5.9 Species connectivity
IThe dispersal of species may occur on small to large scale and contributes to a very important biological process in marine conservation: the connectivity. The connectivity, terrestrial or marine, is the exchange of individuals among populations through the passive transport and/or active movement of individuals at various life stage (i.e. gametes, larvae, juveniles, sub-adults and adults). In marine environment, adult movement (for mobile species) and larval dispersal (for sessile species) is important for the continued presence of species in a location, including the ability of a species to colonize or re-colonize habitats.
The movement of individuals from one place to another occurs for a number of important reasons, which include genetic mixing, outbreeding, the seeking of new habitats, the reduction in competition, and the avoidance of predators. The dispersal scale in marine species is
||Broadcast spawning||Shed eggs and sperm into the water column (e.g. most fish and echinoderms)|
|Brooding||Females collect sperm from water column and fertilize eggs internally (e.g. sponge, cnidarians, molluscs, ascidians)|
|Hermaphrodites||Produce both female and male gametes either simultaneously or sequentially|
||Planktonic larvae||Feed in the plankton|
|Lecithothrophic larvae||Feed on a yolk reserve|
|Duration of planktonic period
||Planic||Larvae that develop in the plankton for hours or week|
|Aplanic||Larvae with no free-living dispersal stages|
|Behaviour of the larvae
||Benthic||Larvae that develop on close association with the substratum|
|Planktonic||Larvae that spend a portion of their development-time swimming freely in surface water|
extremely wide-ranging: from the distance of millimetres to hundreds of kilometres. Dispersal may take individuals great distances from where they started, never to return, or it may remove them from their points of departure only temporarily, to experience new and profitable habitats before returning to where they were born.
The larval dispersal depends on features and strategies adopted by the species during their life cycle. Once larvae have been released from the parent, or hatched from the egg, their role is to live long enough to disperse in the right direction for a distance sufficient to arrive at a suitable place for settlement and recruitment as a juvenile. Several larval biological traits combine to achieve this goal, including:
All these features and strategies have their advantages and disadvantages. Having no larval stage means that the risk of dying from predation or getting lost before settling is removed, but the likelihood of competing and indeed breeding with close relatives is much enhanced. Even closely related species can show very different strategies.
After the spawning output, the larval dispersal could be via currents (then the direction and the distance they travel will depend on the current strengths and direction) and via current plus behaviour (e.g. vertical migration and horizontal swimming). During this stage the larvae have to survive the predators. At last, the larvae are ready to settle in habitats where they can recruit into the adult population and begin the cycle again. It is crucial that this settlement takes place at the most appropriate time and in the best environmental conditions to maximize survival, and in the most suitable specific habitats (Fig.1_Ses5.9_ Kris Beckert, IAN Image Library).
In healthy ecosystems, many habitat patches are connected by larval dispersal of species easily moving between them (for example a fully functional coral reef system). While in a perturbed or degraded system, as individual patches disappear, the connectivity declines, leaving isolated patches outside the spatial range of connectivity. In these situations, if the species have poor power of dispersal, they are more vulnerable to recruitment and settlement failure.
Connectivity is a fundamental aspect to consider when evaluating the status of existing MPAs (Fig.2_Ses5.9_Jane Hawkey, IAN Image Library), since well-connected and highly diverse populations are more resilient to environmental changes and less subject to face local extinction. Connectivity is important to maintain population resilience, to reduce the extinction risk and to preserve diversity.
5.9.1 Pro and cons of the reproductive feature: broadcast spawning. Provide an example.
Pros and cons of the reproductive feature: broadcast spawning
Broadcast spawning is a reproductive feature that includes releasing male and female reproductive cells in their habitat and fertilization occurs outside the female’s body.
Although broadcast spawning seems as a primitive way of reproduction, many animals such as sponges, fish, jellyfish, urchins and others use this method of producing offsprings. This means that they must be doing something right. Some of the benefits include large numbers of descendants, which insures continuation of their species. Furthermore, because fertilization is happening outside the body of a female unit, the chances of finding suitable habitat are much more likely. What’s more, and is very important, is that parents don’t have to invest their energy in raising their children.
However, there are some flaws in this technique, the most notable one is that fertilization doesn’t happen as sperm can get carried away by sea currents or end up being eaten by other animals. Even if sperm reaches the egg cell, the danger doesn’t stop there, larvae could still get eaten or carried away into unsuitable habitats. Furthermore, due to global warming, average temperatures are rising and that negatively affects fertility of animals which in turn lowers chances of successful fertilization and limits the early embryo’s life span.
In conclusion, broadcast spawning as all things in life has some good and some bad sides. In my opinion, broadcast spawning is not a very effective way of reproduction as it hinges on many environmental factors that dictate the course of procreation. Especially in today’s world impacted by climate change.
Sources of information:
• Udžbenik iz biologije za drugi razred gimnazije “Živi svijet 2” Zdravko Dolenec, Gordana Rusak 15.3.2019.
