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Tantulocaridans are a group of small crustaceans characterized by being exclusive ectoparasites of other crustaceans such as copepods, ostracods, and peracarids. The life cycle includes an infective parasitic larva (or tantulus larva), which after establishing itself in the host’s cuticle, undergoes an elaborate metamorphosis, ending with the development of either an adult sexual individual or parthenogenetic females (Huys et al., 1993; Arbizu and Petrunina, 2018; Petrunina and Huys, 2020).
Tantulocaridans have been found from the intertidal zone to beyond 9,000 m deep, including worldwide tropical and polar waters, and they are part of the temporal meiofauna and meroplankton (Boxshall and Huys, 1989; Petrunina and Kolbasov, 2009; Petrunina and Huys, 2020). Most of the records have been reported in the Pacific and Atlantic oceans, especially in high latitudes and the deep sea (Mohrbeck et al., 2010). There are currently 39 species and 23 genera, grouped into five families (Petrunina and Huys, 2020), although recent studies indicate that the current number of described species underestimates the group’s real richness (Arbizu and Petrunina, 2018, Petrunina and Huys, 2020). Regarding their phylogeny, studies of the adult genital openings and 18S rDNA sequence data place tantulocarids as close relatives of thecostracans (Boxshall and Lincoln, 1987; Huys et al., 1993; Petrunina et al., 2013). Currently, there are no scientific publications that record the presence of the order in the Caribbean Sea (Petrunina and Huys, 2020).
The organisms come from sediment samples collected in two offshore research cruises, at more than 2,000 m deep, carried out by the Coordination of Scientific Services (CSC) of the Institute for Marine and Coastal Research - Invemar, within the framework of projects of hydrocarbon exploration in the Colombian Caribbean Sea (Block COL 3, located between the departments of Atlántico and Magdalena), in June 2015 and May 2018 (Figure 1). The sediment was collected using a 0.25 m2 sampling area Box-corer and sieved with a 300 µm pore mesh. To the retained material, a narcotic solution of analytical magnesium chloride (70 g per liter ), 10 % formaldehyde (preservative), and Bengal Rose 1 g per liter (pigment) was added, according to the recommendations of Mason and Yevich (1967); Holme and Mcintyre (1971) and Baguley et al. (2006)).
Figure 1 Collection sites for specimens within the COL 3 hydrocarbon exploration block in the Colombian Caribbean Sea.
The samples were reviewed using an optical microscope and stereoscope Zeiss with an integrated camera, with maximum magnification of 600X. Within the collected macrofauna, a cumacean (2015 sampling) and two tanaidaceans (2018 sampling) were found being parasitized by Tantulocarida. Schemes of the parasites and hosts were made to facilitate the visualization of the main structures used for taxonomic identification. Body size was measured from the cephalon’s anterior margin to the posterior margin of the expanded trunk.
The collected individuals are in process of being deposited within the Makuriwa collection of Invemar. The next works were used as a reference for tantulocaridans identification: Boxshall and Lincoln 1983 and 1987, Huys et al. (1993), Huys and Conroy-Dalton (1997), Ohtsuka (1997), Ohtsuka and Boxshall (1998), Kornev et al. (2004), Knudsen et al. (2009), Kolbasov and Petrunina (2010), Mohrbeck et al. (2010), Petrunina et al. (2013), Huys et al. (2014), and Petrunina and Huys (2020). For the identification of host peracarid crustaceans: Petrescu (1991), Blake and Scott (1997), Larsen and Heard (2004), Roccatagliata (2004), Mühlenhardt-Siegel (2005 and 2011), and Drumm and Bird (2016). The nomenclature system used for the descriptions was that of Huys and Boxshall (1988).
Description of the specimens collected
Specimen 1. Early stage male parasitizing a cumacean (Cumacea: Leuconidae) of genus Eudorella (Figure 2). Depth: 2,786 m. Coordinates: 11°48’3.606” N; 74°44’37.634” W, June 2015.
Figure 2 Eudorella sp. (Cumacea) lateral view, with tantulocaridan attached to the lateral margin groove of cephalon (arrow)
Corporal length around 292 µm; the body is divided into cephalon, thorax, six pedigerous somites, and urosome without appendages.
