Europ. J. Protisto!' 31, 54-57 (1995)March 15, 1995

European Journal of

PROTISTOLOGY

A Stalkless Sub-species of Clathrulina elegansCienkowski (1867) from the Plankton of Loch Ness,Scotland

Judith Young, Johanna Laybourn-Parry*, Roger I. Jones, andMark WaltonInstitute of Environmental and Biological Sciences, University of Lancaster;Lancaster; UK

SUMMARY

During the course of a detailed investigation of the plankton dynamics of Loch Ness,Scotland, we have discovered a stalkless form of Clathrulina living in the plankton. Thisheliozoon was established in culture and its life-cycle events observed. Though broadlysimilar to the sequence of events described by Bardele [4] for Clathrulina elegans, it differsin respect of lacking a stalk and in the number of fission products produced. We proposethat the planktonic form of the species living in the pelagic zone of Loch Ness be regardedas a sub-species, Clathrulina elegans var. planktonicum.

Introduction

The Desmothoracida is an order of freshwater or ma­rine Heliozoea whose members are typified by the pos­session of a scleroprotein test or capsule and, in mostcases, a stalk [6,4]. Reproduction involves the produc­tion of amoeboid swarmers which transform intobiflagellate swarmers. These subsequently metamor­phose into naked heliozoa which go on to secrete a cap­sule and to produce a stalk. The life-cycle ofClathrulina elegans Cienkowski (1876) [5] has beendescribed in detail by Bardele [4]. This species pos­sesses a hollow stalk which results from the produc­tion of a large pseudopod with hundreds ofmicrotubules that act as a template for the secretionof an organic fibrous sheath [3,4].

The ecology of heliozoa is poorly documented com­pared to other protists. Generally they prefer unpol­luted, well-oxygenated waters and occur from thetropics to the poles. While some can tolerate acidic con-

* Present Address: School of Zoology, Faculty of Scienceand Technology, La Trobe University, Bundoora, Victoria3083, Australia

0932-4739-95-0031-0054$3.50-0

ditions, most prefer a pH close to 8.5. Freshwater spe­cies in Northern temperate sites peak in abundance atthe end of autumn when organic residues are abundantand water temperatures are not too high. The majorityare sub-benthic or benthic [6, 7]. A perusal of literaturefrom the beginning of the century reveals that speci­mens of Clathrulina elegans lacking a stalk were re­ported from samples of benthic mud from Loch Nessby Penard [13]. C. elegans was reported again fromLoch Ness in a later study by Murray [14], althoughthe latter author did not note whether specimens hada stalk or not.

Here we describe a stalkless form of Clathrulina ele­gans which appears regularly in the plankton of LochNess each year between early autumn and late spring.We will argue that it should be regarded as a planktonicsub-species, namely Clathrulina elegans var. plankto­meum.

Methods

Loch Ness is the largest body of water in the United King­dom. The maximum depth is 230 m, its length is 39 km andits mean width is 1.45 km. Its waters are stained brown by

© 1995 by Gustav Fischer Verlag, Stuttgart

Stalkless Species of C. elegans . 55

Fig. 1a. The capsulated form of Clathrulina elegans var. planktonicum showing capsule form. Size bar =10 11m. - Fig. lb. Thecapsulated form of C. elegans var. planktonicum, showing the axopodia. Size bar = 10 11m. - Fig. 1c. A scanning EM of thecapsule. Size bar 10 11m. - Fig. 1d. The capsule containing two fission products. Size bar = 10 11m.

inputs of humic material from the catchment and this al­lochthonous input of carbon results in atypical patterns ofprotozooplankton abundance [12]. During a study of theplankton dynamics in loch Ness, the water column wassampled at regular intervals down to 100 m at fortnightly in­tervals during 1991-1993. Samples were fixed in Lugol's io­dine. Fresh material was also collected with a plankton net of30 11m mesh. On some occasions individuals of Clathrulinaelegans were picked by micropipette from net hauls in orderto initiate cultures in the laboratory. We were able to maintainClatbrulina elegans in culture for a few months in plastic tis­sue culture flasks fed on a diet of heterotrophic flagellates inChalkley's medium enriched with grass seed infusion. Cul­tures were incubated in the dark at 10-12 "C, The life-cycleevents were observed using a Zeiss ID03 or Zeiss IM35 in­verted microscope under phase and Nomarski light micro­scopy, with a magnification of between 100 x and 400 x .

