Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

Problems and puzzles in echinoderm demography

Thomas A. Ebert

1. Department of Integrative Biology, Oregon State University, Corvallis, OR 97331, USA;

ebertt@science.oregonstate.edu (*Correspondence).

Received 11-III-2020. Corrected 24-VII-2021. Accepted 24-VII-2020.

ABSTRACT

Introduction: There are problems and puzzles in understanding reproduction, growth and mortality in echino-

derm life cycles. Objective: Explore problems and puzzles in life cycles that are important and challenging.

Methods: The literature is used to elucidate problems associated with all life stages. Results: Sources of larvae

that settle at a site are explored using oceanographic modelling and genetic methods. There are few studies that

have estimated larval mortality in the plankton under field conditions and results differ from experimental results

or patterns of settlement. In a small number of studies, mortality rate of newly settled larvae appears to change

rapidly as individuals grow. There are problems measuring size, and measurement bias that interferes with many

tagging methods used to estimate growth. There also are problems with the use of natural growth lines and com-

monly used software to estimate both growth and mortality from size-frequency data. An interesting puzzle is

that echinoderms may show negative senescence with mortality rate decreasing with size. There is a problem

in fertilization success based on density so there should not be rare species where sexes are separate with free

spawning of gametes yet there seem to be rare echinoderms. Conclusions: All parts of echinoderm life cycles

provide problems and puzzles that are important and challenging.

Key words: echinoderm; life-cycle; plankton; connectivity; growth, mortality; reproduction.

The following review explores problems

and puzzles in the demography of echinoderms

in the context of a complete life cycle for spe-

cies that have planktonic larvae (Fig. 1). This is

not intended to be an exhaustive review of lit-

erature but rather to show areas of echinoderm

biology that are important and require attention

and creative approaches. Some echinoderm

groups have limited number of publications on

particular aspects of life cycles and echinoids

by far have the most detailed studies. In part,

this is due to their importance both as grazers in

near-shore environments and in fisheries. Holo-

thurians also have important fisheries but are

very difficult to study because size is difficult

to measure and they lack hard parts with natu-

ral growth lines. Asteroids also are difficult

to study and in general have limited commer-

cial importance other than as pests or as the

souvenir trade. Ophiuroids in some habitats

have natural lines that can be used to estimate

growth but because they lack any commercial

value have not benefited from many detailed

studies of all parts of life cycles for single spe-

cies. Finally, crinoids have nearly no detailed

studies of their life cycles.

Linkage of sources and sinks: Recogni-

tion of linking sources of larvae with sites of

settlement is not new (e.g. Thorson, 1961)

Ebert, T.A. (2021). Problems and puzzles in echinoderm

demography. Revista de Biología Tropical, 69(S1),

1-13. DOI 10.15517/rbt.v69iSuppl.1.46318

DOI 10.15517/rbt.v69iSuppl.1.46318

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

but the problem is how to determine the links

between sources and sinks and estimate prob-

abilities (Fig. 2). The general framework for

such discussions is connectivity (Wing, Gibbs,

& Lamare, 2003; Cowen, Gawarkiewicz, Pine-

da, Thorrold, & Werner, 2007).

Ocean currents: Modelling ocean cur-

rents can provide a framework for study and a

means of generating hypotheses that need to be

tested (Pineda, Hare, & Sponaugle, 2007). An

example of this approach is for the echinoid

Loxechinus albus (Blanco, Ospina-Álvarez,

Navarrete, & Fernández, 2019). The simulation

presented by these authors used oceanographic

data along the coast of Chile together with a

reasonable estimate of larval development time

for L. albus, 20 days. The simulation started

with establishing blocks along the coast and

seeding these with passive particles and exam-

ining the distribution of particles after a simu-

lation run of 20 days. The pattern of particles

provides reasonable hypotheses for testing the

connectivity of L. albus at sites along the coast.

How one does this is a major problem.

