Part of a large population of Noumea haliclona on its food sponge Darwinella. Mallacoota, northern Victoria, February 1983
PHOTO: Bill Rudman.
At times we see huge numbers of a particular species and then other times there will be none. In other cases common species at a particular spot will disappear for months or even years. Without long term studies of each individual species it is difficult to come up with "general rules" for phenomena such as these. The causes of such population fluctuations, both explosions and declines, are often difficult to determine. The most common and most spectacular examples, probably because they are intertidal and relatively large, concern Sea Hares. Often they are found on beaches packed so thickly that one is forced to walk on the animals to get past [see Mass Mortality].
When we find large sea slugs, mating or laying eggs, then disappearing, it is reasonable to think that the animals have "migrated" to the egg-laying spot then moved away. This type of interpretation has led to many accounts of "breeding migrations" which on further study have been shown to be open to a much more simple explanation. Most sea slugs, being specialist feeders, live where their food is found. In many cases they will settle out of the plankton directly on to a colony of their food, or in the case of herbivores, on to a food plant or place where the plant they feed on is abundant. Since they can grow remarkably quickly if food is abundant, the apparent sudden appearance of a large population of a species may be the result of the rapid growth of juveniles on abundant food rather then the migration of animals from one part of the shore to another. Sea Slugs are hermaphrodite, with a full complement of male and female organs. When they are mature they regularly mate with any available partner. Large aggregations of mating animals are often called "mating aggregations" but we must be careful to realise that this is not a planned "orgy" but the result of their feeding behaviour leading to an aggregation of animals, which in turn leads to mating.
Often population crashes are the result of environmental catastrophes. Here are a couple of examples involving the small chromodorid, Noumea haliclona. In New South Wales it feeds on pink and a yellow forms? [or species?] of the sponge Darwinella. It is often common intertidally in Sydney Harbour, feeding on a yellow Darwinella, but at other times there are none to be seen. In the 1980s, the Sydney Harbour intertidal population of this species was wiped out for over a year by a severe storm which occurred after 2 years of little rainfall. This storm caused huge quantities of freshwater to flow into the harbour turning the top layer of water almost fresh, and killed all the sponge colonies in the intertidal and in the top half metre of the subtidal. On another occasion, in northern Victoria, I found a population of millions of animals all feeding on a pink intertidal form of Darwinella (see photo). In this case severe storms washed all the intertidal sponge away in 2 nights and even a year later the intertidal population had not recovered. I guess the important lesson is that there is no general rule about such things. We need to study species, and perhaps populations, individually, as each will have its own needs and capabilities.
We also need to have an understanding of local conditions. The New South Wales coast, in eastern Australia, is essentially warm temperate , but at times there is a very tropical element in the flora and fauna, as warm water currents, flowing from the tropical north, carry larvae of tropical species down the coast. For opisthobranch larvae to settle and survive they need to find their preferred food organisms, which means that sometime earlier the larvae of their food would have needed to drift south as well. To understanding life histories and population survival therefore, we have to understand not just the life cycle and biology of one species, but the life cycle and biology of its food, its predators and many physical aspects of the environment.
Rudman, W.B., 2004 (August 2) Population fluctuations. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/popfluc
February 8, 2006
From: Kira Alleyne
How is it that opisthobranchs are a bio-indicator species? I am currently doing research on the opisthobranchs of Barbados since the last study was in 1974 and am finding difficulty locating any. I have only been able at the moment (after sampling two locations) to find three specimens; Discodoris evelinae, Elysia crispata and Dendrodoris krebsii. I still have six more locations to sample.
email@example.comAlleyne, K., 2006 (Feb 8) Opisthobranchs as bio-indicator species. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15732
I'm not quite sure what question you want answered. Firstly, what research are you doing? Are you just having a look for opisthobranchs or are you interested in some aspect of the biology or ecology of one or more species? Hopefully the era where people 'do a locality' and write a paper on what they found, is almost over, as without a broader understanding of the biology and variability of individual species, such papers often cause their own problems for future workers.
