rispetto a quelli trovati su Fucus spp. Supponendo che, dall’esposizione ad esemplari di I. balthica, G. gracilis possa acquisire un vantaggio in termini di incremento della propria crescita, è stato condotto un esperimento per verificare l'ipotesi nulla che non vi fossero differenze nella crescita di G. gracilis in presenza e in assenza di I. balthica. L'esperimento è stato condotto utilizzando palloni di vetro aerati, ponendo in ogni pallone 3 frammenti di G. gracilis, appartenenti a fasi vitali e sessi diversi, nelle seguenti tre condizioni: C1: alghe non esposte a isopodi; C2 esposte a 5 isopodi liberi; C3: esposte a 5 isopodi ingabbiati in un tubicino di plastica. Il peso dei frammenti è stato misurato ogni settimana per un mese, al fine di calcolare il tasso di crescita giornaliero. I risultati dimostrano che l'isopode è in grado di brucare i macroepifiti cresciuti sul tallo algale nel corso dell’esperimento. Inoltre, non è stata rilevata alcuna differenza significativa nel tasso di crescita di G. gracilis né tra le diverse condizioni sperimentali né tra i diversi stadi vitali/sessi dell’alga, dimostrando che G. gracilis non raggiunge una crescita maggiore in presenza di I. balthica. Considerando la possibilità che il rilascio di spermazi da parte degli individui maschili di G. gracilis possa essere favorito dalla presenza di I. balthica, è stato ideato un esperimento per testare l'ipotesi nulla che non vi fossero differenze nella quantità di spermazi rilasciati in presenza e in assenza dell'isopode. Sono stati utilizzati acquari contenenti una composizione di frammenti algali provenienti da gametofiti maschili differenti. Il numero di spermazi rilasciati per grammo d’alga è stato ricavato per quattro diverse condizioni sperimentali: C1: alghe non esposte a isopodi; C2: esposte a 5 isopodi; C3: esposte a 10 isopodi; C4: esposte a 20 isopodi. I risultati suggeriscono che la presenza di I. balthica favorisca il rilascio di spermazi da parte dell’alga. In particolare, gli isopodi favoriscono un maggiore rilascio di spermazi rispetto al controllo (C1), ma solo in caso di esposizione alla maggiore densità (C4) tra quelle testate. In conclusione, i risultati di questa tesi hanno contribuito a portare ulteriori prove a sostegno dell'esistenza di un'interazione mutualistica con benefici reciproci tra G. gracilis e I. balthica. Hanno inoltre fornito una nuova visione delle relazioni esistenti tra alghe marine e piccoli invertebrati, suggerendo l'importanza di studiare queste associazioni anche tra altre specie.
QUAL E' IL LIVELLO DI MUTUALISMO NELLE INTERAZIONI TRA IL CROSTACEO ISOPODE IDOTEA BALTHICA E L'ALGA ROSSA GRACILARIA GRACILIS?
FERRARI, VITTORIA
2022/2023
Abstract
Various interactions involving macroalgae and other organisms are known in the marine environment, among these, some represent true examples of mutualism. Biotic pollination can be considered a mutualistic interaction. In the case of terrestrial plants, it is generally defined as the process of transferring pollen from male to female reproductive structures of the same species. However, the use of this term has been extended to marine macroalgae thanks to the study of Lavaut and colleagues (2022), who had used the term “pollination” to refer to the transport of male gametes (spermatia), by isopod crustaceans of the species Idotea balthica, from male to female specimens of the red alga Gracilaria gracilis. These authors suggested the existence of a mutualistic relationship with mutual benefits for the two species. The aim of the present study was to investigate to what extent this interaction can be considered mutualistic and what are the potential benefits obtained by both partners. The association of isopods of the genus Idotea with various algal species (G. gracilis, Fucus spp., Ulva spp.) collected in the field has been evaluated. The results showed that at three locations in northern France, frequency of isopods of the genus Idotea was much higher on G. gracilis than on other algae. Furthermore, isopods associated with G. gracilis were smaller than those found on Fucus spp. Assuming that, from exposure to I. balthica specimens, G. gracilis may gain an advantage in terms of growth increase, a laboratory experiment was planned to test the null hypothesis that there was no difference in the growth of G. gracilis in presence and absence of I. balthica. The experiment was conducted for one month using aerated glass balloons, placing 3 fragments of G. gracilis, belonging to different life stages and sexes, in each balloon under the following three conditions C1: algal fragments not exposed to isopods; C2: exposed to 5 isopods free to move; C3: exposed to 5 isopods caged in a small plastic tube. The results showed that the isopods were able to graze macroepiphytes grown on the algal thallus during the experiment. Furthermore, no significant difference was found in the growth rate of G. gracilis either between the different experimental conditions or between the different life stages/sexes of the alga, demonstrating that G. gracilis did not achieve a higher growth in presence of I. balthica. Considering the possibility that the release of spermatia from male gametophytes of G. gracilis may be favoured by the presence of I. balthica, a controlled laboratory experiment was designed to test the null hypothesis that there was no difference in the amount of spermatia released by G. gracilis in presence and absence of the isopod. Aquaria containing a composition of algal fragments from different male gametophytes have been used. The number of spermatia released per gram of alga was obtained for the four different experimental conditions tested: C1: algal fragments not exposed to isopods; C2: exposed to 5 isopods; C3: exposed to 10 isopods; C4: exposed to 20 isopods. The results suggested that the presence of I. balthica favours the release of spermatia from the alga. In particular, isopods favoured a higher spermatia release compared to the control (C1), but only when algae were exposed to the highest density of I. balthica specimens (C4) among those tested. In conclusion, results of this thesis have brought further evidences to support the already proposed hypothesis of the existence of a mutualistic interaction with mutual benefits between G. gracilis and I. balthica. Furthermore, the obtained results provide new insights into the nature of the relationships existing between macroalgae and small invertebrates, suggesting the importance of studying these associations between other species as well.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12075/16945