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1689025/ 15.3.2019.
5.9.2 Pro and cons of the reproductive feature: brooding. Provide an example.
Brooding or Broodiness is a behaviour of some egg-laying animals to sit on the clutch of
eggs to incubate them. It’s especially present among birds. In essence, incubation is the
process of maintaining uniform heat of the developing eggs.
When it comes to broodies, body heat from the brooding parent provides the needed
temperature. The incubation period varies from 11 to 85 days but generally speaking,
smaller birds tend to hatch faster.
There are many pros of brooding. For instance, the parent, while sitting on the eggs,
protects the eggs from potential predators. Parents can also detect non-fertile and non-
developing eggs and will kick them out of the nest.
When the eggs hatch, parents will feed the newly hatched chick and, in some cases,
teach them what to eat and where to find food. The parent can also expose the chick to
the pathogens and bacteria in the environment in order to build a strong immune
However, there are also some cons of brooding. Even though brooding parents protect
the eggs, they themselves are exposed to danger from predators.
Not all broodies are good parents. Some will abandon their nest partway through. Once
the chicks hatch, some parents will lose interest in caring for them, or worse, maim or
All in all, I find brooding quite interesting and, in my opinion, positive aspects of it
overshadow some negative sides of brooding.
5.9.3 Pro and cons of the developmental mode:gonochorist. Provide an example.
A gonochorist is an individual of a species which belongs to one out of two or more sexes and it retains the same sexuality during its life.
The first advantage of gonochorism is preventing uniparental (one-parent) reproduction. Because of gonochorism, animals are able to exchange genes and be different morphologically and physically. Another advantage is the possibility of independent evolution, which means that a male and female can evolve separately and can develop the best properties for both male and female. Every gonochorist descendant will be different because of gene exchange of male and female individuals. A disadvantage of gonochorism is that 2 individuals of different sexes have to find each other, which is hard if they don't have well-developed movement. Secondly, 2 individuals have to meet at the time of mating, which is also difficult. Another problem of gonochorism is fewer descendants, which will not secure the survival of the species and it can become extinct much easier than hermaphrodite species.
I think gonochorism is much better than hermaphroditism despite the cons of gonochorism. Gonochorists have better evolutionary possibilities and they prevent “cloning”. Because of the gene exchange, every individual will be different and that offers better individual development.
A good example of a gonochorist is the sea urchin; it has sexual dimorphism but has 2 different sexes. Sexual dimorphism means that they have 2 different sexes but they look the same. They have outer fertilization and from fertilized egg cells larva pluteus is developed. Pluteus is a planktonic organism, it floats free in water and because it floats, the sea urchin can colonize larger areas in the sea. The main disadvantage of sea urchins is weak development of movement, but that is why they have outer fertilization and secure survival.
5.9.4 Pro and cons of the developmental mode: hermaphrodites. Provide an example.
Hermaphrofite is an organism with both male and female reproduction organs or some other sexual characteristics. The majority of tunicates, slugs and even earthworms are seen as hermaphrodites. Hermaphroditism can be separated in three divisions: Sequential, simultaneous and pseudohermaphroditism. Almost everything in our lives has its pros and cons, so do hermaphrodites.
Hermaphrodites have their advantages. They don't need to find a different mate to their sex as a result of that , they can preserve their energy and spend it on something much wiser. Furthermore self-fertilization is also impossible (it reduces genetic diversity) which makes certain species more vunerable to predators whose only plan is to eat them, also parasites are quite dangerous as well. Therefore, immense number of species have developed alternative methods to avoid self-fertilization, for example snails whose reproductive organs are indeed complex. Besides that, they are not in danger of being extinct owing to both male and female reproductive organs.
On the other hand, hermaphrodites also have a few cons. For example, more energy is needed to maintain both reproductive systems, so it's crucial for them to manage their energy and spend it wisely. Moreover, another big con is the fact that they've got limited gene diversity so their reproduction is basically limited.
In my opinion some characteristics of hermaphrodites are really interesting, for example their inner structure, it's really weird to imagine having both male and female organs at once. That fact amazes me and it will for the rest of my life.
5.9.5 Pro and cons of the larval nutrition mode: planktonic. Provide an example.
Plankton is a group of organisms that live in water and are not capable of swimming against a
current. Small phytoplankton cells usually live in low nutrient waters called oligotrophic. The reason
for that is that they are more efficient at using smaller amounts of nutrients than large
phytoplankton cells. Also smaller species have a higher cell surface area which allows them to take
up more available nutrients.
The most important elements of phytoplanktons growth are minerals: N (nitrogen), P (phosphorus)
and Si (silica). Regeneration of nutrients sustains the growth of phytoplanktons especially in areas of
low phytoplankton productivity. This process usually happens when organic matter dissolves to
inorganic nutrients using microbes in the plankton.
The traditional model of a short marine food chain contains phytoplankton, copepod and fish.