If you are asking about bio-indicator species as a comment on lack of abundance - For example 'Why would you use an uncommon animal as an indicator species?" Then you may have a point. But your question raises other areas of discussion as well.
Since miners took canaries down coal mines in the 19th century to indicate when poisonous gases were about - [when the canary fell off its perch and died the miners ran] - we have been looking for other animals to give us early warnings about impending disasters. Bio-indicators - as they are known - became a popular tool for researchers justifying the usefulness of their research when applying for grant funds. In desperation - I guess - they think politicians and bureacrats would think it valuable to have bio-indicators for pollution, global warming etc etc. While I sympathise with the problems of grant applicants, I can't say there are many true examples of what could be called a 'bio-indicator'. I certainly don't know of any opisthobranch that is truthfully a bio-indicator - though I am open to suggestions.
Now to your problem of a shortage of opisthobranchs in the Barbados. Opisthobranchs, especially in the tropics, are notoriously unreliable. While in cool temperate waters you can usually find a group of 'old faithfuls', in warmer waters this is not necessarily so. Many species have planktotrophic larvae, so their presence as benthic adults at any one place is dependent on the vagaries of the currents and the health of microscopic planktonic larvae. Their presence also depends on similar uncertainties in the life cycle of their food organism, be it plant, sponge, ascidian, hydroid etc etc which may also be dependent on a planktonic larval stage. When you think about it, it is amazing that sometimes everything clicks, the food organism passes through its larval stages and settles and grows to a suitable size to enable its prey nudibranch larvae to find it, to settle, to grow to maturity and then find a mate to start the cycle over again.
If you search for caribbean in the Forum you will find quite a diversity of species are found in the region. Unfortunately, apart from a few, we know very little about their abundance and natural history. I am afraid most studies of the fauna have been done during short stays by visiting scientists during university holidays. Not to say that such studies aren't valuable, but you take a risk in hoping to find an abundance of animals at a particular time. Also to progress past the 'what I collected on my holidays research' we need some permanent residents to study populations in depth.
April 22, 1998
From: David & Leanne Atkinson
I have been on holidays and diving a lot at Port Stephens over the last few weeks. About 3 weeks ago the nudibranch population more than suddenly halved and has remained static since. The water temp has remained at 20 deg for the last 6 weeks, so I can't blame that. All the "exotic" species have gone and we are left with the species that are here most of the year. Polycera capensis seems to still around in reasonable numbers. Hopkinsia sp. is still around, but numbers have declined.
Any suggestions as to the cause of this phenomenon?
David & Leanne Atkinson.
firstname.lastname@example.orgAtkinson, D. & L., 1998 (Apr 22) Population decline - Port Stephens. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/153
Without longterm studies of each individual species it is difficult to come up with "general rules" for phenomena such as you have observed. The causes of population fluctuations, both explosions and declines, are often difficult to determine. The most common and most spectacular examples, probably because they are intertidal and relatively large, concern Sea Hares. Often they are found on beaches packed so thickly that one is forced to walk on the animals to get past. These are often interpreted as breeding swarms, but although some may be copulating, as adults usually do when they find themselves in close proximity, the usual cause of the great numbers is a recent storm which has washed them off the grass or algal beds on which they were living, up on to the beach.
In the case of your observations at Port Stephens it may just be the end of the life span of some of the species involved. I suspect that many of the tropical species at Port Stephens arrive as larvae from further north when the southerly flowing current or eddies move in close to shore. If the larvae that settled at Port Stephens in the spring arrived at the same time it is possible that their life span or the life span of the colonial animals they feed on has ended somewhat synchronously as well. The populations of non-tropical species are also declining around Sydney as winter approaches. To better understand these fluctuations we need to study the lifecycles of many more species and their food..
