[Date Prev][Date Next][Date Index]

Update Avon Point Biology: Hydras and Peritechs

     Below is a further update on the biology at
the reef at Avon Point. This update is concerned
with two animals the solitary hydras (the colonial
hydras will be handled at some other date)and the 
colonial Peritechs. As always if anyone else is 
working with these animals I would love to hear
from you.
                      John Lavelle
                      December 21, 1998

     As far as I know this is the first time a 
quantitative study has been attempted for population
densities of both Hydras and Peritech in any of the
Great Lakes. In both cases I do not want to imply a
greater degree of accuracy than is the case. When 
the rocks underside area was determined I treated
the rock by it's general shape. That is if it looked
basically like a rectangle, circle, or triangle the
area was figured as if it were that shape. Also I rounded
the rock sizes to the nearest centimeter. In some
cases in 1993 and 1994 I did not make actual counts 
of the animals only an area of the rock and had to 
extrapolate the total from this. 

     From the general characteristics of the animals
I think that these are Hydra americanus (but I could 
be wrong). The following tables shows the Hydras 
population peaking in June of every year. In 1993 
and 1995 there was a smaller population increase
in October (1993) or September (1995). Every year the
density of the Hydras has been decreasing. A shot
at speculation would be that the hydras are decreasing
because of the decreasing numbers of crustaceans.

     Average monthly population density for hydras.
The numbers are the average number of hydras per square cm. 
The rock under surface area is only the area of the rocks 
taken into account, not included are the rocks sides which
may also have a large population of hydras. The hydra were 
only rarely found on the rocks upper surfaces. The figures
were derived from about sixteen samples per month (far fewer
samples were taken in the months of April and Nov.).

              HYDRAS PER SQUARE CM.                      
       1993    1994     1995    1996   1997  1998
April           0                              0
May   0.078   0.0007   0.0016    0      0      0
June  0.203   0.2318   0.4069  0.134    0.067  0.158
July  0.281   0.1123   0.1393  0.093    0.051  0.008
Aug   *       0        0.0091  0.007    0.012  0.002
Sept  *       0.0016   0.0476  0.014    0.014  0.0008
Oct   0.383   0.0034   0.0254  0.012    0.022  0.0044
Nov.                                    0      0.006

* for these two months the hydras were extremely rare.

                    Green Hydras
   In the past 6 years I have only run into these animals

    July 16, 1995  one animal seen. The green hydras' stalk
was 2 mm. long and the 5 tentacles were about 1/4 the length
of the stalk.  

    August 17, 1997. Very short tentacles about 1/4 the stalk


    Since these animals, the "Peritechs" are so little
known to people who are interested in the Lake's biology
I thought a brief description would be prudent to put
here. None of the following is derived from my work. All
the information except that dealing specifically with
Avon Point is from the sources listed below this introduction.
	The two species Zoothamnium and Carchesium belong to 
the order Peritrichida.
	The Peritechs are found in a wide variety of 
        environments and while the Peritechs as an 
        order can be solitary or colonial, sessile 
        or mobile, stalked or free-swimming, we are 
        concerned here only with the colonial sessile 
        members of the order. The colonies of the 
        Zoothamnium found at the Point are palm leaved 
        shaped colonies between 1 and 2.5 mm. in height 
        and about the same size in width. Normally the 
        Zoothamnium are symphorionts to other benthic 
        animals, but here on the shale reef, the colonies, 
        while sometimes found on Zebra mussels, the vast 
        majority are attached to the underside of rocks.  
        Most of the species are bacteria feeders. A bell 
        shaped like vessel is attached to the substrate 
        by means of a stalk, all of the stalks are connected 
        together. The stalks being thus joined together are 
        not independently contractile, the whole colony 
        contracts at the same time. This not true of the 
        Carchesium, which is also present on the reef. The 
        Carchesium's stalks while they join together to 
        form a colony remain independent of each other 
        and each stalk contracts individually. The colony 
        expands by fission, the fission taking place in a 
        definite way giving the colonies of each species 
        a definite shape. The colonies are founded by large 
        bells which detach from the colony and swim away. 
        After swimming for several hours they attached to 
        the substrate and grow a stalk, beginning a new colony.	
        Normally the colony remains active until November 
        or December and then produces a resting cysts to 
        over-winter. Here, at Avon Point, the colonies all 
        but disappear in June and do not reappear until the 
        following Spring. The Carchesium differs from the 
        Zoothamnium in shape, they are not palm-leaf shaped 
        rather they are shaped like a funnel.
  Tom Fenchel. "Ecology of Protozoa" 1987
(Brock/Springer series in contemporary bioscience)     
Science Tech. Inc. Madison Wisconsin. Page 6
  Parker, Sally  "Synopsis and Classification of Living Organisms". 
      Page 628 -  Subclass - Peritricha
  Libbie Hyman " The Invertebrates" ;  Protozoa through 
                 Ctenophora".  1940
        McGraw-Hill New York
 V.A. Dogiel " General Protozoology"   
                        2nd Edition 1965       
 The Clarendon Press  Oxford (Pages 285,278-80,464,471)	

