Data on three Baltic species of Corynosoma Lühe, 1905 (Acanthocephala: Polymorphidae) from Baltic grey (Halichoerus grypus) and ringed seals (Pusa hispida)

We analyzed Baltic Corynosoma material (C. magdaleni Montreuil, 1958, C. semerme (Forssell, 1904) Lühe, 1911 and C. strumosum (Rudolphi, 1802) Lühe, 1904) from grey (Halichoerus grypus) and ringed seals (Pusa hispida) for the variation of hook morphology and for finding possible morphotypes, by using the proboscis profiler (Wayland 2010) and Meristogram (Wayland 2016).

Methods
Methods:

All data analysis was performed using the R language and environment (R Core Team 2019). For species names we followed the nomenclature given by the Catalogue of Life (2019 Annual Checklist: Roskov et al. 2019) and WoRMS (WoRMS Editorial Board 2020).

The studied seals were collected during Swedish grey and ringed seal hunting in 2007-2008 in different parts of the Baltic Sea (Table 1). The thorny-headed worms were collected during the autopsy at the Swedish Museum of Natural History (for more details see Leidenberger et al. 2019). The worms have been preserved in 70% alcohol. Voucher material is deposited in the invertebrate collection of the Department of Zoology, Swedish Museum of Natural History (SMNH), Stockholm, Sweden (# SMNH-162973-162978, SMNH-172304-172310).

Additionally, specimens of C. magdaleni collected by Nickol et al. (2002) from the Finish Baltic Sea, were borrowed from the University of Nebraska State Museum, Manter Laboratory of Parasitology (# HWML 39480). Here, we tested the material mentioned in Table 1 for intra-taxon and inter-taxon variation in hook morphology by using the Proboscis profiler (Wayland 2010).

We measured the length and base measurements of each hook in at least one longitudinal row of hooks per specimen (for more information see Wayland 2010). In total, 11 specimens (3 females and 8 males) of C. semerme were in a suitable condition for measuring hook morphometrics. Dorsal hook rows were measured in two female specimens (the ventral hooks were obscured by debris or damaged). In each of the male specimens only one single row was visible in profile; six of these rows were dorsal, one lateral and the other ventral. Additionally, hook morphometrics, for both a dorsal and a ventral longitudinal row of hooks, were recorded from a single male specimen of C. magdaleni (HWML 39480-1041-3).

The proboscis profiler and meristogram analyse serial variation in hook morphology along longitudinal rows of hooks. An acanthocephalan specimen can only be analysed using these methods if it has at least one longitudinal row of hooks visible in profile. The R package FactoMineR (Lê et al. 2008) was used to conduct principal component analysis (PCA).

The proboscis profiles of C. semerme did not show any evidence of sexual dimorphism (Figs 1, 2). Males of C. semerme and C. magdaleni appear to display similar patterns of serial variation in hook length and base dimensions (Fig. 1). A principal component analysis of male proboscis profiles demonstrated more variation within C. semerme, than between this taxon and C. magdaleni (Fig. 2).

The meristogram (Huffman and Bullock 1975, Wayland 2016) was used to produce a graphical summary of serial variation in hook morphometrics. A meristogram (Huffman and Bullock 1975, Wayland 2016) was generated to provide a graphical summary of serial variation in hook morphology in C. semerme (Fig. 3); insufficient data was available for the other two taxa. Maximum values of all four hook morphometrics (length, base, ratio and area) were found approximately half way along the proboscis.

Comparison of the number of longitudinal rows and number of hooks in each row is known to vary in Corynosoma species depending on the region and host. Cryptic speciation is known as one reason for morphological variability in Acanthocephalans observed within and between geographical locations (e.g. Waindok et al. 2018, Steinauer et al. 2007, Väinölä et al. 1994, Popov and Fortunato 1987). For example Popov and Fortunato (1987) could, on the bases of only the number of longitudinal rows and number of hooks in each row, separate two clear populations of C. strumosum in ringed seals, namely the western-arctic (Barents and East Siberian Sea) and western-pacific one (Bering and Okhotsk Sea). Our analyzed material consisted of too few individuals to find any morphotypes, but by comparing the literature there are numerous variations in hook arrangement described. Plastic morphometric characters are also known between different host species. Hernández-Orts et al. (2017) observed for C. hannae Zdzitowiecki, 1984 intraspecific morphological and biometrical variability depending on the host. A study by Skorobrechova and Nikishin (2014) showed that also the structure of the capsule of C. strumosum was regarded as a morphological expression of the degree of balance of the host-parasite system and was determined by the adaptation to the given paratenic host.

In the majority of acanthocephalans, the proboscis is the primary organ of attachment to the intestinal wall of the definitive host and therefore adaptive radiation is expressed in divergence of proboscis morphology, including hook patterns. However, in Corynosoma both the trunk and proboscis anchor the acanthocephalan to the wall of the intestine (Aznar et al. 1999). We might speculate that adaptation to different regions of the digestive tract might be accompanied by changes in trunk shape and spination. Spine coverage may be linked to trunk shape (Hernández-Orts et al. 2012), but further analyses are needed here.

This meristogram, as well as the row data (available as Suppl. material 1-2) may serve as a useful reference for comparison with further Corynosoma specimens and/or other Corynosoma species in the future. We see great challenges in the access to good and sufficient material for further analysis, while our study of the hooks might contribute and motivate to further investigations on this interesting parasite group. Leidenberger et al. (2020) presented an extensive literature summary on host records and geographical distribution of the three Baltic Corynosoma species, which can be used as guide for further sampling efforts.