Geochemical tags: Natural geochemical

tags have been used for a variety of species to

indicate sources of larvae (e. g. Thorrold et al.,

2002; Levin, 2006; White, Standish, Thorrold,

& Warner, 2008). Use of elemental signatures

in calcareous structures to trace of the trans-

port of echinoderm larvae, however, may have

limited use. Holothurian and asteroid larvae

lack calcareous structures. Ophiuroids lose lar-

val spicules during metamorphosis. Echinoids,

however, retain remnants of larval spicules at

metamorphosis (Gordon, 1926; Emlet, 1985)

but these are very small and may be too small

for analysis. This has not been explored. A

related issue is whether there are elemental dif-

ferences in echinoid skeletons along a coastline

of interest that could be used to identify source

areas for larvae. This is a question that could

be addressed with current technology using

samples from spines that show regenerating

tips and internal regions separated by growth

interruption lines. Spine tips regenerate rapidly

and so represent close to point-samples in time

Fig. 1. Life-cycle graph with uncoupled reproduction

(source) and settlement (sink) with four size classes; all

arrows have a time constant of one year so fx values include

gamete production, survival in the plankton, and post-

settlement processes to an age of one year when individuals

are in size-class 1; gx values are probabilities of growth to

the next size class multiplied by the survival probability; rx

values are the probabilities of not transferring to the next

size class multiplied by the survival probability.

Fig. 2. Four locations (A, B, C, D) showing possible

transfer probabilities, Tx, in a directional current that

include numbers of gametes produced at a site multiplied

by the probability of successful transfer to the same or

other site, the connectivity of sites.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

(i.e. Ebert, 1967; Heatfield, 1971; Gorzelak,

Stolarski, Dubois, Kopp, & Meibom, 2011).

Analysis of regenerating spine tips would be a

rapid way of discovering whether local elemen-

tal signatures might be useful in identifying

sources of echinoid larvae that settle.

Genetic markers: Genetic measures of

connectivity (Hedgecock, Barber, & Edmands,

2007) provide large-scale patterns that show

accumulation of differences over many years.

All echinoderm classes are represented in

such studies: crinoids (Hemery et al., 2012),

asteroids (Marcus, 1977; Keever et al., 2009;

Yasuda et al., 2012), ophiuroids (Cho & Shank,

2010), holothuroids (Uthicke & Benzie, 2003),

and echinoids (Watts, Johnson, & Black, 1990;

Miller, Supernault, Li, & Withler, 2006; Casila-

gan, Juinio-Meñez, & Crandall, 2013).

Connectivity on a finer scale to indicate

sources and sinks is less common and may indi-

cate the nature of the problem of addressing the

source or sources of larvae that settle at a site.

There are studies of the echinoid Strongylocen-

trotus purpuratus in southern California and

Mexico that explore genetic patterns. Edmands,

Moberg, and Burton (1996) using allozymes

found differentiation over short distances could

be as great as those seen at larger scales. A lack

of genetic pattern in recruits of S. purpuratus

was reported by Flowers, Schroeter, and Bur-

ton (2002) with no indication of reproductive

sweepstakes in the sense that for any year some

females provided more offspring than others.

Settlers on brushes with ages of one or two

weeks appeared to have resulted from many

females. Changes in the shape of the coastline

appeared not to generate patterns at least for

species with long generation times so suf-

ficient unusual settlement events can obscure

differences (e.g. Olivares-Bañuelos, Enríquez-

Paredes, Ladah, & De La Rosa-Vélez, 2008).

Major coastal features can create patterns

for some species and indicate that sources

and sinks are within particular subregions

of overall distribution. The echinoid Arbacia

stellata was studied along the central coast of

Baja California, Mexico, and around up into

the middle of the Gulf of California (Olguín-

Espinoza, 2003) using allozyme analysis of ten

polymorphic loci. Breaks were evident associ-

ated with Punta Eugenia and the tip of Baja

California (Fig. 3). Further differentiation was

evident along the east coast of the peninsula to

Fig. 3. Connectivity of Arbacia stellata around Baja California, Mexico based on analysis of ten polymorphic loci. A.

locations of intertidal study sites. B. genetic relationships among populations based on a distance Wagner tree rooted at

the midpoint of the longest path; there are breaks associated with coastline features: Punta Eugenia and Cabo San Lucas;

distance is important within the Gulf of California (after Olguín-Espinoza, 2003).

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

Bahía de los Angeles. The differentiation may

indicate restrictions to gene flow and so aid

in understanding the connectivity of sources

and sinks. Generation times may be very short

for Arbacia species based on rapid growth and

size structure (Olguín-Espinoza, 2003; Barrera,

2018) and short generation times would pro-

mote differentiation.