For electron microscopy, specimens were fixed in glutaral­dehyde and osmium tetroxide. After drying they were sputter­coated with gold to a thickness of 100 nm and examined in aJEOl 840ASEM at an accelerating voltage of 15 kV.

Results and Discussion

The diameter of the stalkless form of Clathrulina ele­gans in the plankton of Loch Ness is between 50­60 11m, which is consistent with that reported forC. elegans by Bardele [4]. The details of the stalklessform are shown in Fig. la-c. None of the specimenswe have seen in the plankton had a stalk, nor didany of the cultures contain individuals with stalks,even after reproduction within the culture. The culturevessel offers a large area of surface for attachment, but

56 . J. Young, J. Laybourn-Parry, R. I. Jones, and M. Walton

opodia are withdrawn. One of the fission productssubsequently adopts amoeboid motion and emergesfrom the capsule, while the remaining fission productredevelops axopodia, Fig. 2. The emergence of theamoeboid swarmer through one of the openings inthe latticed capsule takes around 15 minutes. Whenit entirely emerges it adopts an ovoid form and twoflagella appear. In the culture flasks the flagellatedswarmer swims away from the parent and within threeminutes settles onto the anterior flagellated end of thecell and assumes a spherical shape. The flagella cease tomove and axopodia start to emerge, to produce a smallnaked heliozoon. Within five minutes of settling thefirst signs of capsule secretion are visible. The axopo­dia appear to act as a kind of scaffolding for the secre­tion of the capsule. The entire process from fission tothe beginning of capsule secretion takes around 30 min­utes at 20°C. Apart from the number of fission prod­ucts and the lack of stalk production, the eventsdescribed here are similar to those described by Bar­dele [4] for C. elegans. The reduced number of fissionproducts may have evolved in response to the energylimited planktonic environment. The stalked form ofC. elegans is typically found attached to vegetationin the littoral zone of water bodies (e.g., [8]), a habitatwhich offers a much more food rich environment thanthe plankton.

We have not observed cysts in our cultures. Bardele[4] noted that capsules contained up to eight cysts cov­ered by fine spines in older crowded cultures. Cien­kowski [5] also noted cysts from which single,apparently flagellated swarmers emerged. Undoubt­edly the cyst must playa role in the annual occurrenceof C. elegans var. planktonicum in Loch Ness, thoughwhere the cysts reside is unclear. In a deep lake withoutany significant littoral zone, like Loch Ness, they couldsimply remain suspended in the water column. Severalother modes of asexual reproduction have also beenreported by various authors [8, 9]. However, we didnot observe any other form of reproduction nor didBardele [4] in his very much more detailed investiga­tion.

The numbers of Clathrulina elegans var, planktoni­cum in the plankton of Loch Ness are extremely low;less than one per litre. However, the densities of all theprotozooplankton elements are low in this oligotrophicsystem. It is worth noting that while C. elegans var.planktonicum is largely confined to the wintermonths, Actinosphaerium species occur throughoutthe year in Loch Ness in densities of up to 381-1

[12]. The occurrence of heliozoa in the plankton offreshwater lakes tends to be sporadic [2, 11]. Most re­ports relate to eutrophic and mesotrophic lakes. Incomparison oligotrophic water bodies are poorly re­searched. For the few lakes where seasonal data areavailable heliozoa are a consistent element in the plank­ton, for example in Loch Ness and in a large ultra-oli­gotrophic Antarctic lake [10]. While one cannot makeany solid generalizations from so few data sets, it maybe that the pelagic zone of oligotrophic lakes offers an

1

I

30~!

!