Phytoplanktons have developed certain methods of feeding. A strategy commonly used by many
kinds of planktons is mixotrophy. It can be described as a strategy that combines characteristics of
autotroph organisms (which make their own food using photosynthesis) and heterotroph organisms
(which ingest food that autotroph organisms provided). In addition, some species of ciliates are
known to contain large numbers of chloroplasts.
5.9.6 Pro and cons of the larval nutrition mode: lecithothrophic.Provide an example.
Many marine species use indirect development, a kind of life history that includes a larval stage before adulthood. Larvae have to choose between two nutrition modes, planktotrophic and lecithotrophic with each carrying a set of advantages and disadvantages.
A pro of lecithotrophic species is that they produce fewer but larger eggs made out of yolk which supplies them with high enough food levels for their growth and metamorphosis. In addition,they spend less time in the plankton compared to the planktotrophic larvae, which allows them to develop significantly faster and have bigger chance of survival under hard marine circumstances surrounded by many predators. Lecithotrophic larvae begin to feed after metamorphosis and that is a big pro because it allows them to reach a peak in their ability to catch prey. On the other hand, lecithotrophic larval nutrition mode has some massive disadvantages of demographic nature. They are not able to spread as much across the sea because they produce less eggs and according to some research significantly slower than planktotrophic larvae. Furthermore, the mother loses large amounts of energy into producing egg yolk reserves for her descendants.
To conclude, the lecithotrophic larval nutrition mode is the best option for the survival of an individual organism because of stronger and more dependable food sources in the early stages of life, while on the other hand, for the survival of a specific species I would give advantage to the other nutrition modes due to their greater reproduction rate.
5.9.7 Pro and cons of the duration of the larvae planktonic period:planic.
The duration of the planic planktonic larvae phase can be teleplanic in which the planktonic period lasts for several months, anchiplanic, that lasts from a few hours to a few days, and actaeplanic which lasts for periods of one week to two months.
In the first place, the pros of the planic duration of the larvae planktonic period in most cases is that the larvae and adults have different diets, so this prevents adults from competing with their young for food. An additional advantage is large geographic range and lower risk of extinction, which means greater persistence in geological time. Also, larvae consume a lot of food, but in the adult stage they are mainly responsible for reproduction so they eat less.
However, larvae might limit the ability of adults to shift or expand. While they are in metamorphosis, their larva stage limits mobility, so they are unprotected from the predators. The most important disadvantage is that they are using more energy for metamorphosis and reproduction. An additional disadvantage is the reduced opportunity for local adaptation.
In conclusion, the probability that juveniles will occupy a favorable habitat is enormous, which is an advantage for the larvae, but not for the other organisms in the sea.
5.9.8 Pro and cons of the duration of the larvae planktonic period: aplanic.
In contrast to marine organisms whose offspring go through an extended planktonic stage, the young of direct developers (aplanic) develop directly into juveniles. Direct development does not mean that no changes occur between planktonic stage and adulthood. One very obvious change is the growth of the organism. There are some pros and cons about such development.
Firstly, one of the cons of direct development is the competition among siblings of marine organisms for the same food sources as they have the same diet. Direct development significantly reduces survival advantages. Secondly, there is also competition between parent and offspring.
On the other hand, there are also some pros of direct development. Aplanic marine organisms don't need to use excessive amounts of energy to go through an extended planktonic stage. An additional advantage is decreased probability that the offspring will occupy favourable habitat. Larvae will not limit ability of adults to shift or expand. Those larvae are not a liability.
In conclusion, marine organisms who develop directly into juveniles have a greater competition among the same species, including the competition for food sources and habitat.
However they don't need to have additional stages as that is the case in planic development of some marine organisms. Aplanic development in some species can be an advantage as well as a disadvantage.
5.9.9 The difference between benthic and planktonic larvae (use examples).
The difference between benthic and planktonic larvae
A larva is a juvenile form many animals go through before metamorphosing into adults. The Larvae, among other things, can be devided into planktonic and benthic larvae.
One of the species that has a planktonic larva are urchins. After the fertilization which happens in water, the fertilized eggs develop into a planktonic larvae. For at least a week and often longer it is carried by currents and tides. They are not in control of their faith. Because of that, animals often release a huge amount of larvae to increase the chances that at least one will survive. Finally, far from the place where it was first born it settles to the bottom where it will begin to metamorphose through different stages until it becomes an adult unit. Planktonic larvae often do not look similar to what they will develop into later.
The benthic species of larvae are those that never move far from the bottom. Whether they are attached to it, walk, swim, crawl or dig,they are always quite close to the ground. They often look like the adult they will develop into. They are also known as “crawl-away larvae” because they crawl away from their egg after hatching. I was not able to find an example of a sea animal that produces benthic larvae.
This leads me to the conclusion that there are more species known to us, or at least they are more popular, that have planktonic larvae than those who have benthic larvae.