Gnesioceros sargassicola (Mertens, 1833), 1939

Gnesioceros sargassicola (Mertens, 1833) View in CoL

Gnesioceros sargassicola (Mertens, 1833) Hyman 1939 b View in CoL

Gnesioceros mertens (Diesing, 1850) View in CoL

Gnesioceros mertensi (Diesing, 1850) View in CoL

Pelagoplana sargassicola (Mertens, 1833) View in CoL

Planaria sargassicola Mertens, 1833

Planocera sargassicola (Mertens, 1833)

Stylochoplana sargassicola (Mertens, 1833)

Stylochus mertensi Diesing, 1850

Stylochus pelagicus Moseley, 1877

Stylochus pelagicus Moseley, 1877

Stylochus sargassicola (Mertens, 1833)

Materials

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00634 ; preparations: whole mount (Neo-Mount); disposition: in collection; occurrenceID: C03621CA-8BAA-519D-8584-1830479B4E58; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00635 ; preparations: whole mount (Neo-Mount); disposition: in collection; occurrenceID: CCBB9BA2-3F3B-5B77-ADF5-BE8948FB394F; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00636 ; preparations: whole mount (Neo-Mount); disposition: in collection; occurrenceID: 06BB7B3D-83A1-5DB2-B73C-925C1FDE1A59; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00637-1 ; preparations: Hematoxilin and eosin sagittal section 1-3; disposition: in collection; occurrenceID: E1F34171-35D0-58B5-BE4A-DD14F111F5E8; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00637-2 ; preparations: Hematoxilin and eosin sagittal section 2-3; disposition: in collection; occurrenceID: 387619E6-CF8E-5338-BBA9-7D2BEBDDA07A; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00637-3 ; preparations: Hematoxilin and eosin sagittal section 3-3; disposition: in collection; occurrenceID: 86672D96-6D3C-5F84-89C8-26C9B8680B33; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00638-1 ; preparations: Hematoxilin and eosin sagittal section 1-6; disposition: in collection; occurrenceID: 1F76A4DE-D9EF-5074-A58E-7ECD1E2871C1; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00638-2 ; preparations: Hematoxilin and eosin sagittal section 2-6; disposition: in collection; occurrenceID: 778582C9-394E-590F-940D-EC9E7428FA9C; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00638-3 ; preparations: Hematoxilin and eosin sagittal section 3-6; disposition: in collection; occurrenceID: B7AF631E-61A3-5762-B1B5-618B427367EA; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00638-4 ; preparations: Hematoxilin and eosin sagittal section 4-6; disposition: in collection; occurrenceID: 5B2F493A-CA9C-5FD8-AF3E-ABAC4CA0F40B; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00638-5 ; preparations: Hematoxilin and eosin sagittal section 5-6; disposition: in collection; occurrenceID: D51B89D0-E9BC-53A8-BAB3-8CFCE0F774DB; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00638-6 ; preparations: Hematoxilin and eosin sagittal section 6-6; disposition: in collection; occurrenceID: D1435E05-C9BB-5742-8C1A-BA62B7C5075B; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00639-1 ; preparations: Hematoxilin and eosin sagittal section 1-6; disposition: in collection; occurrenceID: 8B22DF88-F5C8-526C-99AF-EB80E8A7B078; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00639-2 ; preparations: Hematoxilin and eosin sagittal section 2-6; disposition: in collection; occurrenceID: 0844E7DB-F128-5152-8F89-449A37A4507F; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00639-3 ; preparations: Hematoxilin and eosin sagittal section 3-6; disposition: in collection; occurrenceID: D9C5D213-4A97-5DB5-9ABD-E2483B99DC8D; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00639-4 ; preparations: Hematoxilin and eosin sagittal section 4-6; disposition: in collection; occurrenceID: AB452021-45C2-5185-8927-76B0EE97ED35; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00639-5 ; preparations: Hematoxilin and eosin sagittal section 5-6; disposition: in collection; occurrenceID: B76C44F2-0A36-592D-A1CB-39130377CFFB; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Type status: Other material. Occurrence: catalogNumber: CBUMAG: PLA: 00639-6 ; preparations: Hematoxilin and eosin sagittal section 6-6; disposition: in collection; occurrenceID: 06F6E257-8876-5D73-891C-B2F8DE3B30C1; Taxon: scientificName: Gnesioceros sargassicola ; taxonRank: species; Location: country: Colombia; decimalLatitude: 10.13538174; decimalLongitude: - 76.00058405; Record Level: institutionID: 891.780. 111 - 8; collectionID: RNC: 207; institutionCode: Universidad del Magdalena (UniMagdalena); collectionCode: CBUMAG; basisOfRecord: PreservedSpecimen