The other thing to realise is that unlike many land animals, most sea slugs have free swimming larvae which hatch out of the egg ribbon. These larvae can spend many weeks in the plankton and can be swept far from their "place of birth". The makeup of a population at say Port Stephens is therefore very dependent on things that have happened far from Port Stephens. No matter how ideal conditions may be at Port Stephens, if conditions did not favour the production of eggs and larvae larva further up the coast, or if the currents were not close enough to shore to bring the larvae to Port Stephens, then populations at Port Stephens the next year may be dismal... Bill RudmanRudman, W.B., 1998 (Apr 22). Comment on Population decline - Port Stephens by David & Leanne Atkinson. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/153
April 13, 1998
From: Kirsten Benkendorff
I find several species of Opisthobranchs which suddenly appear in the intertidal area, lay their eggs and then disappear. One such species is this communally spawning Discodoris, which I do not yet have a species name for. I was wondering if anyone could enlighten me as to why they might do this. I would have thought that the intertidal area was a harsher environment in which to leave eggs than on subtidal reefs. But not being a diver myself I can't even be sure if this species is usually found in subtidal habitats.
Dept. Biological Science,
University of Wollongong, NSW 2522
email@example.comBenkendorff, K., 1998 (Apr 13) Population fluctuations. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/152
Firstly, Richard Willan has suggested your "Discodoris" is Platydoris galbanus. On your question about short term visitors, have a look at my comments on population decline at Port Stephens [#153]. When you find a couple of large Sea Slugs, mating or laying eggs, then disappearing your reasonable thought is that the animals have "migrated" to the egg-laying spot then moved away. This type of interpretation has led to many accounts of "breeding migrations" which on further study have been shown to be open to a much more simple explanation.
Most Sea Slugs, being specialist feeders, live where there food is found. In many cases they will settles out of the plankton directly on to a colony of their food or in the case of herbivores on to a food plant or place where the palnt they feed on is abundant. Since they can grow remarkably quickly if food is abundant, the apparent sudden appearance of a large population of a species may be the result of the rapid growth of larvae on abundant food rather then the migration of animals from one part of the shore to another. In the case of some Sea Hares they change the type of algae they feed on say from Red Algae to brown or Green algae as they grow. In this case it is crawling youngsters rather then swimming larvae that search out the appropriate plant. If the plant is more common intertidally, then that is where the animals will "migrate" to. Sea Slugs are hermaphrodite with a full complement of male and female organs. When they are mature they regularly mate with any available partner. Large aggregations of animals all mating are often called "mating aggregations" but we must be careful to realise that this is not a planned "orgy" but the result of their feeding behaviour leading to an aggregation of them, which in turn would lead to mating.
If you see a large aggregation, have you any evidence to show that the animals have really moved here recently on "foot"? It is more likely that the animals settled near to where you found them and were just overlooked by observers until they were big enough to be noted, or like in some Aplysia reports they were washed together by storms.
On intertidal versus subtidal living. Some species have a wide distributional range from the shallow intertidal to 30 or 40 meters deep. The Discodoris egg ribbon has been laid on the underside of a large intertidal rock, a good place to hide from predators and to stay wet. Many subtidal animals still lay their eggs in crevices and under rocks, even though they are underwater. Clearly this suggests that laying eggs under stones may protect the settling larvae from more than drying out. The most likely reason for this behaviour is to protect the slugs, and their eggs, from being eaten by fish.
A good example of animals living where they can rather than where they ought to is the small chromodorid, Noumea haliclona. It feeds on a pink and a yellow forms? species? of the sponge Darwinella. At times I have found it commonly intertidally in Sydney Harbour feeding on a yellow Darwinella and on another occasion I found a population of millions of animals all feeding on a pink intertidal form of Darwinella in northern Victoria (see photo). In the Sydney Harbour example the whole intertidal population was wiped out for over a year after a severe storm, after 2 years of little rainfall, caused huge quantities of freshwater to flow into the harbour turning the top layer of water in the harbour almost fresh and causing the death of all the sponge colonies in the intertidal and in the top half metre of the subtidal. In the northern Victorian case severe storms washed all the intertidal sponge away in 2 nights of severe storm and even a year later the intertidal population had not recovered. The subtidal however cannot be considered a refuge. While organisms are protected from drying out in the subtidal they are suddenly open to attack from a wide range of potential enemies, including fish. I guess the important lesson is that there is no general rule about such things. We need to study species, and perhaps populations, individually, as each will have its own needs and capabilities