The problem of connectivity using genetic

methods is difficult and made more so with

echinoderm species that have mixed reproduc-

tion methods such the holothurian Stichopus

chloronotus (Uthicke, Benzie, & Ballment,

1999; Soliman, Takama, Fernandez-Silva, &

Reimer, 2016; Pirog et al., 2017).

Understanding the connectivity of echi-

noderms and other marine species becomes

very important in the development of marine

reserves and fishing regulations. If larvae are

mixed from many sites as indicated by Flowers

et al. (2003) importance for maintaining a spe-

cies may depend very little on particular marine

reserves and more on the overall stock density.

On the other hand, species that have short gen-

eration times and high dispersal may benefit

from both site selection and size of reserves.

Connecting populations may continue to be

best done using a combination of genetics and

oceanographic models as suggested by Hedge-

cock et al. (2007) with a focus on following

cohorts, which is not an easy problem to solve.

Mortality in the plankton: A major prob-

lem in the plankton studies is the estimation

of mortality. It is generally agreed that loss in

the plankton must be very high given the large

number of eggs produced per female and the

number of new individuals age-one that appear

in benthic populations. But what are the mortal-

ity values? Two studies in particular have esti-

mates of planktonic mortality rates under field

conditions. Rumrill (1987) sampled plankton

in an arm of Barkley Sound, Vancouver Island,

British Columbia, Canada, and estimated den-

sities of early developmental stages of the echi-

noids Strongylocentrotus droebachiensis and S.

purpuratus on a daily basis during spawning

episodes. The rate of decline from peak density

was used to estimate daily mortality. A similar

approach was used by Lamare and Barker

(1999) to estimate daily mortality of the echi-

noid Evechinus chloroticus larvae in Doubtful

Sound, South Island, New Zealand.

An experimental approach to larval mor-

tality focused on just the role of predation

(Johnson & Shanks, 2003). In experimental

chambers, daily losses due to predation could

be very low and approaching zero. A differ-

ent approach to estimating larval mortality is

given by analyzing the consequences of larval

cloning where persistence in the plankton dur-

ing development would require low rates of

mortality (Ebert & Janies, 2020). Comparison

of different estimates (Table 1) shows the wide

divergence of estimates.

An additional complication to survival in

the plankton is provided by settlement of S.

purpuratus larvae on brushes in southern Cali-

fornia. Gonads increase in mass during sum-

mer and fall with a drop indicating spawning

(e.g. Basch & Tegner, 2007). Ocean circulation

is complex south of Point Conception. The

Channel Islands provide conditions favorable

to formation of eddies and gyres in the Santa

Barbara Channel (e.g. Harms & Winant, 1998)

that may hold larvae near shore and increase

the settlement season. Larval retention in the

Santa Barbara Channel may be the explanation

for the striking difference between settlement

at Scripps Institution of Oceanography and

Gaviota (Fig. 4). A short period of spawning

and a long period of settlement means that sur-

vival in the plankton may be much better than

indicated by the work of Rumrill (1987) and

Lamare and Barker (1999). We still have a very

poor understanding of survival and sources of

mortality in the plankton.

Post-settlement growth and mortality:

There are few studies of post-settlement growth

and survival of echinoderms under field con-

ditions. Rowley (1990) followed cohorts of

newly-settled S. purpuratus at Naples Reef

near Santa Barbara, California. Based on his

Figure 7, mortality, M day

-1

, was 0.034 in bar-

rens and 0.060 in a kelp bed. Growth between

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habitat types also was different with early

post-settlement growth faster on barrens for

the first 50 days (0.42 mm month

-1

vs. 0.31

mm month

-1

). After 50 days and sizes of 0.8-

1.2 mm, feeding begins on pieces of algae and

growth becomes much faster in the kelp bed

so by the end of a year, age-one sea urchins

feeding on algae had an estimated diameter of

1.7 cm whereas they attained only 0.36 cm liv-

ing on the barrens area. Early mortality of the

asteroid Acanthaster planci (Keesing, Halford,

& Hall, 2018) was estimated as 6.5 and 7.8 %

-1

for 1.0 and 1.6 mm individuals, decreasing

with size to 1.2 % d

-1

(2.7 mm) and 0.45 % d

-1

(5.5 mm). The rapid changes in mortality with

small increases in size differ from the more

constant rates observed by Rowley (1990) for

S. purpuratus. Studies of newly settled echino-

derms are rare and have emphasized the role of

small predators and so differences between the

work on Strongylocentrotus and Acanthaster

may reflect different predators or other factors.