Distribution

Gnesioceros sargassicola was first described by Mertens (1832) from specimens found on floating Sargassum in the Sargasso Sea. It has been recorded on Sargassum mats in the Gulf of Mexico, the Caribbean Sea and the Sargasso Sea ( Plehn 1896, Hyman 1939 b, Faubel 1983, Faubel 1984, van Tussenbroek et al. 2024 a, van Tussenbroek et al. 2024 b) and in the western and central regions of the North Atlantic, in Bermuda ( Hyman 1954), Boa Vista in Cape Verde ( Laidlaw 1906) and off the coast of West Africa ( Moseley 1877).

Additionally, G. sargassicola has been found in littoral environments, including Devil's Foot Island, Quisset Harbor and Florida ( Hyman 1939 a, du Bois-Reymond Marcus and Marcus 1968), as well as in the Cayman Islands ( Prudhoe 1944), Curaçao, Bonaire, Puerto Rico and Saint Barthelemy ( du Bois-Reymond Marcus and Marcus 1968). More recently, it has been reported from Santa Marta, Colombia ( Quiroga et al. 2004) and in the Canary Islands ( Cuadrado et al. 2021). Here, we report for the first time the presence of Gnesioceros sargassicola in floating Sargassum mats off the central Caribbean coast of Colombia.

Taxon discussion

Our specimens align well with the morphological descriptions provided by Hyman (1939 b), du Bois-Reymond Marcus and Marcus (1968). As noted by du Bois-Reymond Marcus and Marcus (1968), our specimens are relatively small in comparison to the littoral specimens reported elsewhere. Hyman (1939 b) also described G. sargassicola var. lata Hyman, 1939 from seaweed roots in Bermuda, characterised by a broad anterior that quickly tapers to the posterior end; however, we did not find any specimen that shared this description. Remarkably, we also observed the " club-shaped " object attached to the female genital pore, as described by Hyman (1939 b), in several specimens. We found between one and three of these objects in mature specimens. These structures are visible in both our whole mounts (Fig. 3 View Figure 3 A and C) and sagittal sections (Fig. 3 View Figure 3 D and E). They appear as elongated structures with a cuticular envelope resembling a honeycomb. While no internal structures or organs were observable in the histological sections, basophilic cells can be identified (Fig. 3 View Figure 3 C).

Hyman (1939 b) suggested that the function of these structures may be related to reproduction, possibly as a secretion from the musculo-glandular ring in the female antrum. Our observations indicate that the cells inside these structures are spermatozoa, suggesting they might correspond to " spermatophores ". This supports the findings of du Bois-Reymond Marcus and Marcus (1968), who reported a similar structure in one of their specimens from Curaçao. These " spermatophores " are likely deposited during copulation; however, to accurately define their nature, further research on the reproductive biology and mating behaviour of this species is necessary. Prudhoe (1985) referreed to these as " true " spermatophores because they are attached to the female pore, in contrast to similar-looking structures left behind in cases of hypodermic insemination in other polyclad species, which have also been called spermatophores. To our knowledge, no other true spermatophores have been reported in polyclads.

Faubel (1983) description of G. sargassicola provided a more comprehensive account of the reproductive structures of the species than previous works. However, a misinterpretation led him and others to believe that the Lang's vesicle was roundish. While the original description by Mertens (1832) is unclear on this point, Graff (1893) refers to it as an accessory vesicle in his work and incorporated it into his illustrations. However, due to the lack of clarity in Graff's figures, Hyman (1939 b) subsequently demonstrated the transverse position of the Lang's vesicle. Our own observations of whole mounts (Fig. 3 View Figure 3 C) further corroborate the crescent-shape of Lang's vesicle. This feature supports the idea that its shape may serve as a valid taxonomic character within the family Gnesiocerotidae Marcus & Marcus, 1966 , with the exception of Comoplana agilis (Lang, 1884) (although see Oya and Kajihara (2020) for a more comprehensive discussion on the family's taxonomic status).

To better illustrate the relevant taxonomic characters of the species, we provide colour photographs of whole mounts and histological sections (Fig. 3 View Figure 3 ). Additionally, we present the first genetic sequence of G. sargassicola from pelagic Sargassum .