Gonadal histology of box jellyfish (Cnidaria: Cubozoa) reveals variation between internal fertilizing species Alatina alata (Alatinidae) and Copula sivickisi (Tripedaliidae)

Abstract

Cubozoans (box jellyfish) are gonochoristic cnidarians with distinct reproductive strategies. This comparative histological study examines the gonad organization of Alatina alata and Copula sivickisi, two box jellyfish species that exhibit different modes of internal fertilization. A. alata reproduces via spermcasting aggregations while C. sivickisi reproduces via copulation; in both cases, internal fertilization occurs in the gastrovascular cavity. Herein, we provide the first histological description of subgastric sacs-structures unique to C. sivickisi. Although previously thought to function as sperm storage sacs, our findings reveal that subgastric sacs are nematocyst nests lacking sperm entirely. Conversely, we discovered that velarial spots in C. sivickisi females correspond to actual sperm storage structures. Histological examination of cubozoan sperm packages revealed that while sperm packages from both species have motile flagella, A. alata males produce nonencapsulated sperm bundles (i.e., "spermatozeugmata"), and C. sivickisi males produce encapsulated packages (i.e., "spermatophores"). Our findings corroborate the presence of several types of nematocysts in C. sivickisi embryo strands and spermatophores, and indicate their provenance to be both female and male gonads respectively, as well as subgastric sacs (i.e., nematocyst nests). In contrast to our findings of velarial spots as sperm storages structures in C. sivickisi females, and of nematocysts in the gonads of both sexes, we report that A. alata medusae lack both sperm storage structures and gonadal nematocysts. Finally, we discuss our findings on reproductive morphology of C. sivickisi and A. alata in light of the respective reproductive behavior of these two cubozoan species.

Gonadal cnidocytes in the cubozoan Tripedalia cystophora Conant, 1897 (Cnidaria: Cubozoa)

“…This is supported by our finding that one of the examined females had a spermatozeugma attached to a hemi‐gonad (Figure a, insert). It has also been suggested that the isorhizas are used to anchor the spermatozeugmata to the female tentacles during mating (Garcia‐Rodriguez et al, ), but these hypotheses are not mutually exclusive. Only ripe sperm cells are included in the spermatozeugmata leaving the immature sperm behind in the hemi‐gonad and each hemi‐gonads can thus form several spermatozeugmata over time (present work, Straehler‐Pohl et al, ; Garcia‐Rodriguez et al, ).…”

“…It has also been suggested that the isorhizas are used to anchor the spermatozeugmata to the female tentacles during mating (Garcia‐Rodriguez et al, ), but these hypotheses are not mutually exclusive. Only ripe sperm cells are included in the spermatozeugmata leaving the immature sperm behind in the hemi‐gonad and each hemi‐gonads can thus form several spermatozeugmata over time (present work, Straehler‐Pohl et al, ; Garcia‐Rodriguez et al, ). This strongly suggests that even though the female of T. cystophora only mates once (Werner, ), the male can have multiple matings like seen for C. sivickisi (Lewis & Long, ).…”

“…Alatina alata also has internal fertilization without mating and males release the sperm in the water. Interestingly, even though the males of A. alata still produce spermatozeugmata cnidocytes are not included (Garcia-Rodriguez et al, 2018). There is some evidence that the released embryos of A. alata have associated cnidocytes but their origin is uncertain since the female gonads lack cnidocytes (Garcia-Rodriguez et al, 2018).…”

“…The female ingests the transferred sperm package, which accordingly enters their gastrovascular cavity (Stewart, ). Interestingly, in males of C. sivickisi the gonads are packed with cnidocytes, which are imbedded in the spermatophore (sensu Garcia‐Rodriguez, Ames, Marian, & Marques, ) where they mature (Garm, Lebouvier, & Duygu, ). This happens despite the sperm is transferred directly to the female and thus do not experience predation (Hartwick, ; Lewis & Long, ).…”

“…Cnidocytes were also found in female gonads of C. sivickisi and they were transferred to the embryo strand (Garm et al, ; Lewis & Long, ). These cnidocytes were of the eurytele type, which possess a stylet and typically contain toxin, putatively protecting the embryo strand from predation after being externalized (Garcia‐Rodriguez et al, ; Garm et al, ). Tripedalia cystophora has internal fertilization like C. siviskisi but interestingly no embryo strand is produced and instead the embryos complete their development inside the female (Stewart, ; Werner, , ).…”