CRUSTACEAN RESEARCH，NO .31: 39-46，2002 Interna structure and nucleus formation in Sacculinαpolygeneα( C r u s t a c e a : Cirripedia: Rhizocephala: Sacculinidae) Jorgen Lutzen Abstract.-The root system and body and in sacculinids it is borne ven- nucleus formation in the colonial rhizo- trally on the abdomen of the host crab; cephalan Sα cculinαpolygeneαL u t z e n & it is solely concerned with sexual repro- Takahashi has been reinvestigated and duction.The externa (or externae in found to correspond to those of other re- asexually reproducing species) is/are lated species. A conflicting opinion of connected to the parasite's trophic org但し these subjects recently proposed by Rus- the interna or root system，which is 10- sian zoologists is due to the fact that cated within the host abdomen and these authors have mistaken anumber of cephalothorax.Primordial externae arise host structures for pa此 s of the parasite's as so-called nuclei that are part of the root system. root system and are located in the crab's abdomen，although they may start their 1ntroduction development in the cephalothorax. A nucleus is located within atumor ，an ex- 1saeva et al.(1999 ，2001) and Shukal戸lk pansion of the root system with an interior & 1saeva (2000) recently reported on the consisting of aloose tissue of widely organization of the root system and the separated stellate cells. mode of asexual reproduction in the north- ， west Pacific sacculinid Sα cculina pol)伊 nea Lutzen &Takahashi ，a parasite of the Materials and Methods littoral crab Hemigrapsus sα nguineus As new material was not available，1 (de Haan).Al though they described the reexamined part of myearlier section late development of the primordial series of Hemigrapsus sα nguineus para- externae correctly，their description of sitized by S .polygene α.The processing of the early development differs signific 田 lt- this material was described by Takahashi ly and fundamentally from results pre- & Lutzen (1998). It comprises 10μm thick sented earlier by Takahashi & Lutzen continuous Paraplast (R) serial sections of (1998)，組d this has prompted the present the abdomens and part of the cephalotho- reinvestigation of myearlier published rax of 10 sacculinized crabs and afew material. non-sacculinized crabs.Because of the The generallife cycle of rhizocepha- considerable thickness of the sections，the lans has been summarized by Hoeg & images are not optimal. 1n addition，1 Lutzen (1985，1995 ，1996) .The complete studied afew slides of cross-sections of life cycle is only known for asingle species ， abdomens of H .sanguineus parasitized by the sacculinid Loxothylαcus pαnopαei S .polygene αt h a t were kindly placed at (Gissler) (see Glenner，2001) .Terms used mydisposal by Dr.Valeria 1saeva，1nsti- in the following text are explained as fol- tute of Marine Biology，Russian Academy lows. The externa is the reproductive of Sciences，Vl adivostok，Russia. 40 J.LUTZEN Results centrally in the visceral mass (Fig.1・ 8) Figure 1presents aseries ofnuclei and Figures 2-4 illustrate structures that tumors arranged to illustrate the growth are relevant for the Discussion. An expan- of the early nucleus from adiameter of cα. sion of the root system is always present 20μm to almost 50μm and the accompa- beneath the stalk of the externa (Fig. 2) . nying expansion and change in shape of Within the cephalothorax of parasitized the tumor.Th eillustrated stages are each crabs numerous rootlets lie between the chosen from among cα. 300 examined diverticula of the host's digestive gland stages of the same size class (仕 omten sec- (Fig. 3). Rootlets are particularly abun司 tioned crabs) .In the present study，which dant in the first segments of the abdomen aimed to document the development of and may even invade acluster of sub- the earliest nuclei，it was deemed un- gastrodermal glands，which occur in a necessary to present larger (and older) zone circumscribing the boundary be- tumors/nuclei. Subsequent development tween the crab's midgut and hindgut takes exactly the same course as in other (Fig. 4). The glands，which in the illus- species of three genera of sacculinids trated caSe are only super五cially invaded (Delage，1884: Sacculinα; Smith，1906: by rootlets ，consist of many small acini Drepαnorchis; George，1959: Heterosaccus). composed ofrelatively few large cells with The smallest recorded nuclei (15-25 darkly staining cytoplasm and peripher- μm ) and tumors always occurred at the ally placed nuclei . tips of rootlets，which as aresult had of- ten swollen into small buds (Fig.1 ・1，2，3). Discussion Accompanying bud-formation，the cuticu- The material used by the Russian au- larized epidermis surrounding these thors differs from myown only in that small tumors typically increased in theirs was collected in Vostok Bay，Siberia ， height and stainability，a fact which con- Sea of Japan，while my specimens are siderably helps in the detection of these from the Amakusa Islands，Kyushu ，Ja- small