Clearly, more field studies of newly-settled

echinoderms are needed.

Growth and survival: Past early growth

and survival there are general problems of

estimating growth, which lead to problems

TABLE 1

Estimates of mortality (M) of echinoderm larvae in the plankton; Johnson and Shanks (2003) measured losses in

replicated chambers due to predation over one-day periods; Ebert and Janies (2020) are calculations based on the

assumption that a population of larvae could be sustained in the plankton by budding only with development

time of 25 days

Species Location days M day-1 Reference

Evechinus chloroticus

Doubtful Sound, South Is., New Zealand 14 0.171 Lamare & Barker, 1999

Evechinus chloroticus

Doubtful Sound, South Is., New Zealand 17 0.242 Lamare & Barker, 1999

Evechinus chloroticus

Doubtful Sound, South Is., New Zealand 30 0.209 Lamare & Barker, 1999

Evechinus chloroticus

Doubtful Sound, South Is., New Zealand 10 1 Lamare & Barker, 1999

Strongylocentrotus droebachiensis

Barkley Sound, Vancouver Is., Canada 13 0.156 Rumrill, 1987, 1990

Strongylocentrotus droebachiensis

Barkley Sound, Vancouver Is., Canada 17 0.063 Rumrill, 1987, 1990

Strongylocentrotus purpuratus

Barkley Sound, Vancouver Is., Canada 11 0.266 Rumrill, 1987, 1990

Dendraster excentricus

Coos Bay, OR, & Friday Harbor, WA, USA 1 0.001 Johnson & Shanks, 2003

Oreaster sp. with one bud

Gulf Stream-Florida Current 25 0.028 Ebert & Janies, 2020

Oreaster sp. with two buds

Gulf Stream-Florida Current 25 0.044 Ebert & Janies, 2020

Fig. 4. Differences between settlement per week (wk) of Strongylocentrotus purpuratus on scrub brushes at Scripps

Institution of Oceanography (SIO) and Gaviota; Gaviota is in the Santa Barbara Channel where gyres sometime form

(Harms & Winant, 1998); original data from Ebert, Schroeter, Dixon, and Kalvass (1994).

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

of estimating mortality. The first problem is

measuring size. For some echinoderms the

problem is obvious: holothurians are very dif-

ficult to measure. Both length and weight are

subject to changes in size of individuals in

response to being handled or their resting state.

For example, Herrero-Pérezrul, Reyes-Bonil-

la, García-Domínguez and Cintra-Buenrostro

(1999) working with the holothurian Isosticho-

pus fuscus measured length underwater with a

tape to avoid disturbing the sea cucumbers and

then brought individuals to the surface to deter-

mine weight. They found a positive allometric

relationship between length and weight but

with a wide spread of data points so individuals

that were 20 cm long weighed between 100 and

500 g. Isostichopus fuscus that measured 30 cm

long weighed between 300 and 800 g. Measur-

ing size of holothurians is difficult and it is not

clear whether length is a more consistent mea-

sure than weight or whether contracted length

is better than relaxed length. Inter-radial ossicle

width is related to contracted body length in

Holothuria (Halodeima) atra (Ebert, 2010). An

inter-radial ossicle 4 mm wide can be associ-

ated with a length of 6 to 22 cm. A further com-

plication of the relationship is due to asexual

reproduction by fission that will result in large

ossicles in short sea cucumbers or very small

ossicles in the regenerating posterior segment.