            1994   1995  1996   1997     1998
April         +                          0.09
May         1.41   0.12  0.28  0.955    0.45  
June        0.03   1.3*  1.42  1.62     0.004
July        0      0     0.09  0.049    0.02
Aug         0      0     0.08  0.445    0.042
Sept        0      0     0.03  0.204    0.064
Oct         0      0     0     0.218    0.045
Nov         0      0            0        0

+ The Zoothamnium were found abundantly in this month
  but, regretfully I didn't count the entire number of
  colonies found on the rocks. Instead I took samples
  of using of a area of the the rocks underside. This
  was a mistake in hindsight (but than how was I to know 
  I was going to keep doing this for four years).
  The notes do tell that rocks were found with 11 
  zoothamnium per roughly an area of 30 mm.sq. at their
  densest and 2 zoothamnium per 30 mm. sq. elsewhere on 
  the rocks. 

* The Peritechs were only found on two rocks in June.
  The following table lists all the rocks sampled
  for the month. The second rock of June 1st had an
  extraordinary number of colonies but after that 
  date not a single colony was seen.

     Below is an example of how the Zoothamniums abruptly
disappear from the Lake's bottom and it also shows how 
the colonies are not uniformly found on the rocks. These
colonies some to have a preference for certain rocks.
What it is they prefer I have no idea. 

         Area    Number 
         Sq.cm  of colonies
June 1    48      51
         110    1460
June 7    99       0
          44       0
          40       0
June 11   70       0
         153       0
Jun14    112       0
          54       0
          40       0
June 18   85       0
June21    88       0
          30       0
June25    48       0
          77       0
June 28   84       0

    Because the data collected for 1992 and 1993 was not 
consistent in form I can not breakdown the number of colonies
found per square cm. But it is important to note the changes
that have occurred in their population densities over the
years. To that end I put the summariares covering those years
below. I've also included notes on 1994 as this year had
a period when the Carachism colonies were found in great

     In 1992 the colonies abruptly disappeared from the 
samples on June 7. On May 31 a rock sample had its entire 
underside covered by colonies at a density of 40 colonies 
per sq.cm. On the sample taken on June 7 and those taken 
after this date no colonies were seen until August 2 and 
than they were widely scattered and rare. The colonies were 
occassionally found attached to free floating algae and 
unattached drifting alone. As stated above the colonies 
were able to fold up within one second and 10 seconds 
was the time needed to re-extend itself. The colonies 
were between 1 and 2 mm. high, and about the same size in 
width. The Zoothamnium behaved in one way differently in 
May of 1992 then the colonies found in 1993/1994. These 
May colonies did not respond to touch by folding up. 
On May 10 a large field of zoos having a density of 
30 stalks per sq. cm had the colonies within the field 
folding up apparently at random. A colony would snap shut 
while those all around it remained opened. This lack of 
response to stimuli is strikingly seen from another rock 
taken from a depth of 12 feet. The bottom of this rock had 
a Zoothamnium field with a density of 60 stalks per sq. cm. 
Amid the colonies were several brown hydras, one hydra moved 
into the dense field of zoos. There was no response by the 
colonies to the hydra moving among them, they did not close 
up when touched by the hydra, and those that were closed 
opened even with the hydra right over them.

	From the first sample, taken May 9, the Zoothamnium 
responded to touch by snapping shut. The colonies did not 
come close to consistently reaching the population density 
found in 1992. The greatest density found in 1993 was 25 
colonies per sq. cm, the normal density on the rocks sampled 
that had Zoothamnium fields was more like between 10 and 15 
colonies per sq. cm. Unlike the year before, in 1993 the 
colonies did not abruptly disappear, rather they slowly 
decreased in numbers. While decreasing in numbers though 
they did manage to stick around for a lot longer time, 
their density on June 26 was 20 colonies in a 7 cm. sq. area. 
The colonies disappeared from the samples after June 30. 
Carachism were more abundant in 1993 then 1992. In 1992 the 
Carachism were rarely found and when found the colonies were 
widely scattered. 1993 the Carachism colonies were not only 
more common but they too were found in fields, the densest 
cluster seen was 13 colonies in a 1.5 cm. sq. area.


     The year could be considered the year of the Carachism. 
In 1992 and 1993 the Carachism were generally found widely 
scattered.  In 1994 they were, for a brief period, the dominant 
organism on the reef. Whether this high population was due to 
the sampling being done earlier and the Carachism is normally 
at such levels in the early Spring or some new condition in the 
Lake's environment caused their high density is unknown.