stages.In favorable sections ，the pan. In both cases the host crabs were H. nucleus' small prospective mantle cavity sα nguineus collected at the shore. was often seen to have fused with the in- side of the bud's epidermis (Fig.1・ 2) . The orgαnLZαtion of the root system In later stages the tumors expand pro- The generallayout of the root system portionally much more than the nuclei. in sacculinids is very simple (Delage， Nuclei ofcα.50μm in diameter often have 1884; Smith，1906; I仕uger，1940; Hoshino， tumors with dimensions of 250 x300μm 1965). Beneath the point where the stalk (Fig.1・8). In all growth stages，the nucle- of the externa penetrates the integument us is always positioned superficially of the host，the root system has asac- within the tumor. Opposite the nucleus， shaped enlargement，from which alarge larger tumors usually extend into many number of rootlets issue .By branching smaller or larger rootlets . profusely，the rootlets penetrate in be- Even the smallest buds contained a tween many organs of the host.An organ minute vesicle associated with asmall ， that is especially heavily infiltrated by compact body，which represent the earli- the rootlets is the digestive gland (hepato・ est rudiments of the mantle cavity and pancreas)，but the rootlets never enter the visceral mass，respectively .In nuclei of cα. tubular diverticula ofthe gland.The epi- 50μm in diameter or larger，it was pos- dermis ofthe rootlets consists of asimple ， sible to recognize acuticular lining in the cuticle-producing epithelium，which is of- mantle cavity and arudimentary ovary ten associated with asubjacent layer of INTERNA STRUCTURE IN SACCULINA POLYGENEA 41 日g. 1. Sacculinαpoか'genea.Eight successive stages (1-8) in the development and expansion of nuclei and tumors.ct ，contact between the epithelia of mantle ca吋ty and bud; ep，合 ontal epithe- lium ofbud，exhibiting increased height and"stainability; mc，prospective mantle cavity lined by cuticle (cu); nu，nucleus; tu，tumor; vm，prospective visceral mass. 10μm thick paraplast sections stained with hematoxylin and eosin. Scale bar 50μm. 42 J.LUTZEN two distinct systems:n 田 TOWdistal rootlets and much larger transport trunks (or transport canals). They believed that the trunks connected the externae and/or their primordia with the rootlets. Although they did not present any histolo回cal sec- tions of rootlets issuing from the trunks， they did provide cross-sections of the lat- ter (Shukalyuk & Isaeva，2000 ，fig. 1d; Isaevaetαl. ，2001 ，fig .2E). In these figures the diameter of the trunks (cα.125-200 μm ) is seen to exceed that of the rootlets by several times. However，it is evident that the authors mistook the tubular diverticula of the host's digestive gland with what they construed to be transport trunks. According to their description，the lining of the purported trunks is made up of alongitudinally folded epithelium，but this is exactly characteristic of the diges- tive gland diverticula.The diameter and average wall thickness of the diverticula in H .sα nguineus (Fig. 3) also matches that ofthe “trunks". Furthermore，a careful examination by me of several sectioned externae in situ has shown that although the hemocoelic canal penetrating the stalk of the externa expands to abasin Fig. 2. Sαcculina polygeneα. Expansion of underneath the stalk basis，the rootlets the root system (ex) underneath the stalk of issue directly 企omthis expansion (Fig. 2) ， the externa and ramification of rootlets (r) as has been reported also in other species . 企omit; the expansion is continuous with the Even if it rnay give off afew larger roots as hemocoelic canal (hc) running through the well they are always short and their stalk. 10μm thick paraplast section stained ， with hematoxylin and eosin .Scale bar 100μm. walls are much thinner than the “trans- po吋 trunks"，and smooth，never folded.A single diverticulum and many rootlets were illustrated side by side in cross- axial cells (Bresciani & Hoeg，2001) .The section by Hoshino (1962:fig .2 ，p l. 1，fig.1; s仕ucture of the root sys旬 m and the histol- Pα chygrα:psus crαssipes Randall parasit- ogy of the roots of Sacculinαpolygeneαa s ized by S. confrαgosαB o s c h m a )and by reported by Takahashi &Lutzen (1998)， Takahashi & Lutzen (1998: 日g.5; present and those of S .plan αB o s c h m a reported species). by Liu & Lutzen (2000)，agree with this Figure 5(center and right frame) in description (Fig. 2). Isaeva et al .(2001) sums up their interpre- Shukalyuk & Isaeva (2000) and Isaeva tation of the root system in S .polygene α. et αl. (2001) arrived at quite another It falsely shows how the rootlets fuse with conclusion when investigating the mor- the “transport trunks"，which is ，of course， phology of the root system in S .polygenea . impossible.The error becomes all the They reported it to be differentiated into more incomprehensible in light ofthe fact INTERNA STRUCTURE IN SACCULINA POLYGENEA 43 that the digestive gland diverticula in H .