-r:»..----: -~t-r ,> X

. : r ./ I

/ / .. / /

Fig. 2. The life-cycle events of Clathrulina elegans var, plank­tonicum.

this did not elicit stalk production. Hence we considerthis stalkless variety of C. elegans to be a sub-specieswhich is morphologically adapted to a pelagic modeof life. We propose the name Clathrulina elegans var.planktonicum.

The sequence of events in the life-cycle of the sub­species are summarised in Fig. 2. In all the specimenswe have observed reproducing, only two fission prod­ucts occurred (Figs. Id & 2). This differs from theevents described by Bardele [4], Foulke [8] and Archer[1] for C. elegans, in which more than two fission prod­ucts are developed. Immediately prior to fission the ax-

ideal environment for heliozoa in terms of both bioticand abiotic conditions.

Acknowledgements

This work was funded by a Natural Environment ResearchCouncil grant. We are grateful for the help of Adrian Shinewith field sampling and Dr. Ken Oates for his assistance withelectron microscopy.

References

1 Archer W. (1877): Resume of recent contributions to ourknowledge of "Freshwater Rhizopoda". Part III. Heliozoa(Desmothoraca) and Monothalamia (Monostomata).Quart. ]. Micro. Sci., 17, 67-80.

2 Arndt H. (1993): A critical review of the importance ofrhizopods (naked and testate amoebae) and actinopods(heliozoa) in lake plankton. Mar. Microb. Food Webs,7,3-29.

3 Bardele C. F. (1971): On the morphogenetic role of micro­tubules during stalk formation in Clathrulina elegans(Desmothroraca). ]. Protozool., 18 (Suppl.), 18.

4 Bardele C. F. (1972): Cell cycle, morphogenesis, and ultra­structure in the pseudoheliozoan Clathrulina elegans. Z.Zellforsch., 130, 219-242.

Stalkless Species of C. elegans . 57

5 Cienkowski L. (1867): Dber Clathrulina, eine neue Acti­nophryengattung. Arch. mikr. Anat., 3, 311-316.

6 Febvre-Chevalier C. (1985): IV Class Heliozoea Haeckel1866. In: Lee].]., Hutner S. H., and Bovee E. C. (eds.): Anillustrated guide to the protozoa, pp. 302-317. Society ofProtozoologists, Lawrence, Kansas.

7 Febvre-Chevalier C. (1990): Phylum Actinopoda, ClassHeliozoa. In: Margulis J. 0., Corliss J. 0., and ChapmanD.]. (eds.): Handbook of protoctista, pp. 347-362. Jones& Bartlett, Boston.

8 Foulke. G. (1884): Some phenomena in the life-history ofClathrulina elegans. Proc. Acad. Nat. Sci. (Philadelphia),17 -19.

9 Hertwig R. und Lesser E. (1974): Uber Rhizopoden unddenselben nahestehende Organismen. 3. Teil Heliozoa.Arch. mikr. Anat., 10, (Suppl.), 147-236.

10 Laybourn-Parry ]., Marchant H. ]., and Brown P. E.(1992): Seasonal cycle of the microbial plankton inCrooked Lake, Antarctica. Polar Biol., 12,411-416.

11 Laybourn-Parry]. and Rogerson A. ( 1993): Seasonal cy­cle of protozooplankton in Lake Windermere, England.Arch. Hydrobiol. (in press).

12 Laybourn-Parry]., Walton M., Young]., Jones R. I., andShine A. (1995): The protozooplankton of a large oligo­trophic lake, Loch Ness, Scotland. ]. Plankton Res. (inpress).

13 Penard E. (1904-5): Sur les Sarcodines du Loch Ness.Proc. Roy. Soc. Edinb., 25, 593-608.

14 Murray]. (1904-5): The rhizopods and heliozoa of LochNess. Proc. Roy. Soc. Edinb., 25, 609-613.

Key words: Heliozoea - Desmothoracida - Clathrulina - Loch Ness

Judith Young, Institute of Environmental and Biological Sciences, University of Lancaster, Lancaster, LA1 4YQ, United King­dom