Although measuring sea urchins seems

simple, the presence of spines and associated

tubercles can lead to size-specific biases. For

example, red sea urchins, Mesocentrotus fran-

ciscanus, from San Nicolas Island, California,

were measured live and individuals were again

measured following cleaning in 5 % sodium

hypochlorite bleach. The difference between

cleaned and live plotted against diameter mea-

sured live (Fig. 5) shows that there is substan-

tial measurement bias so a sea urchin that was

measured with a diameter of 9 cm, when mea-

sured following cleaning may have been only

8.3 cm but also may have been very close to 8.3

cm when measured wet. The important point is

that there is bias in measurement and the level

of bias can exceed the amount to growth over

a period of study such as a year or longer. The

implication is that any tagging method that uses

external tags such as Floy or dart tags or inter-

nal tags like PIT or coded wire will produce

flawed growth curves that will tend to have an

estimated maximum size much smaller than

actual growth.

Natural growth lines have been used

to estimate age for many echinoderm spe-

cies: ophiuroids (Gage, 1990; Dahm, 1996;

Quiroga & Sellanes, 2009), echinoids (Deutler,

1926; Moore, 1935; Nichols, Sime, & Bishop,

1985; Cabanac & Himmelman, 1996; Ouréns,

Flores, Fernández, & Freire, 2013), and holo-

thuroids (Sun, Hamel, Gianasi, & Mercier,

2019). There are several problems with using

natural lines. The first is the dependence on

worker variation in what lines are counted. For

example, two studies of the sand dollar Echi-

narachnius parma (Cocanour, 1969; Cabanac

Fig 5. Bias in measurement of Mesocentrotus franciscanus

collected in 1990 at San Nicolas Island, California (Ebert

& Russell, 1992); sea urchins were measured live (wet)

and again following cleaning with sodium hypochlorite

that removes spines. A. measurements by M.P. Russell. B.

measurements by T.A. Ebert; the trend is there is a greater

probability that large sea urchins will have greater bias in

wet measurements.

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& Himmelman, 1996) show differences in the

estimates of age and growth (Fig. 6) that are

the result of what lines are classed as annual.

A second problem is found in old individuals

that are growing so slowly that annual lines

cannot be resolved. This problem usually is

not addressed and the result is that age of large

individuals is seriously underestimated and

hence a growth model that is selected may be

flawed and estimates of mortality underesti-

mated. Some authors, however, have recog-

nized and commented on the problem (Brey,

Pearse, Basch, McClintock, & Slattery, 1995;

Shelton, Woodby, Hebert, & Witman, 2006).

Validation of lines with tetracycline show clear

support for annual growth of small and medium

sized sea urchins but tetracycline lines can be

so close to the edge of small ossicles of large

individuals that they have been considered to

be errors (Ouréns et al., 2013).

Mortality estimation: Estimating mor-

tality is difficult and usually is done using a

combination of growth and size. A general

approach is to use a length-converted catch

curve where growth parameters are used to

change size-frequency into age-frequency. The

decline in age-categories are used to estimate

mortality. Common programs for doing this are

FISAT II, ELEFAN in R, and TropFishR. These

methods use the von Bertalanffy growth model

and so have an asymptotic size, S∞, which has

problems associated with estimation (Schwam-

born, 2018). The problem can be illustrated

using data from tagged Echinometra mathaei

when the von Bertalanffy model is used. When

von Bertalanffy growth parameters were esti-

mated from tagging, asymptotic size was esti-

mated to be at the mode of large sea urchins

(Fig. 7). To estimate mortality, growth has to

be known for individuals on the descending

limb of the size-frequency distribution, which

is not possible with the parameters determined

using von Bertalanffy growth. A solution is to

estimate a different maximum size and this is

done using the Powell-Wetherall plot method

(Powell, 1979; Wetherall, 1986). With a larger

asymptotic size, a new value for the parameter

K can be determined by drawing a line through

the mean of the growth data, the dashed line in

Figure 7. The new growth parameters can now

be used to estimate annual mortality, M, using

a length-converted catch curve. The estimate of

M is 0.146 yr-1, which is the same, 0.145 yr-1,

as determined using the Richards function and

combining the size data from 1976 and 1977

(Ebert, 1982). These estimates are similar to

the means of 1976 and 1977 estimated using

means of size data and different growth models

(Ebert, 2013): 0.136 yr-1 for von Bertalanffy

and 0.141 yr-1 using the Tanaka model. The

von Bertalanffy growth parameters derived

using the Powell-Wetherall method are poor

compared with the actual tagging data. The

estimates of annual mortality, however, are not

unreasonable. This is interesting and deserves

Fig. 6. Number of growth lines counted in the sand dollar,

Echinarachnius parma, showing the difference between

counts by different workers. A. Cocanour (1969). B.

