Diploproctodaeum arothroni Bray and Nahhas, 1998

Hassanine, Reda M. El-S., 2006, The life cycle of Diploproctodaeum arothroni Bray and Nahhas, 1998 (Digenea: Lepocreadiidae), with a comment on the parasitic castration of its molluscan intermediate host, Journal of Natural History 40 (19 - 20), pp. 1211-1222 : 1213-1217

Diploproctodaeum arothroni Bray and Nahhas, 1998

( Figures 1 , 2 )

Final host. Arothron hispidus Linnaeus (Tetraodontidae) . Site: intestine.

Intermediate host. Crassostrea cuccullata Born ( Bivalvia: Ostreidae ). Site: gonads and digestive gland.

Locality. Mangrove swamps on the Egyptian coast of the Gulf of Aqaba.

Material. Voucher specimens are deposited in the Helminthological Collection of the Red Sea Fishes, Marine Science Department, Faculty of Science , Suez Canal University, Ismailia, Egypt .

Material examined. The holotype specimen in the Natural History Museum, London, reg. no. 1997.8.29.1.

Egg Eggs oval, thin-walled, non-operculated, 58–64×30–35. As they do not hatch in seawater,

infection must be accomplished by the ingestion of eggs and the emergence of miracidia in the digestive tract of C. cuccullata . This was confirmed when eggs of the same shape and size were observed in the digestive tracts of many infected individuals of this oyster; also, some egg shells were observed in their faeces. Thus, there is most probably no freeswimming miracidium in the life-cycle of D. arothroni .

Larval forms (from the oyster gonad)

Mother sporocyst ( Figure 1A, B ). Young mother sporocyst (1–2 weeks after infection) (based on 40 specimens) ( Figure 1A ): body oval, thin-walled, opaque white, 760–910×610–712 (835×661). Germinal balls aggregated in round compact mass in one half of body; the other half appeared to be transparent.

Mature mother sporocyst (3–5 weeks after infection) (based on 40 specimens) ( Figure 1B ): body sausage-shaped, slightly swollen anteriorly, thin-walled, opaque white, 2003– 2697×397–483 (2350×440). Germinal balls aggregated in persistent compact band extending throughout anterior third of body; some balls move posteriorly and give rise to rediae (usually 8–13 per sporocyst), which are usually at different stages of development; largest 250×90, smallest 110×61. Lateral birth-pore observed at 624–693 (659) from anterior end. No germinal balls observed in mature sporocysts before and during the gametogenic activity of the oyster, but some were persistent after this period. The walls of the sporocyst are capable of contraction and distension, but no movement was observed.

Daughter redia ( Figure 1C, D ). Young daughter redia (6–8 weeks after infection) (based on 40 specimens) ( Figure 1C ): body elongate, cylindrical, 2104–2570 (2337) in length, 270– 337 (304) in width at its middle and characterized by presence of distinct lateral projection at 250–302 (276) from anterior extremity in place where the birth pore is situated in mature redia. Mouth antero-terminal. Pharynx small, muscular, pyriform, 100–121×71–88 (80), connected directly to small saccular caecum extending posteriorly to near lateral projection and filled with granular material which is probably from gonads of oyster. Few spheroidal germinal balls usually present at anterior end of body. Developing cercariae 18–24 in number and usually crowded together in anterior half of body.

Fully mature daughter redia (9–11 weeks after infection) (based on 40 specimens) ( Figure 1D ): body vermiform, usually curved, 2721–3350 (3036) in length, 406–516 (461) in maximum width at level of birth pore. The latter 719–870 (795) from anterior end and easily seen during emergence of cercariae. Mouth antero-terminal. Pharynx pyriform, 105– 133×73–89 (119×81), connected directly to small sub-triangular caecum extending posteriorly to midway between anterior end and birth pore. Germinal balls completely absent. Cercariae 18–25 in number, at different stages of development, usually crowded together in anterior half of body.

Cercaria ( Figure 1E ). After expulsion from redia, cercaria remains for about 2 weeks within the gonad of oyster before their emergence, which occurs at night.

Fully mature cercaria (emerges from oyster 14–15 weeks after infection) (based on 40 specimens): trichocercous cercaria. Body oblong, 257–299×190–222 (278×206). Tegument covered with minute sharp spines. Tail plumose, broad, supported by a medial tubular structure, moderately long, 624–714 (699) in length, bears 22 sharply pointed setae on each side which decrease in size posteriorly; first 10 setae, 88–94 (91); next four, 78–85 (82); next two, 70–73 (72); next two, 55–61 (58); last two, 22–30 (26) in length. Oral sucker sub-terminal, oval, 37–43×45–54 (40×50). Ventral sucker oval, sessile, situated in middle of body, smaller than oral sucker, 27–31×30–38 (29×34). Sucker-width ratio 1:0.68–0.75. Prepharynx practically absent. Pharynx moderately large, with wavy anterior border, 22–27×30–35 (25×33). Oesophagus absent. Intestinal caeca relatively wide, curved, extend backwards to abut posterior body-wall, may give appearance of having ani. Cystogenous glands numerous, relatively large, round, scattered throughout body, negative to neutral red. Penetration glands absent. Testes two, oval, symmetrical, near posterior extremity, sub-equal, 50–58×41–47 (54×44). Anlagen of ovary median, just pre-testicular. Genital ducts not visible. Excretory vesicle tubulo-saccular, short, lined by granular cells, extends anteriorly as far as rudiments of ovary. Main excretory tubules join antero-lateral margins of excretory vesicle and extend anteriorly to mid-level of ventral sucker. Flame-cell formula 2[(3+3)+(3+3)]524. Excretory pore postero-terminal.