The cephalon lacks cephalic appendages, a characteristic that is shared by the entire class (Petrunina and Huys, 2020). The oral disc (do) is seen in the anteroventral position (Figure 3) and is the site of attachment between the parasite and the host; the lateral surface of the cephalon is ornamented by three longitudinal lamellae that extend from the beginning of the oral disc to the middle of the cephalon (Figure 3, bottom). The cephalic shield is long (twice the width) reaching the edge of the thorax, and it represents 19 % of the total length. The cephalic stylet (e) is the structure by which the parasite pierces the integument of the host (Boxshall and Lincoln, 1987). In this case, it is very thin and delicate, almost straight (Figure 3, bottom). The cephalic pores are not visible with a light microscope.
Figure 3 Early stage male, lateral view; ab: abdomen; cu: umbilical cord; do: oral disc; e: stylet; ec: cephalic shield; LI-LIII: lamellae; ter: thoracic tergites. Developing sexual adult (♂) and umbilical cord drawn in dotted lines in image below.
Thorax with six short longitudinal ridges-like dorsal tergites, 5 and 6 tergites shorter than the others (Fig. 3, bottom). The entire thorax is expanded to allow the growth of a sexual tantulocaridan, which receives nutrients from the host through the umbilical cord (cu), which is robust and bifurcated. This cord connects with the cephalic area of the developing sexual tantulocaridan (Figure 3, upper and lower), which has thoracopods with setae, a characteristic only seen in males (Huys et al., 2014; Arbizu and Pertrunina, 2018).
The larva has six pairs of thoracopods; the first five are biramous, and the sixth is uniramous. No more detailed structures are distinguishable under a light microscope. The urosome consists of the last thoracic segment, which is very short, plus a long non-segmented abdomen (Figure 3, superior and inferior). The abdomen longer than wide and ends in four lamellae; the posterior ventral margin ornamented with a pair of thorn-like processes. Only the tantulus larvae that develop a male retains the thoracopods and the urosome, while the larvae that develop sexual and parthenogenetic females lose them (Huys et al., 2014).
Specimen 2. Early female stage parasitizing a Tanaidacea: Tanaellidae of the genus Tanaella. Depth: 2,420 m. Coordinates: 11°38’20.614” N; 75°1’51.737” W, May 2018.
Specimen length around 196 µm, attached to the propodus of the fourth right leg (Figures 4 and 5). Its body has the typical tantulocaridan segmentation described in figure 2, with the exception that neither pedigerous somites nor urosome are seen, characteristic of organisms that carry sexual or parthenogenetic females. Figures 4 and 5 show the expanded trunk and inside a mass of tissue with tagmata, suggesting the development of a sexual female.
Figure 4 Host tanaidacean Tanaella sp. (♀) lateral view, with tantulocaridan attached to the fourth pereopod, right side (arrow).
Specimens 3 and 4. Two early stage females parasitizing a tanaidacean (Tanaidacea: Tanaellidae) of the genus Tanaella. Depth: 2,387 m. Coordinates: 11°38’0.205” N; 75°2’13.108” W, May 2018.
Tantulocaridans length between 250 and 260 µm, attached to the propodus of the fifth and sixth pereiopods on the left side. Neither pedigerous somites nor urosome are seen, but it can be distinguished the development of tagmata within both individuals, suggesting the development of sexual females (Figure 6). Due to its small size, the recognition of other diagnostic structures was not possible.
COMMENTS
Although tantulocaridans can use a wide range of crustaceans as host species, reports of host cumaceans and tanaidaceans are scarce. So far, only two species are known to parasitize cumaceans (both form the family Cumoniscidae), and five species that parasitize tanaidaceans (families Microdajidae and Onceroxenidae) (Petrunina and Huys, 2020). Additionally, and according to the available information, tantulocaridans present a high level of specificity with the host (ibid.), which it means it is very likely that the parasites reported here belong to two different species. However, there are difficulties related to the identification, which is based mainly on the morphology of the thoracopods, the body tagmosis and the ornamentation of the different tagmata (Huys, 1991 and Mohrbeck et al., 2010), only possible with the use of electron microscopy.
This is the first time that these parasites have been recorded in macrofaunal crustaceans in the Caribbean Sea. One of the possible causes is that, based on personal experience, the screening carried out to separate the benthic fauna from the sediment detaches the parasites from the hosts and, due to their small size, they are not retained in the sieves. The use of finer sieves can favor the collection of these individuals in future samplings.