Cabanac and Himmelman (1996); the consequence of

differences results in different estimates of size at age and

different growth models that would be appropriate.

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additional exploration using different growth

models with the length-converted catch curve.

Senescence: The model used to describe

adult mortality, M, is the simple decaying

exponential:

This equation is used because information

is lacking to require a more complicated model

that would include changes in mortality rate

with age or size; M is a constant. A consequence

of having a constant mortality rate regardless of

age or size is that there is no senescence or neg-

ligible senescence (Finch, 1990). A question is

whether echinoderms show changes in mortal-

ity with age or size and there is evidence that

they do. Survival rate may actually improve

rather than decline with age. Vaupel, Baudisch,

Dölling, Roach, and Gampe (2004) coined the

term negative senescence and proposed that

candidates should have indeterminate growth,

begin reproduction early in life and then con-

tinue to reproduce for as long as they live.

Data for echinoderms to test the presence of

these characteristics are few. Three sea urchin

species have sufficient data that show changes

in size-specific reproduction, growth, and mor-

tality (Ebert, 2019). An example, S. purpura-

tus, shows indeterminate growth, increased

reproductive output with size, and decreased

mortality with size (Fig. 8). It is unknown

how common this pattern might be for other

echinoderms. Problems include demonstrating

indeterminate growth, which requires using a

method that can measure very small growth

increments, which as indicated above prob-

ably must be done using chemical tagging of

all sizes. Size-specific reproduction requires

gonad changes for all sizes and analysis done

with raw data rather than using restricted sizes.

Changes in mortality can be estimated using

the method of Van Sickle (1977) but there may

be problems that need exploration such as how

variance in size changes with time and how

this might impact the estimate of mortality.

Determining whether negative senescence is

common in echinoderms is challenging.

Rarity and fertilization success: Can

echinoderm species with separate sexes and a

planktonic stage, also be rare? Collections at a

location may contain species that are uncom-

mon or rare but merely reflect local conditions

such as shown by ophiuroids collected under

intertidal rocks at False Point, La Jolla, Cali-

fornia (Muscat, 1975). Not all of the species

in Table 2 have planktonic larvae. Both, the

ophiuroids Ophioplocus esmarki and Amphi-

pholis squamata are brooders and Ophiactis

simplex has mixed reproduction that includes

fission. Also, the intertidal under rock habitat

is not preferred by some species. For example,

Fig. 7. A. A problem with the length-converted catch curve

using the right limb of the size-frequency distribution when

the von Bertalanffy growth model is used (solid line in A);

the original size at which growth is 0 is D∞, 4.82 cm and

the associated growth constant, K = 0.52 yr-1; D∞, 4.82 cm

is at the mode of the size distribution, B. and so cannot be

used to convert sizes to ages; dashed line in A is the result

of estimating D∞ by the Powell-Wetherall method, which

gives D∞, 6.69 cm and associated value of K = 0.17 yr-1 so

the sizes of the right limb can be converted into age classes.

Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

Ophiothrix (Ophiothrix) spiculata is very abun-

dant subtidally and O. simplex is abundant in

sponges so being uncommon or rare can just

result from sampling in marginal habitats.

What remains is Ophiocnida hispida. Is it rare

or just not sampled in a preferred habitat? Little

is known about O. hispida. Austin & Hadfield

(1980) simply say that it occurs in the inter-

tidal of southern California. It is present farther

south (Granja-Fernández et al., 2014) but few

specimens were collected in survey. Little is

known about the biology of O. hispida but it

probably has separate sexes with free spawn-

ing of gametes. These are traits that Vermeij &

Grosberg (2018) argue are not compatible with

persistence because of problems of fertilization

when individuals are widely spaced.

The newly discovered asteroid Oreaster

(Janies et al., 2019) appears to be rare as a

benthic stage but it is possible that population

maintenance is through budding in the plankton

as indicated above. Rarity is a problem and

a usual conjecture is that the species may be

much more common in some place that has yet

to be discovered. Problems with fertilization

can be related to rates of adult mortality. If

mortality rates are low then low rates of fertil-

ization can be compensated for by having many

spawning events over a lifetime. Most years

may be unsuccessful but occasionally success-

ful fertilization occurs. Mortality rates are the

result of mechanisms that promote life and

hence under forces of selection. Problems with

fertilization close the life-cycle (Fig. 1) started

at the beginning of this manuscript.