Metacercaria ( Figure 1F ). Formation of metacercaria was observed in petri dish filled with seawater. Period of the free-swimming cercaria in seawater is very brief and transitory; emerged cercariae appeared to be at rest for only a few seconds, and then moved rapidly in clockwise, helical motion near the bottom of the dish. On contact with substratum (e.g. pieces of filamentous algae and other aquatic vegetation placed in dish), the cercaria spreads setae of its tail in different directions, and begins series of writhing movements which result in detachment of tail, causing emission of secretions from the cystogenous glands. This colourless transparent material forms a flexible membranous cyst around the metacercaria; some of the cyst material is anchored to the substratum. This is probably similar to what happens in nature, since the cercaria has no penetration glands to penetrate a second intermediate host. Metacercariae remain alive for about 1 week and no progenesis was observed during this period. Generally, it is closely similar to cercaria in all characteristics but differs in having a widely pyriform body and excretory vesicle filled with relatively large granules. Because the lagoon bottom is very muddy, metacercaria were not seen on the aquatic vegetation, which is usually covered by a layer of mud. Four-day-old metacercariae encysted on filamentous algae fed to uninfected A. hispidus developed into fully gravid worms 8–9 weeks after infection. Thus, the complete life cycle of D. arothroni extends over about 24 weeks.

Adult ( Figure 2 )

(Based on 20 fully mature specimens.) Body thick, distinctly scoop-shaped, 1968–2952 (2460) long, 1061–1570 (1315) wide at level of ventral sucker. ‘‘Scoop’’ large, mainly glandular, sub-circular, complete posteriorly, 1023–1680 (1352) long, representing 51–57 (54)% of body length. Forebody 708–1140 (924), representing 36–38 (37)% of body length. Tegument contains minute, closely set spines which decrease in size and number posteriorly.

Pre-oral lobe 139–208 (174). Oral sucker well developed, sub-terminal, round, 260– 390×288–435 (325×362). Ventral sucker sub-spherical, just pre-equatorial, 204–282 in diameter. Sucker-width ratio 1:0.64–0.71.

Prepharynx extremely short. Pharynx fairly large, strongly muscular, scallop-shaped due to the presence of five low protuberances on its anterior margin, 204–306×232–370 (255×301). Intestinal bifurcation about midway between suckers; intestinal caeca wide, extend backwards to abut posterior end open with separate ani.

Testes two, oval, contiguous, obliquely arranged midway between ventral sucker and posterior end, sub-equal, 260–410×204–306 (255×335). Post-testicular region 288–472 (380) long, representing 14–16% (15%) of body length. External seminal vesicle curved sinistrally, sub-median, in anterior region of second half of body. Cirrus-sac thick-walled, 464–702×105–135 (583×120), sinistrally sub-median, extends from short distance posterior to ventral sucker to near pharynx, contains funnel-shaped internal seminal vesicle, well-developed prostatic complex and a relatively long ejaculatory duct. Genital pore sinistrally sub-median, just posterior to level of pharynx.

Ovary follicular, median, consisting of 46–57 compact follicles situated between posterior testis and uterus. Seminal receptacle oval, antero-lateral to posterior testis. Uterus pre-ovarian, inter-caecal, moderately long. Distal uterine coil highly muscular, modified to form distinct metraterm. Vitelline follicles numerous, small, extending in lateral fields from posterior end of body to pharyngeal level, confluent in post-testicular region. Eggs oval, thin-shelled, non-operculated, numerous, moderately large, 58–62×30– 36 (60×33).

Excretory vesicle I-shaped, extends anteriorly as far as to ovary; excretory pore terminal.

Figures

Figure 1. Stages in the life cycle of Diploproctodaeum arothroni Bray and Nahhas, 1998. (A) Young mother sporocyst; (B) mature mother sporocyst; (C) young daughter redia; (D) fully mature daughter redia; (E) fully mature cercaria; (F) metacercaria. Scale bars: 500 Mm (A–D); 200 Mm (E, F).

Figure 2. Fully gravid worm of Diploproctodaeum arothroni Bray and Nahhas, 1998. Scale bar: 500 Mm.