Conclusion: Work is needed for all echi-

noderm groups to resolve problems and puzzles

to understand growth and survival at all life

stages from connectivity across geographic

regions through processes in the plankton,

early post-settlement growth and mortality,

TABLE 2

Ophiuroids under intertidal rocks. False Point, La Jolla,

California, 1973-74; area = 1 750 m2 (Muscat, 1975)

Species Total

Ophionereis annulata

2 513

Ophioplocus esmarki

2 401

Amphipholis squamata

618

Ophiothrix (Ophiothrix) rudis

545

Ophioderma panamense

209

Ophiopteris papillosa

101

Ophiothrix (Ophiothrix) spiculata

Ophiactis simplex

Ophiocnida hispida

Fig. 8. Strongylocentrotus purpuratus growth (A. Sunset

Bay, Oregon); maximum and minimum gonad sizes (B.

Sunset Bay and Gregory Point Oregon); size structure

and annual size-specific mortality rate, M, (C. Sunset Bay

1964-2009); S. purpuratus has indeterminate growth, no

reduction of game production with size (age) and shows

decreasing mortality, M yr-1, with size; these are attributes

of negative senescence.

Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 1-13, March 2021 (Published Mar. 30, 2021)

problems with measuring size and determin-

ing patterns of growth and mortality, changes

in mortality with age or size that may indicate

negative senescence, and problems of rarity.

All of these problems are inter-related in the

total life cycle.

Ethical statement: authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are

fully and clearly stated in the acknowledge-

ments section. A signed document has been

filed in the journal archives.

ACKNOWLEDGMENTS

I thank the organizing committee of the 4

Latin American Echinoderm Conference and in

particular Dinorah Herrero for inviting me to

La Paz to give the lecture on which this paper

is based. Work with settlement in southern

California was with S. Schroeter and J. Dixon.

Work with growth of S. purpuratus in Oregon

was with J.C. Hernández and S. Clemente.

Portions of the work were supported by the US

National Science Foundation, the Washington

Department of Fish and Wildlife, the US Fish

and Wildlife Service, Directors Sea Urchin

Advisory Committee of the California Depart-

ment of Fish and Game and personal funds.

RESUMEN

Problemas y acertijos en la demografía

de los equinodermos

Introducción: Existen problemas y acertijos en la

comprensión de la reproducción, el crecimiento y la morta-

lidad en los ciclos de vida de los equinodermos. Objetivo:

Explorar los problemas y acertijos en los ciclos de vida

que son importantes y desafiantes. Métodos: La literatura

es usada para dilucidar los problemas asociados con todas

las etapas de vida. Resultados: Las fuentes de larvas que

se asientan en un sitio se exploran usando modelos ocea-

nográficos y métodos genéticos. Existen pocos estudios

que han estimado la mortalidad larval del plancton bajo

condiciones de campo y los resultados difieren de los

resultados experimentales o los patrones de asentamiento.

En un número pequeño de estudios, la tasa de mortalidad

de las larvas recién asentadas parece cambiar rápidamente

a medida que los organismos crecen. Existen problemas

para medir tamaños y el sesgo de medición interfiere con el

uso de muchos métodos de marcado para estimar el creci-

miento. También hay problemas con el uso de las líneas de

crecimiento natural y con los programas comúnmente usa-

dos para estimar tanto el crecimiento como la mortalidad a

partir de datos de frecuencia de tamaño. Un acertijo intere-

sante es que los equinodermos pueden mostrar senescencia

negativa con una tasa de mortalidad que disminuye con el

tamaño. Existe un problema con el éxito de la fertilización

basado en la densidad, por lo que no debería haber especies

raras cuando los sexos están separados y existe un desove

libre de gametos, sin embargo, parece haber equinodermos

raros. Conclusiones: Todas las partes de los ciclos de vida

de los equinodermos proveen problemas y acertijos que son

importantes y desafiantes.

Palabras clave: equinodermos; ciclo de vida; plancton;

conectividad; crecimiento; mortalidad; reproducción.

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